Sex-based Differences in the Efficacy and Tolerability of Selective Serotonin Reuptake Inhibitors in Adults with Major Depressive Disorder: A Systematic Review and Meta-analysis

Abstract

Purpose of the Review:

Sex-based differences in antidepressant outcomes may influence treatment decisions in major depressive disorder (MDD). This systematic review and meta-analysis evaluated sex-based differences in the efficacy and tolerability of selective serotonin reuptake inhibitors (SSRIs) in adults with MDD.

Collection and Analysis of Data:

PubMed, Cochrane Library, and Scopus were searched electronically, supplemented by citation searching. Two reviewers independently screened records and assessed full-text articles for eligibility. Of 3,523 records identified, 100 duplicates were removed, and 237 full-text articles were assessed, resulting in 10 included studies published between 2005 and 2025. Eligible studies included at least one SSRI arm with extractable sex-stratified outcome data specific to SSRIs. Risk of bias was assessed using RoB 2, the Newcastle–Ottawa Scale, and design-appropriate appraisal tools. Certainty of evidence was evaluated using GRADE. Pooled estimates were generated using random-effects meta-analysis.

Results:

Ten studies (2005–2025) were included. Remission (six studies, 3,865 participants) showed OR: 1.18 (95% CI: 0.96–1.44; I² = 0%); response (two studies, 3,006 participants) OR: 1.39 (95% CI: 0.87–2.22; I² = 51.8%); and continuous symptom improvement (three studies, 3,050 participants) standardized mean difference 0.40 (95% CI: 0.04–0.75; I² = 74.9%), each favoring female. Certainty ranged from moderate (remission) to low (response, continuous improvement).

Conclusions:

SSRI efficacy outcomes showed a consistent female-favoring direction, clearest for continuous symptom improvement, but estimates were limited by heterogeneity, imprecision, and incomplete sex-stratified reporting. The evidence does not support sex-based prescribing; adequately powered, prospectively sex-stratified studies are needed.

Key Messages:

• Question: Do treatment outcomes with SSRIs differ between adult males and females with major depressive disorder (MDD)?

• Findings: Across pooled analyses, SSRI efficacy outcomes showed a consistent direction favoring females, with the strongest signal for continuous symptom improvement.

• Meaning: Sex may influence SSRI treatment response in MDD, underscoring the need for adequately powered, prospectively sex-stratified studies.

Major depressive disorder (MDD) is a common, disabling, and heterogeneous psychiatric disorder that imposes a substantial global health burden.^1–3 The World Health Organization estimates that 4.0% of the global population experiences depression, including 5.7% of adults, and recent global burden analyses show that depressive disorders remain more burdensome in women than in men.^1,3 In the National Epidemiologic Survey on Alcohol and Related Conditions-III survey, the 12-month and lifetime prevalence of Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5) MDD were 10.4% and 20.6%, respectively; the corresponding estimates were 13.4% and 26.1% in women compared with 7.2% and 14.7% in men, underscoring both the frequency of MDD and its sex-related distribution in adults. ² Despite the availability of established treatments, marked between-patient variation in symptom profile, illness course, and treatment outcome continues to complicate routine management.^3,4

Selective serotonin reuptake inhibitors (SSRIs) remain among the most commonly used first-line pharmacotherapies for adults with MDD because of their broad evidence base and generally favorable balance between efficacy and tolerability. ⁴ Contemporary guidelines also emphasize individualized treatment selection, since adverse effects, acceptability, comorbidity, and patient preference may influence adherence and longer-term outcomes even within the same antidepressant class. ⁴ In this context, clinically relevant sex-based differences in SSRI efficacy or tolerability could influence expectation setting, monitoring, and treatment planning.^5–7

The current literature, however, remains fragmented and difficult to interpret.^6–8 Some reports suggest that women may respond more favorably to serotonergic antidepressants in certain settings. Still, the overall evidence has been inconsistent across study designs, antidepressant comparators, outcome definitions, and patient populations.^6,8 Biological plausibility for sex-based differences is supported by literature implicating monoaminergic, inflammatory, neurotrophic, and endocrine pathways, while reproductive stage and menopausal status may further modify treatment response.^5,7 Interpretation has also been limited by incomplete sex-disaggregated reporting and the frequent reliance on secondary subgroup analyses rather than studies designed primarily to test sex-based differences.^6,8

A focused synthesis limited to SSRI-specific, sex-stratified evidence is needed. We narrowed this review to SSRIs for three reasons. Current guidelines place SSRIs as the first-line class for adult MDD, so sex differences within this single class speak most directly to everyday prescribing. ⁴ A single-class analysis removes a weakness of earlier work, which pooled SSRIs with tricyclics and SNRIs and could not separate drug-class effects from sex. ⁶ Ovarian hormones influence serotonergic function, so the biological case for a sex difference is strongest for serotonergic drugs. Confining the question to this class sharpens that signal rather than obscuring it. We carried out this systematic review and meta-analysis to evaluate whether SSRI treatment outcomes differ between adult men and women with MDD. Remission was the primary efficacy outcome. Response, continuous symptom improvement, and tolerability were the secondary endpoints.

Methods

Protocol and Reporting Standards

This systematic review and meta-analysis was conducted in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA 2020, Supplementary table-PRISMA checklist) statement and the methodological principles of the Cochrane Handbook for Systematic Reviews of Interventions.^9,10 The review was prospectively registered in PROSPERO-CRD420251026151. Subsequent protocol refinements, including extending the evidence window to 2025 and treating pooled secondary analyses as pooled-study units, were agreed upon by all authors to ensure methodological consistency.

Eligibility Criteria

Inclusion Criteria

Eligible studies included randomized and non-randomized clinical trials, and secondary analyses of pooled trial data, as well as prospective observational cohorts that provided extractable SSRI-specific sex-stratified quantitative data in adults aged 18 years or older with MDD or a major depressive episode diagnosed using accepted diagnostic criteria, including DSM-IV, DSM-IV-TR, DSM-5, ICD-10, or clearly stated equivalent diagnostic frameworks.^11–14 Studies were eligible if they included at least one SSRI treatment arm and reported separable SSRI-specific efficacy and/or tolerability data by sex. Studies published in peer-reviewed English-language journals between January 1, 2005 and December 31, 2025 were eligible. Throughout, “sex” denotes biological sex (male or female) as recorded in the source studies; none of the included studies reported gender identity.

Exclusion Criteria

Studies were excluded if SSRI effects could not be separated from non-SSRI antidepressants in the reported outcomes, including studies in which the SSRI was used only within augmentation, switching, or combination regimens. Case reports, narrative reviews, conference abstracts without extractable data, animal studies, and studies restricted to non-MDD populations or to children and adolescents were also excluded. Studies restricted to bipolar depression, psychotic or delusional depression, or primary substance use disorders were excluded, as were studies confined to treatment-resistant or augmentation populations; studies of unipolar MDD with commonly co-occurring anxiety remained eligible.

Search Strategy

PubMed, Cochrane Library, and Scopus were searched electronically, supplemented by citation searching of included articles and relevant reviews. A hand search was performed of the psychiatry and psychopharmacology journals that most frequently published SSRI-specific, sex-stratified trials in our scoping searches, namely the Journal of Affective Disorders, the Journal of Clinical Psychopharmacology and the Journal of Psychopharmacology, to identify additional relevant studies published from 2005 onwards. Searches were restricted to human studies published in English between January 1, 2005 and December 31, 2025. The window opened in 2005 to capture the period during which SSRIs were established as first-line agents and sex-disaggregated trial reporting became more consistent; primary trial data from before 2005 are also represented in the included pooled reanalyzes of earlier randomized trials. Search terms combined controlled vocabulary and free-text terms related to sex factors, MDD, SSRIs, treatment outcomes, and adverse effects. The detailed electronic search strategies for each database are provided in Supplementary Appendix S2.

Study Selection

Titles and abstracts were screened independently by two reviewers (AKP, AT), and the same two reviewers independently assessed full texts for eligibility; disagreements at either stage were resolved by consensus or, where needed, by a third reviewer (FAK, MR, or IAK). The review protocol was developed and registered under the guidance of the supervising authors (FAK, MR, IAK). The initial screening process was conducted using the Rayyan AI software. ¹⁵ Duplicate records were removed before screening, and the selection process was documented using a PRISMA flow diagram. ⁹ The study selection process is presented in Figure 1.

Figure 1. The Study Selection Process.

Data Extraction and Data Management

Data were independently extracted by two reviewers (AKP, AT) into a standardized Microsoft Excel sheet and cross-verified against the source articles; any discrepancies were resolved by consensus with the authors (FAK, MR, or IAK), and both reviewers rechecked all key data before analysis. Extracted data included author, year of publication, study design, participant characteristics, sample size by sex, SSRI exposure, comparator, follow-up duration, outcome definitions, validated rating scales, dichotomous event counts, and continuous outcome data (means and standard deviations) where available (Supplementary Table S1). Pooled secondary analyses, such as Khan et al. and Thase et al., were retained as pooled-study estimates rather than decomposed into underlying trial rows to avoid pseudo-replication. Studies with reconstructed subgroup-derived data or narrative-only outcome reporting were handled separately and incorporated only where methodologically appropriate. At the end of the data extraction phase, all key data were reviewed and quality-checked by two reviewers. The full extracted dataset used for the meta-analysis is provided in Supplementary Table S2.

Risk of Bias and Certainty Assessment

Risk of bias was assessed according to the study design. Trial-type studies were appraised using the Cochrane Risk of Bias 2 tool. ¹⁶ The single observational cohort was assessed using the Newcastle–Ottawa Scale. ¹⁷ Detailed RoB2 domain assessments are provided in Supplementary Table S3, with graphical summaries using the ROBVIS online tool shown in Supplementary Figures S1–S2. Overall certainty of evidence for the principal pooled efficacy outcomes was assessed using the GRADE approach (Supplementary Table S4). ¹⁸

Outcome Measures

The primary efficacy outcome was remission following SSRI treatment. Secondary efficacy outcomes were treatment response and continuous symptom improvement on validated depression scales, including the Hamilton Depression Rating Scale (HAM-D/HDRS), the Montgomery–Åsberg Depression Rating Scale (MADRS), the Quick Inventory of Depressive Symptomatology (QIDS), the Beck Depression Inventory (BDI/BDI-II), the Patient Health Questionnaire-9 (PHQ-9), and equivalent validated study-defined measures.^19–21 Response was defined preferentially as a reduction of at least 50% from baseline on a validated depressive symptom scale, or as the study-defined response threshold where a standard 50% criterion was not explicitly reported.^22,23 Remission was defined according to study-specific thresholds on validated depression scales, most commonly criteria such as HAM-D-17 ≤ 7, MADRS ≤ 10, QIDS-SR16 ≤ 5, or equivalent operational definitions.^22,24–26 Continuous symptom improvement was defined as the change from baseline to the selected endpoint on a validated depressive symptom scale. Because remission and response depend on a defined cut-off, we gave preference to clinician-rated scales for these two outcomes where a study reported more than one instrument, and used each study’s designated primary scale for the continuous outcome; when multiple time points were available, the principal study-defined acute-treatment endpoint was preferred. Tolerability outcomes included adverse effects, discontinuation due to adverse effects, sexual dysfunction, and other clinically relevant side effects. These were synthesized narratively rather than pooled, except where sufficiently comparable sex-disaggregated quantitative data were available.

Statistical Analysis

Random-effects meta-analysis was used for all pooled outcomes because clinical and methodological heterogeneity across populations, SSRIs, outcome definitions, and study designs was anticipated. ¹⁰ Dichotomous outcomes were pooled as odds ratios (ORs) with 95% confidence intervals (CIs), and continuous outcomes were pooled as standardized mean differences (SMDs) with 95% CIs to accommodate different validated depression rating scales. ¹⁰ Statistical heterogeneity was quantified using the I² statistic. ¹⁰ Sensitivity analyses were conducted to examine the robustness of pooled estimates by excluding pooled secondary analyses, high-risk studies, large influential studies, or reconstructed subgroup-derived estimates where applicable. To address the inclusion of multiple study designs, we pre-specified a design-based sensitivity analysis that restricted each pooled outcome to randomized evidence, excluding non-randomized interventional studies and observational cohorts.

For the primary outcome (remission), small-study effects and possible publication bias were assessed using the Doi plot and the Luis Furuya-Kanamori (LFK) index, which remain interpretable when fewer than 10 studies are available; this was not undertaken for response or continuous symptom improvement, where too few studies were pooled. ⁵⁵ For the same reason, formal subgroup analyses and meta-regression by individual SSRI, dose, treatment duration, baseline severity, treatment setting, and reproductive status were planned but were not feasible; pre-specified sensitivity analyses were used instead to examine the stability of the pooled estimates and likely sources of heterogeneity (Supplementary Appendix S4). Meta-analyses were performed in R version 4.5.2 (R Foundation for Statistical Computing, Vienna, Austria) with the meta, metafor, metasens, ggplot2, dplyr, readxl, forestplot, and robvis packages. ²⁷

Results

Study Selection

The database searches returned 3,503 records, comprising 98 from PubMed, 11 from the Cochrane Library, and 3,394 from Scopus, with a further 20 records identified through citation and journal searching. After 100 duplicates were removed, 3,423 records were screened by title and abstract. Of these, 269 reports were sought for retrieval, and 237 were assessed in full-text for eligibility; 10 studies were included in the review: Nine randomized or pooled trial analyses and one observational study (Figure 1).

Study Characteristics

The basic characteristics of all the included studies are summarized in Table 1. The 10 included studies were published between 2005 and 2025 and comprised randomized or non-randomized trials, pooled secondary analyses, and one prospective observational cohort. The evidence base included pooled multicenter reanalysis, direct SSRI arm trial data, the large STAR*D sex analysis, one primary care observational study, and more recent SSRI-specific follow-up or comparator data from Gougoulaki et al. and Shamabadi et al.^8,28–36 In every included study, the SSRI was the only antidepressant administered, whether given alone, against a single active comparator, or against placebo; none was an augmentation, switching, or combination-therapy trial. For STAR*D, only Level 1 (citalopram monotherapy) data were extracted, and the subsequent augmentation and switching steps were not used. ⁸ RCTs and observational studies were not combined in any pooled estimate: the observational cohort contributed to the narrative synthesis only. A total of 4,924 participants with MDD were included, with individual study mean age ranging from 18 to 80 years across studies. Studies reporting age data (n = 9) had weighted mean ages ranging from 30 to 53 years, with an overall weighted mean age of 41 years (SD = 7.9). Risk of bias was assessed according to the study design. Of the seven trials evaluated using RoB 2, four had a high overall risk of bias, and three had some concerns. The most frequent issues involved deviations from intended interventions and missing outcome data. The observational study (Pinto-Meza et al.) scored 8/9 on the Newcastle–Ottawa Scale, indicating good methodological quality. Pooled secondary analyses (Khan et al. and Thase et al.) were appraised narratively and judged to have moderate and moderate-to-high concern, respectively.

Table 1.

Characteristics of Studies Included in the Systematic Review and Meta-analysis.

Study	Country	Study Design	Age (Years)	Diagnostic Criteria; Outcome Measures	Sample	SSRI Exposure/Comparator	Follow-up and Outcomes
Khan et al. 2005	USA	Pooled secondary analysis of 15 randomized, double-blind, placebo-controlled trials	18–70 (SSRI subgroup mean ≈44)	DSM-IV; HAM-D-17, MADRS	Depressed outpatients; pooled SSRI subgroup 130 (71 F/59 M)	SSRIs (fluoxetine, paroxetine CR, sertraline, citalopram, escitalopram) vs. placebo/SNRI	Trial-end (LOCF); response, remission
Thase et al. 2005	Multicenter (pooled)	Pooled reanalysis of eight randomized trials; active comparator	Adults (analyzed by age <50/≥50)	DSM-IV; HAM-D-17	Adults with MDD; SSRI pool by sex and age: younger 350 F/200 M, older 131 F/68 M	SSRIs (fluoxetine, paroxetine, fluvoxamine) vs. venlafaxine; placebo	6–12 weeks (parent-trial dependent); remission
Berlanga and Flores-Ramos, 2006	Mexico	Randomized, double-blind; active comparator	18–40	DSM-IV; HDRS, BDI	Adults with MDD; 86 (48 F/38 M)	Citalopram vs. reboxetine	8 weeks; HDRS, BDI
Pinto-Meza et al. 2006	Spain	Prospective observational cohort	18–75	DSM-IV (SCID); PHQ-9, EuroQol VAS	Adults initiating antidepressant treatment; 301 SSRI-treated (242 F/59 M)	Citalopram, fluoxetine, paroxetine, sertraline (no comparator)	3 and 6 months; PHQ-9, EuroQol VAS
Kennedy et al., 2006	Canada	Randomized, double-blind; active comparator	18–65	DSM-IV; HDRS-17, Sex FX, IRSD-F	Adults with MDD; 141 (68 F/73 M)	Paroxetine vs. bupropion SR	8 weeks; HDRS-17, sexual function
Naito et al., 2007	Japan	Non-randomized comparative interventional clinical trial	Adults (fluvoxamine arm mean ≈51)	DSM-IV; MADRS	Japanese MDD patients; total 125; fluvoxamine arm 54 (32 F/22 M)	Fluvoxamine vs milnacipran	Weeks 1, 2, 4, 6; MADRS
Young et al. (STAR*D), 2009	USA	Secondary sex analysis of STAR*D Randomized Controlled Trial	18–75 (mean ≈41)	DSM-IV; HRSD-17, QIDS-SR16	Adults with nonpsychotic MDD; 2,876 (1,833 F/1,043 M)	Citalopram (single arm; no comparator)	Up to 12–14 weeks; response, remission
Vermeiden et al. 2010	Netherlands	Randomized, double-blind (post hoc sex analysis); active comparator	18–65	DSM-IV; HRSD-17	Depressed inpatients; 138 (93 F/45 M); fluvoxamine arm 68	Fluvoxamine vs. imipramine	Endpoint; HRSD-17
Gougoulaki et al. 2021	UK	Randomized, open-label (non-masked) (GENPOD); active comparator	18–74 (mean ≈39)	ICD-10 (CIS-R); BDI-II	Adults with depression in primary care; 601; citalopram arm 298 (200 F/98 M)	Citalopram vs reboxetine	6 and 12 weeks; BDI-II, tolerability
Shamabadi et al. 2025	Iran	Randomized, parallel-group, double-blind; active comparator	18–60 (mean ≈32)	DSM-5 (SCID-5-CV); HDRS-17	Outpatients with MDD; 80 analyzed (40 F/40 M)	Sertraline vs. citalopram	Weeks 4 and 8; HDRS-17

BDI: Beck Depression Inventory, BDI-II: Beck Depression Inventory-II, CIS-R: Clinical Interview Schedule–Revised, DSM: Diagnostic and Statistical Manual of Mental Disorders, F: Female, GENPOD: Genetic and Clinical Predictors of Treatment Response in Depression trial, HAM-D-17/HDRS-17/HRSD-17: 17-item Hamilton (Rating) Scale for Depression, ICD-10: International Classification of Diseases, 10th revision, IRSD-F: Investigator-rated Sexual Desire and Functioning Scale, LOCF: Last observation carried forward: M: Male, MADRS: Montgomery–Åsberg Depression Rating Scale, MDD: Major depressive disorder, PHQ-9: Patient Health Questionnaire-9, QIDS-SR16: 16-item Quick Inventory of Depressive Symptomatology–Self-report, RCT: Randomized controlled trial, SCID: Structured Clinical Interview for DSM, SNRI: Serotonin–norepinephrine reuptake inhibitor, SR: Sustained release, SSRI: Selective serotonin reuptake inhibitor, STAR*D: Sequenced Treatment Alternatives to Relieve Depression, VAS: Visual analog scale.

Quantitative Synthesis

Remission

Remission was pooled from six study rows comprising 3,865 participants. The overall random-effects estimate numerically favored females, but did not reach statistical significance (OR: 1.18; 95% CI: 0.96–1.44; I² = 0%). The largest weight in this model came from the STAR*D sex analysis, with additional contributions from pooled secondary analyses and smaller trial-based studies (Figure 2).^8,28–30,32

Figure 2. Forest Plot of Sex-based Differences in Remission After SSRI Treatment in Adults With Major Depressive Disorder. Random-effects Meta-analysis Shows Pooled Odds Ratios Comparing Remission Outcomes Between Females and Males Across Included Studies.

Response

Response was pooled from two studies comprising 3,006 participants. The pooled estimate again favored females, but remained imprecise (OR: 1.39; 95% CI: 0.87–2.22; I² = 51.8%) (Figure 3). This analysis was driven by the STAR*D sex analysis and the pooled SSRI dataset reported by Khan et al.^8,28

Figure 3. Forest Plot of Sex-based Differences in Treatment Response After SSRI Treatment in Adults With Major Depressive Disorder. Random-effects Meta-analysis Showing Pooled Odds Ratios Comparing Treatment Response Between Females and Males Across Included Studies.

Continuous Symptom Improvement

Continuous symptom improvement was pooled from three studies comprising 3,050 participants. Females showed greater pooled symptom improvement than males (SMD 0.40, 95% CI: 0.04–0.75), representing the clearest pooled efficacy signal in the review. However, between-study heterogeneity was substantial (I² = 74.9%) (Figure 4). The main model included Khan et al. Berlanga and Flores-Ramos, and the STAR*D sex analysis.^8,28,30

Figure 4. Forest Plot of Sex-based Differences in Continuous Symptom Improvement After SSRI Treatment in Adults With Major Depressive Disorder. Random-effects Meta-analysis Showing Standardized Mean Differences Comparing Symptom Improvement Between Females and Males.

Sensitivity Analyses

Sensitivity analyses for remission preserved the female-favoring direction across exclusions, though statistical significance varied with removal of pooled secondary analyses, high-risk trials, or the largest-weight study. For continuous symptom improvement, the female-favoring direction persisted across exclusions. Still, the pooled estimate showed greater instability when restricting or expanding the model, particularly when excluding Khan et al. or STAR*D.^8,28,34 Sensitivity analyses evaluating the robustness of pooled estimates are presented in Supplementary Figures S3–S8.

Certainty of Evidence

Using the GRADE approach, the certainty of evidence was moderate for remission and low for response and continuous symptom improvement. Remission was downgraded for imprecision (CI crossed the null). Response and continuous symptom improvement were downgraded for inconsistency and imprecision due to substantial heterogeneity and reduced stability of the sensitivity analysis. For remission, publication bias was assessed using the Doi plot and LFK index (LFK = −2.46; Supplementary Figure S9); the asymmetry was driven mainly by the heavily weighted STAR*D dataset and was interpreted with caution. ³⁷ Full GRADE judgments are in Supplementary Table S4

Qualitative Synthesis

The qualitative synthesis broadly supported the pooled efficacy findings while highlighting important limitations in reporting of sex-specific tolerability. The strongest narrative support for a female-favoring efficacy pattern came from larger SSRI datasets, particularly the STAR*D sex analysis, which remained the principal source of sex-specific acute-treatment data for remission, response, and symptom change. ⁸ Additional subgroup-based evidence from Thase et al. and Vermeiden et al. suggested that age and menopausal status may modify antidepressant outcomes within serotonergic treatment pathways.^29,34

Tolerability findings were less standardized and more clearly domain-specific than efficacy findings. Kennedy et al. was the most informative study for sexual adverse effects, while Gougoulaki et al. provided more contemporary randomized evidence on SSRI follow-up, discontinuation, and symptom burden in primary care.^32,35 Pinto-Meza et al. added real-world observational support from a female-majority primary care cohort with longer follow-up than most trial-based studies. ³¹ Naito et al. and Shamabadi et al. contributed supportive sex- and subgroup-oriented evidence, but their data were less readily poolable because of subgroup complexity or incomplete tabulation of sex-specific clinical endpoint counts.^33,36 Overall, the narrative evidence suggested that sex-related differences in SSRI outcomes may extend beyond efficacy alone, but tolerability data remained too heterogeneous and inconsistently reported to support a robust quantitative synthesis.

Discussion

This systematic review and meta-analysis identified a consistent female-favoring pattern across pooled SSRI efficacy outcomes in adults with MDD, with the clearest signal seen for continuous symptom improvement.^8,28–30 This is an important finding because continuous symptom change can capture clinically meaningful benefit more sensitively than threshold-based outcomes such as remission or response. In everyday practice, improvement in overall symptom burden may influence functioning, treatment satisfaction, and willingness to continue therapy even before a patient crosses a formal remission threshold. The present findings are therefore consistent with sex being associated with SSRI outcome, particularly when treatment benefit is evaluated across the full spectrum of symptom change rather than by binary cutoffs alone.

The remission and response analyses, though not definitive, were directionally aligned with this pattern, both favoring females with no opposing signal. In the STAR*D sex analysis, women showed better citalopram outcomes than men across major acute-phase measures within the same treatment framework, supporting the possibility that sex-related differences are real even when pooled categorical endpoints remain conservative.^8,22,38 From a prescribing perspective, these findings do not justify sex-specific SSRI selection at present, but they do support considering sex as one of several variables that may shape expected response and follow-up strategy.

The wider literature strengthens this interpretation. Earlier pooled and review-level work suggested that women may derive greater benefit from serotonergic antidepressants than men, particularly when compared with less serotonergically dominant comparators.^6,28,29 Berlanga and Flores-Ramos also reported a more favorable outcome pattern with citalopram than with reboxetine in women, again pointing toward a possible serotonergic advantage. ³⁰ More recently, a pooled analysis of nine sertraline trials including 1,832 participants reported that sertraline was effective in women overall and that the signal appeared stronger in women of reproductive age than in postmenopausal women, supporting the possibility that both sex and hormonal stage influence serotonergic treatment response. ³⁹ At the same time, the literature is not uniformly positive. Gougoulaki et al. did not identify a clear female advantage for citalopram in the GENPOD dataset after adjustment for baseline factors, and their tolerability findings were also more neutral than the earlier literature might suggest. ³⁵ Pinto-Meza et al. and Vermeiden et al. further indicate that the observed sex effect may vary by clinical setting, menopausal status, and outcome definition.^31,34 The present review, therefore, adds a focused SSRI-specific synthesis to a field that has often been discussed more broadly across antidepressant classes.

Several biological mechanisms could plausibly contribute to this pattern. Reviews of sex differences in depression and antidepressant response suggest that serotonergic signaling, inflammatory regulation, neurotrophic pathways, and endocrine modulation differ between males and females in ways that may influence SSRI efficacy.^5,7,40 Estradiol may be particularly relevant. Experimental and translational literature indicates that estradiol can increase expression of tryptophan hydroxylase-2 and the serotonin transporter, while reducing expression of the 5-HT1A receptor and monoamine oxidase A and B, thereby shifting serotonergic tone in a direction that could modify both SSRI efficacy and tolerability. ⁴¹ These interactions help explain why sex-related differences are often discussed more strongly in relation to serotonergic agents than with some non-serotonergic comparators. Clinical studies also support a role for hormonal context. In older depressed women treated with sertraline, estrogen replacement therapy was associated with better antidepressant outcome than sertraline alone. ⁴² In a pilot study of postmenopausal women with MDD, adjunctive transdermal estrogen did not clearly improve final remission but appeared to accelerate early improvement during sertraline treatment. ⁴³ Taken together, these data suggest that ovarian-hormone context may not only influence whether response occurs, but may also influence the pace and clarity of SSRI benefit. An updated review concluded that women and men can differ substantially in antidepressant pharmacokinetics and pharmacodynamics, and another review suggested that women may, on average, have higher antidepressant exposure, and may require lower doses than men; however, routine sex-based dose adjustment is not currently recommended because the clinical relevance of these differences is not yet fully established.^44,45

Pharmacokinetic and pharmacodynamic factors may also contribute. Sex differences in body composition, protein binding, cytochrome activity, and drug distribution can alter antidepressant exposure even when men and women receive the same prescribed dose. ⁴⁴ More specific pharmacogenetic data now support the clinical relevance of this pathway. In a retrospective cohort of 283 patients treated with citalopram or escitalopram, increasing CYP2C19 activity score was associated with better tolerability, and genotype-based metabolizer categories separated patients into lower- and higher-tolerability groups. ⁴⁶ In a separate inpatient study, CYP2C19 slow metabolizers had 36% lower antidepressant efficacy, a 75% lower response rate, 43% more intense central nervous system adverse effects, and 22% more intense gastrointestinal adverse effects than normal metabolizers. ⁴⁷ These studies were not designed primarily as sex-stratified analyses, so they do not prove that genotype alone explains the female-favoring signal observed in this review. However, they strongly support the broader argument that metabolic phenotype can influence SSRI efficacy and tolerability and may interact with sex-related biology rather than operate independently of it.

Sex differences may not be confined to efficacy. Although sex-disaggregated tolerability data were limited, the available studies suggest meaningful differences in the adverse-effect pattern. Kennedy et al. and Clayton and Keller showed that sexual adverse effects with serotonergic treatment are common and may differ in pattern between women and men, implying that tolerability should be assessed at the symptom-domain level rather than only through crude overall adverse-event counts.^32,48 A broader prospective multicenter study of 1,022 outpatients found high rates of antidepressant-associated sexual dysfunction across agents, again reinforcing that adverse-effect burden is substantial and clinically relevant. ⁴⁹ In a meta-analysis of antidepressant treatment during and after menopausal transition, antidepressants significantly improved depressive symptoms (Hedges’ g: 0.44, 95% CI: 0.32–0.57) and increased both response (OR: 2.53, 95% CI: 1.24–5.15) and remission (OR: 1.84, 95% CI: 1.32–2.57) rates, but were also associated with a higher risk of discontinuation due to adverse events. ⁵⁰ This is directly relevant to the present review because it shows that a female-favoring efficacy signal can coexist with important tolerability burdens in female populations. Gougoulaki et al. provide a more contemporary and more neutral perspective, suggesting that not all tolerability domains necessarily worsen in women. ³⁵ Overall, the evidence supports the idea that women may differ from men in the SSRI adverse-effect profile, but a single uniform female ADR pattern still cannot be defined with confidence.

Other clinical and environmental factors may further modify antidepressant outcome. Symptom subtype, psychosocial stress, treatment setting, adherence behavior, and reproductive status may all interact with sex in ways that influence apparent SSRI response.^31,51,52 In the GENDEP study, stressful life events before treatment predicted greater improvement in cognitive symptoms with escitalopram but not with nortriptyline, suggesting that environmental stressors may interact with antidepressant class. ⁵³ In primary care, interpersonal and behavioral factors also appear relevant. In a prospective study of 89 patients treated with sertraline or escitalopram, optimism predicted better early response (HR: 1.22, 95% CI: 1.08–1.38) and better remission probability at 12 weeks (HR: 1.25, 95% CI: 1.1–1.4), while poor adherence and perfectionism predicted worse outcomes. ⁵⁴ Ulbricht et al. further showed that women were more likely to cluster in depressive symptom profiles associated with lower remission probability, suggesting that symptom phenotype itself may mediate part of the observed sex effect. ⁵¹ However, they support a clinically realistic model in which sex, symptom phenotype, stress, adherence, and social context all contribute to the outcome that is ultimately observed in antidepressant studies.

This review has several strengths. It addressed a focused SSRI-specific question rather than combining pharmacologically heterogeneous antidepressant classes. It used explicit sex-stratified extraction rules, outcome-specific synthesis, sensitivity analyses, and structured certainty assessment. It also integrated both efficacy and tolerability findings while conservatively handling pooled secondary analyses, which improved interpretability and reduced the risk of artificial precision.^17,28,29

However, substantial limitations circumscribe the certainty and generalizability of our findings. Our search was limited to PubMed, Scopus, and the Cochrane Library, supplemented by citation searching and a hand search of selected journals; Embase and PsycINFO were not searched, which may have resulted in the omission of studies indexed only in those databases. Only a small number of studies could be pooled for each efficacy outcome, and the evidence base combined direct trials, pooled secondary analyses, and one observational cohort. Several trial-type studies also carried a high risk of bias or some concerns for this specific sex-based question.^16,22 Tolerability outcomes were less standardized and less frequently reported in sex-disaggregated form than efficacy outcomes, which restricted quantitative synthesis in that domain. Continuous symptom improvement showed substantial heterogeneity, and remission and response were still heavily influenced by a small number of key datasets, particularly STAR*D. Our findings apply to SSRI monotherapy and should not be extrapolated to augmentation, switching, or combination regimens, where the SSRI’s specific contribution cannot be isolated. SSRIs are not pharmacologically interchangeable, and within-class differences in metabolism, such as differing reliance on CYP2C19 and CYP2D6, were not modeled. ⁴⁴ The included data were also predominantly acute-phase, so the findings may not extend to maintenance treatment. Because our synthesis drew on aggregate study-level data, we could not account for participant-level confounders such as menopausal and reproductive status, hormonal contraception, baseline severity, comorbid anxiety, dosing, and adherence, each of which may shape SSRI response differently in women and men.^31,34,44,54 Incomplete sex-specific reporting, therefore, remains one of the major barriers to stronger inference in this area.

The clinical implications should be measured but not dismissed. These findings do not support sex-specific SSRI prescribing rules at present. They do, however, support more deliberate attention to sex when clinicians discuss expected improvement, monitor tolerability, and interpret early treatment response. For researchers, this systematic review identifies critical evidence gaps requiring prospectively designed investigations. Future studies should move beyond post hoc subgroup reporting and adopt prospectively sex-stratified designs with standardized outcome definitions, clearer adverse-effect phenotyping, and explicit reporting by age, reproductive stage, and metabolizer-related factors where possible.^{7,35,46,47,52}

Conclusions

This systematic review and meta-analysis identified a consistent female-favoring direction across SSRI efficacy outcomes in adult MDD, most evident for continuous symptom improvement. These findings suggest that sex may modify SSRI treatment outcomes, but this signal should be interpreted cautiously: the pooled estimates were limited by substantial heterogeneity, imprecision, reliance on secondary subgroup analyses, risk of bias in several studies, and incomplete sex-stratified reporting. The evidence does not currently support sex-based prescribing decisions. Adequately powered, prospectively sex-stratified studies with standardized reporting of efficacy and tolerability are needed before the magnitude and clinical relevance of any sex difference can be established.