Prospective, randomized, placebo-controlled, two-arm study to evaluate the efficacy of coadministration of garlic as a hydrogen sulfide donor and tadalafil in patients with erectile dysfunction not responding to tadalafil alone – A pilot study

Abstract

OBJECTIVE:

The objective is to evaluate the efficacy of coadministration of garlic (as a hydrogen sulfide [H₂S] donor) and tadalafil for patients with ED using a placebo-controlled, prospective, randomized, two-arm pilot study in patients responding poorly to tadalafil alone.

MATERIALS AND METHODS:

The patients with complaints of ED (with normal penile Doppler) who failed to maintain sustained improvement in erectile function with tadalafil were recruited after excluding those with comorbidities. The study sample was randomized into two groups. Group A received garlic 5 g twice a day orally and Group B received a placebo twice daily orally for 4 weeks. Both groups continued tadalafil 5 mg in the night for 4 weeks. Their erectile function was assessed at the beginning and at the end of 4 weeks using the International Index of Erectile Function (IIEF-EF), erectile function domain and compared. A value of P ≤ 0.05 was considered statistically significant.

RESULTS:

Nineteen patients in Group A (mean age 37.5 ± 10.6 years) and 16 patients in Group B (mean age 39.6 ± 9.6 years) participated in the pilot study conducted from May 2022 to August 2022. The participants treated with garlic (as an H₂S donor) as a coadministrant had statistically significant improvement in IIEF-EF score (P ≤ 0.0001) at the end of 4 weeks compared to placebo.

CONCLUSIONS:

Garlic (as an H₂S donor) as adjunctive therapy was beneficial in our study participants responding poorly to tadalafil alone.

Introduction

Erectile dysfunction (ED), defined as the persistent inability to attain and/or maintain penile erection sufficient for sexual satisfaction,[1] is one of the most common complaints seen in patients attending a sexual medicine clinic. With the introduction of phosphodiesterase type 5 inhibitors (PDE5i) as a treatment modality, the treatment of ED was revolutionized.[2] However, recently, we have found many patients with ED who show initial improvement in erectile function with PDE5i but fail to sustain similar improvement over a longer period.[3] Although the switching of the PDE5i is tried in some cases, the success is limited, even in patients with normal penile vasculature in the Doppler study.[4] Although the exact mechanism for such a response to PDE5i is unknown, the coadministration of PDE5i with other erotogenic agents is known to be of benefit when the PDE5i alone is ineffective.[5]

Recently, it was found that hydrogen sulfide (H₂S) can cause an erectile response in the rat model through large-conductance Ca²⁺-activated K⁺ channels. It was also found that this erectile response was independent of nitric oxide, a pathway used by PDE5i.[6] Such responses are theoretically possible with the use of H₂S donors such as garlic, which has been used by humans since immemorial without any adverse events. It is important to note that garlic is known to cause vasodilation in humans following oral ingestion.[7] Considering this, we hypothesized that the coadministration of garlic as an H₂S donor could be used to potentiate the effect of tadalafil, a PDE5i in those patients of ED (with normal penile vasculature on penile Doppler study) who show poor erectile response to tadalafil after an initial good response likely due to tachyphylaxis.[4] Based on this hypothesis, we conducted a prospective, randomized, placebo-controlled, two-arm pilot study to evaluate the effect of the coadministration of garlic and tadalafil in patients with ED (with normal penile vasculature on penile Doppler study) who started showing poor response to tadalafil, after an initial good response.

Materials and Methods

The Institutional Review Board approved the study protocol (IRB. No. IEC/2022-23/004 dated 21-03-2022). It was registered with the Clinical Trial Registry of India with number CTRI/2022/07/044306 as the prospective, randomized, placebo-controlled, two-arm pilot study before initiating the study.

Participants

The inclusion of the study participants was done only after obtaining their signed written informed consent. The study participants were those sexually active heterosexual healthy male patients who visited the sexual medicine clinic of our hospital with complaints of ED, with normal penile Doppler study and responded to tadalafil initially (at least for 3 months), but failed to sustain the response to tadalafil. Those who failed to respond to dose escalation from 5 mg up to 20 mg of tadalafil were also included in the study. For the study, favorable response or clinical improvement in erectile function was considered at least a 2-point improvement in mild ED, 5-point improvement in moderate ED (includes mild to moderate and moderate categories), and 7-point improvement in severe ED in the International Index of Erectile Function (IIEF-EF-6 item) score, erectile function domain.[8] The participant was included in the study if he had improved erectile function based on the above criteria following tadalafil administration initially but started reporting declining IIEF-EF scores after at least 3 months of sustained improvement with tadalafil.[9] The patients with comorbidities, sexual dysfunctions other than ED, on treatment for any medical, psychiatric, or surgical conditions, undergone surgery for genitalia, history of radiation to the pelvis or pelvic surgeries, and abnormal penile Doppler study findings were excluded from the study. The exclusion was based on the clinical and laboratory evaluation of the participants by the investigator and the concerned specialists. The participants received no remuneration for participating in the study. The participant was considered sexually active if he had performed or attempted at least one sexual activity in a week.

Randomization and study design

The participants were divided into two groups (Group A and Group B) based on the computer-generated random numbers. The participants in Group A received tadalafil 5 mg daily orally along with a juice (quantity of 150 ml) prepared from pressing fresh garlic cloves, resulting in a solution containing 33.33 mg/ml of garlic (on an average of two average-sized garlic cloves (5 g) when pressed, gives the requisite concentration), orally twice a day for 4 weeks. The participants in Group B received tadalafil 5 mg daily along with a juice (quantity of 150 ml) containing a placebo, which was similar to garlic juice for all the parameters except for garlic content, orally twice a day for 4 weeks. Participants in both groups were asked to cleanse their mouths with alcohol-free herbal mouthwash after 20 min of juice intake twice daily for 4 weeks. This mouthwash was prepared in-house with guava leaves, betel leaves, ajwain (carom seed), peppermint oil, and clove oil.[10] The garlic juice and the juice-containing placebo were prepared by the in-house pharmacy in our institute and dispensed to the participants based on their allocation to the groups once every 3 days. The content of allicin in the prescribed garlic juice was kept between 0.65 and 0.69 mmol, which was ensured using a spectrophotometric assay.[11] The participants were asked to keep the juice in the domestic refrigerator for 3 days at 3°C temperatures and were asked to consume it twice daily as per the advised dose. The compliance of the participants was confirmed by cross-checking the treatment diary and the number of remaining tadalafil tablets as well as the volume of garlic juice/placebo juice.

The erectile function of all the participants was assessed at the beginning of the study and after 4 weeks based on the IIEF score, which is a validated tool to evaluate ED.[12] The severity of ED was assessed based on IIEF-EF (6 items) scores as severe (6–10), moderate (11–16), mild to moderate (17–21), mild (22–25), and no ED (26–30).[13] For the study, mild-to-moderate ED and moderate ED categories were clubbed as moderate categories of ED. As this was a continuation of the treatment process with the addition of a new drug to the already existing treatment in a participant who was already on treatment for ED, only 4-week follow-up was considered for evaluation of the efficacy of the treatment under study.

Outcomes

The change in erectile function following the coadministration of the agent (garlic/placebo) along with PDE5i at the end of 4 weeks was the proposed primary outcome measure. The minimal clinically important difference was considered as at least a 2-point improvement in participants with mild ED, a 5-point improvement in participants with moderate ED (including mild to moderate and moderate categories), and a 7-point improvement in participants with severe ED in the IIEF-EF-6 item score, erectile function domain.[8] The drop-out rate from the study due to adverse events was the proposed secondary outcome measure.

Sample size calculation

The calculation of the sample size for this pilot study was based on calculated using a power of 95%, type I error rate of 5%, and 0.8 standardized effect size with an upper confidence limit of 80%.[14] Based on this, the pilot study’s total sample size was calculated as 20 (10 in each arm).

Statistical analysis

Epitools Epidemiological Calculators (©2018 Avuset, Australia) were used to perform the statistical analysis. Mean and standard deviation were calculated for categorical variables. Regression analysis was carried out to assess the effect of age as well as the baseline erectile function of the participant on the erectile function of the participant at the end of the study. A P ≤ 0.05 was considered statistically significant.

Results

The study was conducted from May 2022 to August 2022. To begin with, Group A had 25 participants and Group B had 24 participants. However, five participants from Group A and eight participants from Group B were lost for follow-up. One participant from Group A stopped intervention due to the development of dyspepsia. Finally, 35 participants were analyzed of which Group A had 19 participants and Group B had 16 participants. These details are shown graphically in Figure 1. The mean age of the participants in Group A was 37.5 ± 10.6 years (range 22–62 years). The mean age of the participants in Group B was 39.6 ± 9.6 years (range 28–65 years). There were 14 participants in Group A and 10 in Group B with moderate ED at the time of the inclusion in the study. There were five participants in Group A and six participants in Group B with severe ED at the time of inclusion in the study. Both the groups were similar statistically as far as the age and degree of ED were concerned.

Figure 1.

Participant flow diagram

The change in erectile function after 4 weeks of intervention in the participants in both groups is shown in Table 1. The line diagram depicting the change in erectile function in each participant is shown in Figure 2. The statistically significant minimal clinically important difference was observed based on the IIEF-EF (6 items) score at the end of 4 weeks of intervention in Group A in the participants complaining of moderate ED [Table 2]. On regression analysis, neither the age of the participant (P = 0.6163) nor the baseline erectile function (P = 0.1161) had a statistically significant effect on the erectile function at the end of 4 weeks of coadministration of garlic juice with tadalafil.

Table 1.

The details of International Index of Erectile Function-erectile function scores in the two groups of participants

	IIEF-EF score at 0 weeks (mean±SD)	IIEF-EF score at 4 weeks (mean±SD)	P (for IIEF-EF score between 0 and 4 weeks)	CI (for IIEF-EF score between 0 and 4 weeks)	t-statistic (for IIEF-EF score between 0 and 4 weeks)
Group A (n=19)	12.3±2.5	18.9±2.3	≤0.0001	−8.1240–−5.0160	−8.574
Group B (n=16)	11.6±2.4	12.9±1.9	0.0884	−2.9372–0.2172	−1.761
P (for IIEF-EF score between Group A and Group B)	0.3646	≤0.0001	-	-	-
CI (for IIEF-EF score between Group A and Group B)	−2.4419–0.9219	4.5078–7.4322	-	-	-
t statistic (for IIEF-EF score between Group A and Group B)	−0.919	8.307	-	-	-

Statistically significant P values are underlined and in italic. CI=Confidence interval, SD=Standard deviation, IIEF-EF=International Index of Erectile Function-erectile function

Figure 2.

Line diagram showing the change in erectile function in participants in (a) Group A and (b) Group B. IIEF-EF = International Index of Erectile Function

Table 2.

The details of a minimal clinically important difference in the International Index of Erectile Function-erectile function score in two groups of participants and their statistical significance

	Group A (n=19)	Group B (n=16)	P-value for difference between the groups	CI for difference between the groups
Number of participants with mild ED (based on IIEF-EF score) who showed MCID after 4 weeks	0	0	-	-
Number of participants with moderate ED (based on IIEF-EF score) who showed MCID after 4 weeks	7 out of 14	0 out of 10	0.0253	0.0617–0.9383
Number of participants with severe ED (based on IIEF-EF score) who showed MCID after 4 weeks	5 out of 5	1 out of 6	0.0811	0.6714–283.79

Statistically significant P values are underlined and in italic. CI=Confidence interval, MCID=Minimal clinically important difference, IIEF-EF=International Index of Erectile Function-erectile function, ED=Erectile dysfunction

Few adverse events were reported by the participants. Seven participants from Group A and nine participants from Group B complained of mild headaches and myalgia, but none of them stopped the intervention. Three participants from Group A and one participant from Group B complained of dyspepsia. Of these, one participant from Group A dropped out of the study. Palpitation was reported by two participants in Group A and five patients from Group B. However, none of them dropped out of the study. None of the participants dropped out of the study due to a peculiar smell, bad breath, or peculiar body odor attributed to the consumption of garlic.

Discussion

We present the outcome of a prospective, placebo-controlled, two-arm, randomized pilot study that studied the efficacy of coadministration of 150 ml of garlic juice (as an H₂S donor) prepared from pressing fresh garlic cloves resulting in a solution containing 33.33 mg/ml of garlic, twice a day and tadalafil 5 mg once a day in patients complaining of ED (with normal penile Doppler study) who responded poorly to tadalafil alone. The study showed that coadministration of garlic juice (as an H₂S donor) and tadalafil resulted in a statistically significant improvement in erectile function after 4 weeks in patients complaining of ED (with normal penile Doppler study) who responded poorly to tadalafil alone.

The role of H₂S as a gasotransmitter involved in regulating endothelial functionality has come into focus due to recent research findings.[15] The vasoprotective effect of H₂S is well established at present.[16] Based on these effects, many researchers have assessed the role of H₂S in the physiology of penile erection and it was found that the H₂S had vasodilator and pro-erectile response in animal experiments.[6,17,18] d’Emmanuele di Villa Bianca et al. demonstrated that H₂S-mediated smooth muscle cell relaxation occurs in the human corpus cavernosum as well.[19] Combination therapy of H₂S and PDE5i is proven to be beneficial in treating ED in animal experiments.[20] Qiu et al. suggested that such combination therapy could be beneficial in humans when they respond poorly to PDE5i alone.[21] Oral ingestion of garlic was known to have vasodilation in humans as well.[7] Taking a cue from this, we used the strategy of coadministration of tadalafil and garlic, which is commonly used in our kitchen, as an H₂S donor in our study and found that the erectile function was improved significantly compared to placebo in men with ED who responded poorly to tadalafil alone. In our study, it was important to note that the participants with moderate ED had a better chance of showing the minimal clinically important difference on an IIEF-EF (6 item) scale when tadalafil and garlic juice were co-administered, suggesting a higher chance of success in patients with ED severity of moderate degree or less.

Garlic, which belongs to the Amaryllidaceae family, is known for its allicin content, which is a thiosulfinate.[22] This allicin in garlic decomposes into diallyl disulfide, diallyl sulfide, and diallyl trisulphide.[23] These compounds are converted into H₂S in the human red blood cells in the presence of free thiols, which can be used by the endothelial cells.[24] This H₂S is biologically active in the corpus callosum, where it causes its vasodilator effect mediated by large-conductance Ca²⁺-activated K+ channels.[6] We used freshly prepared garlic juice as a combined treatment modality with tadalafil in our study because only freshly prepared garlic juice contains allicin. Other preparations like garlic oil, fried garlic, and boiled garlic are unlikely to affect endothelial function because of the absence of stable allicin.[24]

Initially, we planned to advise our patients to chew the fresh garlic clove, as it releases the maximum amount of allicin. However, we had to change this plan due to the higher chance of a burning sensation in the mouth following chewing raw garlic and, hence, the likely chance of dropping out of the study. The majority of the adverse events reported by our study participants were likely to be due to tadalafil rather than garlic juice.[25] The use of herbal mouthwash helped to get rid of the smell of garlic following intake of the garlic juice, and we think this also helped to reduce the dropout of the participants from the study. We expected a drop out of the participants from the study due to the excretion of allicin from the sweat glands, which can result in a typical smelly sweat. However, none of our participants dropped out of the study due to this reason.

Coletta et al. showed that angiogenesis and vasodilatation are initiated by the mutually dependent action of H₂S and nitric oxide (NO).[26] In our study, it is likely that the garlic juice acted through the H₂S pathway and tadalafil acted through the NO pathway, producing the mutually beneficial action on penile vasculature, thus helping our patients to achieve better erectile function in our patients with ED who had poor response to tadalafil alone.

Strengths and weakness of the study

Despite being a very effective treatment modality for ED, the treatment dropout rate is very high for PDE5i.[3] We could address these patients’ concerns successfully with our novel treatment strategy. We are the first to successfully treat such patients with ED in clinical settings with medical management, which is one of the strengths of our study. Although we could successfully execute our treatment strategy in our patients, the process of preparation of fresh garlic juice is difficult in day-to-day clinical practice. Unless someone comes out with a garlic preparation, that can keep highly inflammable, evaporative H₂S in its precursor form with a longer shelf life, the practical difficulty in prescribing this treatment remains. This is the major limitation of the study. However, garlic has been one of the most commonly used spices in the kitchen since time immemorial, and hence, daily consumption should not be a concern for therapeutic purposes, as a slight overdose is unlikely to result in major toxicity. We would have preferred to use allicin as a H₂S donor instead of garlic, but the absence of any mechanism at present to keep pungent evaporative allicin in a stable state was the major limitation. Although the sample size is adequate for a pilot study, the sample size itself can be a limitation when the study is considered with respect to the prevalence of ED in men. A multicenter trial with a larger sample size can overcome this limitation as well. We would have preferred double-blinding, which would have added strength to the study. However, the blinding was not possible due to the typical pungent odor of the garlic, because of which the participants would have guessed allocation easily.

Conclusions

Garlic (5 g) in the form of juice (which acted as an H₂S donor) taken orally twice a day with tadalafil 5 mg improved erectile function in our study participants with ED when there was a poor response to tadalafil alone. The likely chance of success was more in participants with ED severity of moderate degree or less. This action of garlic was attributable to the mutually dependent effect of H₂S and NO on penile erection. This novel treatment strategy could bring relief to the majority of patients with ED who respond poorly to PDE5i despite having normal penile vasculature if the results can be replicated in a larger multicenter trial.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.