Acute kidney injury induced by topical hair straightening products: A systematic review

Abstract

BACKGROUND

Hair straightening products containing formaldehyde, glycolic acid, and glyoxylic acid may be nephrotoxic, as several studies have reported acute kidney injury (AKI) induced by these chemicals.

AIM

To investigate the clinical features, complications, and treatment of AKI resulting from topical exposure to hair-straightening products.

METHODS

The study protocol was registered with PROSPERO under the registration number CRD420251010513. PubMed, Web of Science, and Scopus were searched from inception to April 3, 2025, for case reports and case series describing patients diagnosed with AKI-defined as an increase in serum creatinine of ≥ 0.3 mg/dL, following exposure to hair-straightening products. Means and standard deviations were used to describe continuous variables and frequencies and percentages were reported for dichotomous variables.

RESULTS

The search yielded 168 potentially relevant articles, of which six case reports and two case series met the inclusion criteria, collectively providing data on 34 patients for 36 incidents (in one case report, three AKI episodes occurred in the same patient). In 20 incidents, the hair product was identified as “formaldehyde-free”, while in 16 incidents, the chemical composition was unknown. All patients were female (mean age: 28.53 ± 11.72 years; range: 10-58 years) and the median time for the development of AKI was 2 days. The mean serum creatinine level at admission was 5.24 ± 2.83 mg/dL (range: 1.9-13.2 mg/dL). The most common presenting symptoms were vomiting (n = 29/36; 80.6%), nausea (n = 25/36; 69%), and abdominal pain (n = 13/36; 36%). Complications included one patient who developed severe dyspnea with bilateral lung infiltrates and another who developed severe hypertension and hyperkalemia. Twenty-one incidents were managed conservatively, five required steroid therapy, three required hemodialysis, and three required both hemodialysis and steroids. All patients recovered and were discharged.

CONCLUSION

The findings of this systematic review highlight the need for caution when using hair-straightening products due to their potential to cause AKI.

Core Tip: This systematic review is the first comprehensive synthesis of reported cases linking topical hair-straightening products to acute kidney injury (AKI). Analyzing 36 incidents of AKI in 34 female patients, the current study highlights that topical application of hair straightening chemicals can trigger AKI through mechanisms involving calcium oxalate nephropathy and/or interstitial nephritis. Most patients recovered with conservative or steroid-based therapy. The study underscores the need for clinician awareness and regulatory scrutiny, revealing that hair straightening products may not be as safe as presumed.

INTRODUCTION

Several studies have documented the detrimental effects of occupational (occurring in the workplace as part of one’s job duties), cosmetic (personal treatments for beautifying purposes), or intentional (deliberate ingestion, inhalation to self-harm or poisoning attempts) exposure to hair products such as paraphenylenediamine-containing hair dyes on kidney function over the past two decades[1-5]. Previous investigations have also indicated that formaldehyde-based hair-straightening products may induce acute kidney injury (AKI)[6,7]. Given the substantial evidence from animal studies demonstrating formaldehyde-induced nephrotoxicity[8-11], formaldehyde in hair-straightening formulations was replaced with C2 carboxylic acids, such as glycolic and glyoxylic acids, under the assumption that these compounds represented safer alternatives[12]. These formulations are marketed as “formaldehyde-free” hair-straightening products[13].

However, emerging evidence suggests that formaldehyde-free hair-straightening products may also be associated with AKI[7,13-15]. The use of these products has been linked to biopsy-confirmed acute tubulointerstitial nephritis[16,17], tubular necrosis[18], and inflammatory changes within the renal tubules[19,20]. Additional studies have reported potential pulmonary[21] and developmental toxicity[22] following respiratory or oral exposure to these formulations. Although the precise mechanisms underlying the nephrotoxicity of formaldehyde-free products remain under investigation, preliminary experimental studies in animals suggest that topical exposure may induce hyperoxaluria and calcium oxalate nephropathy[23-25]. Moreover, multiple case reports have documented oxalate crystal deposition within the renal tubules of affected individuals[15,19,20].

Notably, glycolic acid-the principal active constituent of formaldehyde-free hair-straightening products-has been shown to release formaldehyde when exposed to high temperatures, such as during blow-drying or flat-ironing. The released formaldehyde can be inhaled or absorbed through the scalp, resulting in systemic exposure and potential nephrotoxicity[13]. The straightening procedure typically involves the topical application of the product followed by high heat exposure (flat iron temperatures often exceeding 150-200 °C in salon practice), a combination that facilitates the thermal release of formaldehyde and increases both inhalation and dermal absorption. Poor salon ventilation and repeated procedures further contribute to cumulative exposure. Therefore, this systematic review aimed to investigate the clinical features, complications, and management of AKI resulting from topical exposure to hair-straightening products.

MATERIALS AND METHODS

This systematic review was conducted and reported in accordance with the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement[26]. The study protocol was registered with PROSPERO under the registration number CRD420251010513 (available at: https://www.crd.york.ac.uk/PROSPERO/view/CRD420251010513).

Search strategy

A comprehensive search strategy was developed to identify relevant studies from three electronic databases-PubMed, Web of Science, and Scopus, from their inception to April 3, 2025. The search combined terms and themes related to AKI and hair straightening chemicals as follows: Theme 1: “AKI” OR “acute kidney failure” OR “acute kidney insufficiency” OR “acute kidney damage” OR “acute renal injury” OR “acute renal failure” OR “acute renal insufficiency” OR “Acute renal damage” OR “Nephropathy”. Theme 2: “Hair straighteners” OR “Hair straightening chemicals” OR “Hair straightening products” OR “Hair relaxers” OR “Hair relaxing chemicals” OR “Hair relaxing products” OR “Keratin hair treatment” OR “Formaldehyde free” OR “Formaldehyde-free” OR “Glycolic acid” OR “Glyoxylic acid”. In addition, the bibliography of relevant articles was also searched manually. The detailed search strategy is described in Supplementary Table 1.

Screening and selection

Study screening and selection were conducted in three stages: Deduplication, title and abstract screening, and full-text review. The Zotero application was used for deduplication. Articles published in English or in other languages with an English abstract were considered for inclusion, following the patient/population, intervention, comparison and outcomes framework as outlined below: Population: Human participants. Intervention: Topical hair straightening products. Comparator: None. Outcome: Clinical features, complications, and management.

Study design

Case reports, case series, or letters to the editor describing patient presentations. AKI was defined as an abrupt decline in renal function within 48 hours, according to the AKI Network criteria[27]. AKI was considered hair-straightening-induced when alternative causes were excluded through clinical, laboratory, and imaging evaluations, and symptoms developed within 48-72 hours of chemical exposure[19]. In cases where a single patient experienced multiple AKI episodes, all episodes were included. Case reports describing AKI associated with hair-straightening products administered via non-topical routes, as well as animal studies, were excluded from the review.

Study quality

Study quality was assessed independently by two authors using a standardized tool adapted from Murad et al[28]. This tool has been previously applied in systematic reviews of case reports[29]. It comprises four items, each rated with a binary response to indicate the potential presence of bias. The overall quality of each study was classified as “good”, “moderate”, or “poor” corresponding to a low, moderate, or high risk of bias, respectively, in accordance with the criteria of Smith et al[29]. Any discrepancies between the two reviewers were resolved by consensus following consultation with the senior author.

Data extraction

Data were extracted using a standardized template and included: (1) Patient demographics (patient age and gender) and study characteristics (country and year of publication); (2) Comorbidities; (3) Clinical features of the illness; (4) Diagnostic findings; (5) Management modalities; and (6) Clinical outcomes.

RESULTS

Search results

The search strategy yielded 168 potentially relevant articles from PubMed (n = 31), Web of Science (n = 42), and Scopus (n = 95). After removing 30 duplicates, 138 articles remained for title and abstract screening (Figure 1). Of these, 39 articles were deemed eligible for full-text review. Ultimately, six case reports and two case series met the inclusion criteria and were included in this systematic review.

Figure 1.

Preferred Reporting Items for Systematic Reviews and Meta-analyses flow diagram for the study selection process.

Epidemiology

The six reports of individual cases and two case series provided data on a total of 34 patients (Table 1). The individual cases originated from the United Arab Emirates, Switzerland, France, and Algeria (one each), and Israel (two reports). Case series were published from Israel (one series comprising 26 patients) and Egypt (one series comprising two patients). Thus, Israel accounted for the largest proportion of reports, representing 33% of individual cases and 50% of the case series. All cases were presented to hospital from 2019 to 2023, except for the two cases reported by Ahmed et al[17] and one case reported by Zergui et al[14], for which the year of presentation was not specified.

Table 1.

Characteristics of the included case reports and case series

Ref.	Study type	Patient’s age/gender	Comorbidities	Clinical features	Serum creatinine (mg/dL)	Renal sonography	Renal biopsy	Treatment	Outcome	Hair straightening method
Abu-Amer et al[20], 2022	Case report	41/F	Hypothyroidism, sleeve gastrectomy	Weakness, nausea, vomiting	3.46	Yes	Yes	Prednisolone 1 mg/kg	Recovered. creatinine 0.9 after one week	Formaldehyde-free product
Bashir and Khater[16], 2023	Case report	40/F	Hypothyroidism	Scalp rash, hypertension, hyperkalemia	3.5	No	Yes	Pulse steroids	Recovered. Creatinine 0.9 at 4-month follow-up	Formaldehyde-free product
Huber et al[15], 2024	Case report	42/F	Healthy	Weakness, nausea, vomiting, flank pain, diarrhea, flank tenderness, hypertension	6.61	Yes	Yes	Conservative management	Recovered. Creatinine 0.87 at 3-month follow-up	Unknown
Mitler et al[18], 2021	Case report	13/F	Healthy	Abdominal pain, nausea, vomiting, oliguria, tachycardia, hypertension, dehydration, abdominal tenderness	3.56	Yes	Yes	Hemodialysis for four consecutive days	Recovered. Creatinine normalized 6-month follow-up	Formaldehyde-free product
Robert et al[41], 2024	Case report incidence 1	26/F	Healthy	Scalp burning sensation, scalp ulcers, vomiting, diarrhea, fever, back pain	2.1	Yes	No	Unknown	Recovered. Serum creatinine returned to normal (0.78) at the last follow-up	Containing 10% glyoxylic acid but no glycolic acid
	Incidence 2	25/F		Scalp burning sensation, scalp ulcers, vomiting, diarrhea, fever, back pain	2.4	Yes	No	Unknown	Recovered. Serum creatinine returned to normal (0.78) at the last follow-up	Formaldehyde-free product (containing 10% glyoxylic acid but no glycolic acid)
	Incidence 3	24/F		Scalp burning sensation, scalp ulcers, vomiting, diarrhea, fever, back pain	1.9	Yes	No	Unknown	Recovered. Serum creatinine returned to normal (0.78) at the last follow-up	Formaldehyde-free product (containing 10% glyoxylic acid but no glycolic acid)
Zergui et al[14], 2025	Case report	25/F		Nausea, vomiting, weakness	3.2	Yes	No	Saline, calcium gluconate, insulin, 25 g glucose, sodium bicarbonate	Recovered. Serum creatinine returned to 1.2 at the two-week follow-up	Formaldehyde-free product (contained glyoxylic acid)
Ahmed et al[17], 2019	Case series of 2 cases case 1	10/F	Healthy	Vomiting, tachycardia, dyspnea, hypertension, scalp rash	13.2	Yes	Yes	Hemodialysis for three consecutive days; 20 mg/kg/dose methylprednisolone for three days followed by oral prednisone (60 mg/m2/day)	Recovered. Discharged with improved RFTs	Formaldehyde-free product
	Case 2	17/F	Healthy	Vomiting, syncope, scalp rash	9.9	Yes	Yes	Hemodialysis for three consecutive days; corticosteroids 40 mg/day	Recovered. Discharged with normal RFTs	Formaldehyde-free product
Bnaya et al[19], 20231	Case series of 26 cases case 1	24/F	Healthy	Abdominal pain, nausea	5.82	No	No	No KRT, no steroids	Recovered	Unknown
	Case 2	22/F	Healthy	Abdominal pain, nausea, vomiting, scalp rash	2.76	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 3	30/F	Nephrolithiasis	Nausea, vomiting, scalp rash, flank pain	4.81	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 4	29/F	Healthy	Nausea, vomiting, scalp rash	2.09	No	No	No KRT, no steroids	Recovered	Unknown
	Case 5	21/F	Epilepsy	Abdominal pain, vomiting, scalp rash, headache	11.83	No	Yes	Steroids	Recovered	Unknown
	Case 6	58/F	Healthy	Nausea, flank pain, syncope	7.54	No	Yes	Steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 7	14/F	Healthy	Nausea, flank pain, headache	3.8	No	No	No KRT, no steroids	Recovered	Unknown
	Case 8	31/F	Healthy, smoking	Abdominal pain, vomiting, diarrhea	8.14	No	No	Temporary hemodialysis	Recovered	Unknown
	Case 9	29/F	Nephrolithiasis	Vomiting, scalp rash	6.8	No	No	No KRT, no steroids	Recovered	Unknown
	Case 10	52/F	Psoriasis, nephrolithiasis	Abdominal pain, flank pain, nausea, vomiting	3.4	No	No	No KRT, no steroids	Recovered	Unknown
	Case 11	24/F	Healthy	Abdominal pain, nausea, fever	2.17	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 12	33/F	Healthy	Nausea, vomiting, chills	6	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 13	13/F	Healthy	Abdominal pain, nausea, vomiting	3.56	No	Yes	Temporary hemodialysis	Recovered	Unknown
	Case 14	17/F	Psoriasis	Abdominal pain, nausea, vomiting	7.1	No	Yes	Temporary hemodialysis and steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 15	17.5/F	Healthy	Abdominal pain, nausea, vomiting, fever, scalp rash	2.9	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 16	24/F	Healthy	Abdominal pain, nausea, vomiting, scalp rash	4.2	No	No	No KRT, no steroids	Recovered	Unknown
	Case 17	36/F	Healthy	Nausea, vomiting, scalp rash	6	No	No	No KRT, no steroids	Recovered	Unknown
	Case 18	21/F	Healthy	Abdominal pain, nausea, vomiting	3.2	No	No	No KRT, no steroids	Recovered	Unknown
	Case 19	41/F	Hypothyroidism, sleeve gastrectomy	Nausea, vomiting	3.46	No	Yes	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 20	19/F	Healthy	Nausea, flank pain, headache	5.09	No	Yes	Steroids	Recovered	Unknown
	Case 21	44/F	Hypercoagulability state (prothrombin variant)	Vomiting, flank pain	7.8	No	No	No KRT, no steroids	Recovered	Unknown
	Case 22	50/F	Healthy	Nausea, vomiting	7.3	No	No	No KRT, no steroids	Recovered	Unknown
	Case 23	21/F	Healthy	Nausea, vomiting, flank pain, scalp rash	3.68	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 24	28/F	Atopic dermatitis, gastroesophageal reflux disease	Nausea, vomiting, flank pain, scalp rash	2.83	No	No	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 25	32/F	Healthy	Abdominal pain, nausea, vomiting	9.03	No	Yes	No KRT, no steroids	Recovered	Formaldehyde-free product (containing glycolic acid)
	Case 26	30/F	Healthy	Flank pain	7.36	No	No	No KRT, no steroids	Recovered	Unknown

¹In the case series of Bnaya et al[19], 2023, analyses of hair-straightening products marketed in Israel revealed varying concentrations of glyoxylic acid (range: 0.56%-17.9%) in some of the products. Ethylene glycol and diethylene glycol were not detected. The concentration of formaldehyde was < 200 ppm in most tested products.

KRT: Kidney replacement therapy; F: Female; RFT: Renal function tests.

Patients’ demographics and clinical features

A total of 36 episodes of AKI occurred in 34 patients, as some experienced more than one episode. The mean age of the patients was 28.53 ± 11.72 years (range: 10-58; < 20 years: n = 8; ≥ 20 years: n = 28). The mean serum creatinine level at admission was 5.29 ± 2.85 mg/dL (range 1.9-13.2 mg/dL). All patients were hemodynamically stable. The most common presenting symptom was vomiting (n = 29/36; 80.6%), followed by nausea (n = 25/36; 69%), abdominal pain (n = 13/36; 36%), scalp rash (n = 13/36; 36%), and flank pain (n = 10/36; 28%) (Figure 2). Less commonly reported symptoms included oliguria, abdominal tenderness, dehydration, flank tenderness, chills, dyspnea, and hyperkalemia (each reported in 2.8% of cases).

Figure 2.

Clinical features in the cases.

In most cases (n = 24/36; 66.7%), patients were previously healthy with no comorbidities (Figure 3). Among those with pre-existing conditions, three patients had hypothyroidism (8.3%), three had a history of nephrolithiasis (8.3%), two had psoriasis (5.6%), one had atopic dermatitis (2.8%), and one had epilepsy (2.8%). The hair-straightening products used were identified as “formaldehyde-free” in 20 incidents, while the chemical nature was unknown in 16 incidents (Table 1).

Figure 3.

Comorbidities in the cases.

Complications

In the study by Bnaya et al[19], one patient developed severe dyspnea with bilateral pulmonary infiltrates, whereas the patient described by Bashir and Khater[16] presented with severe hypertension and hyperkalemia. Both patients subsequently achieved full clinical recovery.

Diagnosis

All cases met the diagnostic criteria for AKI (i.e., a rise in serum creatinine ≥ 0.3 mg/dL). The mean serum creatinine level at admission was 5.24 ± 2.83 mg/dL (range: 1.9-13.2 mg/dL) (Table 1). All cases fulfilled the criteria for hair-straightening-related AKI, as symptoms developed within 48-72 hours of chemical exposure, and other potential causes such as viral, immune-mediated, or pharmacological etiologies were excluded through clinical, laboratory, and immunologic evaluations. Renal sonography and biopsy were performed in 9 (25%) and 13 (36 %) of the 36 AKI incidents, respectively. The findings of these investigations are summarized in Tables 2 and 3.

Table 2.

Renal ultrasonography findings of included studies

Ref.	Sonography findings
Abu-Amer et al[20], 2022	14.4-cm bilateral echogenic, edematous renal parenchyma
Huber et al[15], 2024	Normal kidneys with slight corticomedullary dedifferentiation, absence of pyelocaliceal dilation, and normal vascular flow
Mitler et al[18], 2021	15-cm edematous renal parenchyma
Robert et al[41], 2024 (3 incidents in the same patient)	No evidence of obstructive uropathy
Zergui et al[14], 2025	Normal-sized kidneys; findings consistent with acute kidney injury of non-obstructive etiology
Ahmed et al[17], 2019 (case 1)	Bilateral grade 1 nephropathy
Ahmed et al[17], 2019 (case 2)	Bilateral grade 1 nephropathy

Table 3.

Renal biopsy findings of included studies

Ref.	Microscopic findings
Abu-Amer et al[20], 2022	The biopsy demonstrated features of both oxalate nephropathy and interstitial nephritis (intratubular oxalate crystals, interstitial edema, mixed inflammatory infiltrates with eosinophils, and foci of tubulitis)
Bashir and Khater[16], 2023	Acute interstitial nephritis
Huber et al[15], 2024	Findings were consistent with oxalate nephropathy rather than interstitial nephritis (numerous intratubular calcium oxalate crystal deposits and mild interstitial edema without inflammatory infiltrates)
Mitler et al[18], 2021	Oxalate nephropathy (microcalcifications/calcium oxalate crystals)
Ahmed et al[17], 2019 (case 1)	Acute tubulointerstitial nephritis
Ahmed et al[17], 2019 (case 2)	Acute tubulointerstitial nephritis
Bnaya et al[19], 2023 (case 5)	The biopsy demonstrated features of both oxalate nephropathy and interstitial nephritis (calcium oxalate crystals, small mononuclear interstitial infiltrate with spare eosinophils)
Bnaya et al[19], 2023 (case 6)	The biopsy demonstrated features of both oxalate nephropathy and interstitial nephritis (oxalate crystals, small mononuclear interstitial infiltrate)
Bnaya et al[19], 2023 (case 13)	The biopsy demonstrated oxalate nephropathy (microcalcifications/calcium oxalate crystals)
Bnaya et al[19], 2023 (case 14)	Oxalate nephropathy
Bnaya et al[19], 2023 (case 19)	The biopsy demonstrated features of both oxalate nephropathy and interstitial nephritis (oxalate crystals, multifocal mixed inflammatory infiltrates with multiple eosinophils)
Bnaya et al[19], 2023 (case 20)	Findings were consistent with oxalate nephropathy rather than interstitial nephritis (calcium oxalate crystals; interstitium with mild edema and sparse eosinophils)
Bnaya et al[19], 2023 (case 25)	Oxalate nephropathy

Treatment

Of the 36 AKI incidents, five were treated with steroids alone, three with hemodialysis alone, and three with hemodialysis followed by steroids. In 21 incidents, AKI resolved without the need for kidney replacement therapy (KRT) or steroid treatment (Table 1). There were no fatalities, and all patients were successfully treated and discharged.

Study quality

All case reports and case series included in this review were determined to have a low risk of bias and were rated as high quality (Table 4).

Table 4.

Quality of the included case reports and case series

Ref.	Was the exposure (hair-straightening products) adequately ascertained?1	Was the outcome (AKI) adequately ascertained?2	Were other alternative causes that may explain the outcome (AKI) ruled out?3	Were all important data cited?4	Risk of bias5
Abu-Amer et al[20], 2022	Yes	Yes	Yes	Yes	Low
Bashir and Khater[16], 2023	Yes	Yes	Yes	Yes	Low
Huber et al[15], 2024	Yes	Yes	Yes	Yes	Low
Mitler et al[18], 2021	Yes	Yes	Yes	Yes	Low
Robert et al[41], 2024	Yes	Yes	Yes	Yes	Low
Zergui et al[14], 2025	Yes	Yes	Yes	Yes	Low
Ahmed et al[17], 2019	Yes	Yes	Yes	Yes	Low
Bnaya et al[19], 2023	Yes	Yes	Yes	Yes	Low

¹Ascertainment of the exposure, i.e., hair straightening products: e.g., AKI developed during the 48-72 hours following hair-straightening product exposure, and the patient experienced systemic signs such as abdominal pain, vomiting, and scalp eruption/ irritation, etc. within 24 hours of hair-straightening product exposure.

²Ascertainment of the outcome (acute kidney injury), i.e., raised serum creatinine was demonstrated or a definition of acute kidney injury was used, e.g., acute kidney injury was defined according to Acute Kidney Injury Network criteria, etc.

³Causality: Other etiologies for acute kidney injury, such as viral, immune, and pharmacological, were excluded after clinical, laboratory, and immunological evaluation.

⁴Important data was defined as the history of use of hair straightening products, time from exposure, clinical signs and symptoms, serum creatinine at the time of admission, and outcome.

⁵Risk based on the responses of the first four questions (4 yes indicates low risk; 3 yes indicates moderate risk; 2 or less yes indicates high risk).

AKI: Acute kidney injury.

DISCUSSION

This systematic review indicates that although typically non-fatal, topical hair straightening products can cause AKI even in otherwise healthy individuals. Fewer than 10% of the patients had a history of hypothyroidism or nephrolithiasis; however, it remains unclear whether these comorbidities represent risk factors for hair straightening-induced AKI. Among the 36 AKI incidents included in our analysis, only 2 (6%) were associated with severe complications.

In addition, tubular presence of oxalate crystals was noted in 10 out of the 13 cases (77%) in our study population for whom renal biopsy results were available. Furthermore, 70%-80% of the patients presented with nausea and vomiting, which are typical symptoms of oxalate nephrolithiasis[30]. This supports the hypothesis that renal toxicity of hair-straightening products may be mediated by hyperoxaluria and calcium oxalate nephropathy, as suggested by animal studies. For instance, Robert et al[23] demonstrated that the topical application of a commercially available hair straightening cream, a 10% glyoxylic acid cream, and a 10% glycolic acid cream induced signs of renal injury in mice, albeit with some differences: While all three creams increased urine glycolate compared to the control creme, only the commercial crème and the 10% glyoxylic acid cream elevated plasma creatinine and urine oxalate levels, thereby inducing hyperoxaluria[23].

Given that 23% of the patients in our study population with renal biopsy findings presented with AKI without evidence of oxalate crystal formation, it is likely that additional factors contribute to the pathogenesis of hair-straightening product-induced AKI. Inflammatory changes or nephritis were noted in all biopsy cases lacking oxalate crystal deposition, consistent with previous reports[16-19]. Although nephritis is often seen in oxalate nephropathy[31], other studies have indicated that it is not a universal finding[32-34]. Moreover, there is direct evidence that internal administration of polyglycolic acid, which hydrolyzes to release glycolic acid, can induce inflammatory conditions such as acute peritonitis via activation of the complement system[35]. However, the inflammatory potential of topically applied glycolic acid remains controversial[36,37]. Thus, it is plausible that a local inflammatory response plays an important role in renal toxicity induced by hair-straightening products.

The hypothesis that oxalate nephropathy and inflammation contribute to the development of AKI following the use of hair-straightening products is further supported by the clinical outcomes of the patients included in this systematic review. All patients recovered and were successfully discharged. Most (27/36; 75%) were managed conservatively, a few required KRT (6/36; 16.7%) and treatment details were not reported for 8.3% of cases (3/36). While KRT is a common therapeutic measure for AKI, steroids are rarely used[38].

Limitations

Although the current systematic review is the largest and most up-to-date effort to examine the clinical presentation, complications, diagnosis, treatment, and prognosis of patients with AKI induced by hair-straightening products, it has several important limitations. Our data synthesis relied entirely on case reports and series, and the absence of randomized controlled trials and retrospective cohort studies limits the ability to establish causality. In addition, although a validated methodology was employed to assess the quality of the included studies, the risks of information, misclassification, selection, reporting, and ascertainment biases could be minimized but not completely eliminated[39,40]. Notably, the potential for publication bias can not be ignored, as more severe cases are more likely to be reported. With regard to the oxalate accumulation hypothesis, none of the studies assessed blood or urinary oxalate levels for diagnostic confirmation, and oxalate deposition may also occur in other causes of acute tubular necrosis identified on biopsy.

CONCLUSION

The findings of this systematic review highlight the need for caution when using hair-straightening products, particularly among individuals at risk of AKI. Clinicians should also consider AKI in the differential diagnosis of patients presenting to the emergency department shortly after exposure to hair-straightening products. Moreover, these products should be recognized as a potential etiological factor in cases of unexplained AKI among frequent users. As heat may trigger the release of formaldehyde from so-called formaldehyde-free formulations; reducing flat-iron temperatures and shortening the duration of heat application may decrease the amount of formaldehyde generated. Finally, the establishment of prospective monitoring systems or product safety registries is warranted to generate robust evidence that can better inform regulatory decision-making and enhance public awareness regarding the potential risks associated with hair-straightening product use.