Oral Finasteride, also known as Propecia, is an oral medication that has been prescribed by physicians for decades for hair loss. It works in the majority of patients by reducing hair loss and stimulating hair growth, however there can be side effects. One of the most concerning side effects are the sexual side effects, which occur only in 1.8% to 3.8% of patients who take it. As a result, many physicians have been switching to topical Finasteride as an alternative. In recent years, there have been many studies on the efficacy and side effects of topical Finasteride. In this article, we will review the clinical trials and scientific data to evaluate the side effect profile of topical Finasteride.

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In one of the first single-blind, placebo-controlled clinical trials of topical finasteride, Mazarella et al.¹ (1997) published a study of 52 subjects ranging from 19 to 28 years of age. The patients were treated with topical finasteride 0.005% vs placebo for 16 months. These were the results:

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• There was a significant reduction in the rate of hair loss after 6 months of treatment.
• There were no reports of local or systemic side effects in the patients who received topical Finasteride for treatment.

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In another double blind, randomized clinical trial study conducted with 45 male patients, Hajheydari et al.² (2009), compared oral Finasteride 1mg vs topical Finasteride 1% for a total of 6 months.

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• Authors concluded that “the therapeutic effects of both Finasteride gel and Finasteride tablet/oral were relatively similar to each other.”
• One person had erythema of the scalp after application of the topical Finasteride.

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In a prospective pilot study conducted in 15 patients for 9 months by Rafi and Katz³ (2011), patients applied a topical solution called NuH Hair, which is a novel topical combination of Finasteride, Dutasteride, and Minoxidil, with the option to add oral Finasteride, Minoxidil and/or Ketoconazole shampoo for 9 months.

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• Significant growth was demonstrated after 3 months of use of the topical solution.
• There were no reports of sexual side effects or irritation or redness reported.

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In a randomized clinical study by Tanglertsampan C.⁴  (2012), 33 male patients ranging between 27 to 49 years old were enrolled to compare the efficacy and safety of a 24 weeks application of Minoxidil 3% lotion versus Minoxidil 3% combined with 0.1% Finasteride lotion.

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• This study showed that Finasteride combined with Minoxidil is superior to minoxidil alone by itself.
• Contact dermatitis and redness was observed in 4 patients (24%) using the Finasteride + Minoxidil combination. No sexual side effects were observed.

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Caserini et al. conducted two pharmacokinetic studies on topical finasteride. In a randomized clinical study of 23 males, Caserini et al.⁵ (2014) compared topical Finasteride 0.25% twice daily vs oral Finasteride 1mg once daily for seven days. They measured Dihydrotestosterone (DHT) and Testosterone levels in the serum (systemic DHT levels).

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• Similar reduction in serum DHT was observed by the use of topical 0.25% Finasteride twice daily  (68-75% ) and oral Finasteride 1mg (62-72%).
• No sexual side effects were observed.

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In another randomized clinical trial, Caserini et al.⁶ (2015) did another study involving 50 male patients studying DHT in both the serum and the scalp.

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• This pharmacokinetic study confirmed that topical Finasteride 0.25% (1ml application twice daily) and oral Finasteride 1mg (once daily) both have similar reduction in scalp (47-51%) and serum DHT levels (74-76%). No changes in testosterone were observed.
• In the portion of the study where patients used 1ml of topical finasteride 0.25% twice daily, there were 2 patients (11%) that had the following side effects. Elevated ALT levels, pollakiuria and testicular pain.
• About 37-54% when 0.1ml, 0.2ml, 0.3ml and 0.4ml of topical 0.25% Finasteride was used, showing relatively similar results to 1ml twice a day application of topical 0.25% Finasteride. Serum DHT was reduced by 24% with the use of 0.1ml, 26% with 0.2ml, 44% with 0.3ml and 48% with use of 0.4ml of 0.25% topical Finasteride. This testing suggests that lower dosage of topical Finasteride may lead to lower reduction of serum DHT levels and thus lower sexual side effects (assuming that serum DHT is solely responsible for the sexual side effects).

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Oral vs Topical Finasteride

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In a retrospective assessment and prospective crossover cohort study of 50 male between 20 to 40 years old by Chandrashekar et al.⁷ (2017), all 50 patients used Minoxidil 5% and oral 1mg Finasteride for 2 years.They then stopped and their treatment differed.

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• Results showed that 80% to 84.4% of these patients maintained good hair density while on the topical combination of Minoxidil 5% + Finasteride 0.1% treatment for 1 year.
• No sexual side effects were reported.

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The studies⁸ above demonstrate that topical finasteride has clinically proven, evidence-based, scientific data to show its efficacy in growing hair and that it has a lower side effect profile compared to oral finasteride. Out of the 7 clinical studies reported above, only one study showed one event with sexual side effects (testicular pain). This is 1 out of 268 subjects in all of the studies, which equates to a rate of 0.37%. In contrast, oral finasteride is associated with sexual side effects in 1.8% to 3.8% of patients, which means that in the above 268 subjects, there should have been about 5 to 10 subjects experiencing some form of sexual side effects. However, this was not the case. Only 1 person had sexual side effects as opposed to 5 to 10 individuals.

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Pharmacokinetic studies by Caserini et al. suggest that lower doses of topical finasteride result in lower reduction of serum DHT compared to oral finasteride and thus lower sexual side effects. No study has conclusively shown the reason of how finasteride can cause sexual side effects. Certainly serum DHT is a contributing factor but may not be the only factor. The reason for this conclusion is because there are studies that patients were on high dosage of topical finasteride and still had no sexual side effects.

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In conclusion, the majority of studies on topical finasteride show no to only a few sexual side effects. However, this does not mean that sexual side effects can not happen. It just suggests that risk of sexual side effects are lower due to the fact that the finasteride is not taken orally. Additional larger clinical studies are needed to address the sexual side effects of topical finasteride.