Finasteride Hair and Skin Changes: What the Clinical Evidence Actually Shows

What finasteride does to the follicle

Finasteride is a competitive, specific inhibitor of type II 5-alpha-reductase, the enzyme that converts testosterone into dihydrotestosterone (DHT) [2]. Type II is found mainly in the prostate, seminal vesicles, epididymides and hair follicles and drives about two-thirds of circulating DHT. Type I is predominant in the sebaceous glands of most regions of skin, including the scalp, and accounts for about one-third [2]. Binding studies put finasteride's selectivity at roughly 100-fold for type II over type I [2].

A 1 mg tablet suppresses serum DHT by about 65% within 24 hours [2]. Mean testosterone and estradiol each rise by about 15% and stay inside the normal range, and the drug has no affinity for the androgen receptor [2]. The type II enzyme complex it binds turns over slowly, with a half-life of about 30 days, so the fall in DHT lasts well beyond the 5 to 6 hour plasma half-life of the tablet [2].

What the hair trials actually measured

The core evidence is two 1-year placebo-controlled trials in 1,553 men aged 18 to 41 with male pattern hair loss; 1,215 men continued into blinded 12-month extensions for a second year [1]. Hair was counted in a 1-inch diameter circle (5.1 cm2) on balding vertex scalp, where the men began with a mean of 876 hairs.

Finasteride produced counts 107 hairs above placebo at 1 year and 138 above at 2 years (P<0.001 for all comparisons), while the placebo group kept losing hair [1]. The label adds the longer view: counts rose significantly by 6 and 12 months, peaked during the first two years, then declined slowly but stayed above baseline through five years [2]. At five years the gap between groups was 277 hairs, though that comparison rested on 219 men still taking finasteride and 15 on placebo [2].

How hair was judged matters. The endpoints were hair counts, patient self-assessment, investigator assessment and expert review of photographs [2]. Shaft caliber and texture were not among these endpoints.

The trials enrolled men with mild to moderate hair loss, not complete baldness [2], and the label states that efficacy in bitemporal recession, the receding temples, has not been established [2].

How long before hair changes appear

The label's own wording is that daily use for three months or more is generally necessary before benefit is observed, and that continued use is needed to sustain it [2]. Significant hair-count increases were seen at 6 and 12 months [2], with the maximum improvement coming during the first two years [2].

The dose is one 1 mg tablet a day, with or without food [2]. The 5 mg tablet (Proscar) is a separate product, approved for symptomatic benign prostatic hyperplasia, not for hair loss [3].

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Does finasteride change skin oiliness, acne or seborrheic dermatitis?

Type I 5-alpha-reductase is predominant in sebaceous glands, and finasteride is roughly 100-fold more selective for type II [2]. The hair-loss trials did not measure sebum production, scalp oiliness, acne counts, or seborrheic dermatitis severity, so they do not establish whether finasteride changes these skin findings [2].

Scalp condition was handled as a confounder rather than an endpoint. To keep seborrheic dermatitis from confounding the hair-growth assessment, every man in the Propecia studies, whether on finasteride or placebo, was instructed to use a specified tar-based shampoo during the first two years [2].

Does finasteride work in women?

Propecia is indicated for men only, and pregnancy is a contraindication [2]. The Propecia label identifies handling of crushed or broken tablets during pregnancy as a risk to a male fetus [2].

A 2006 study enrolled 37 premenopausal women with female pattern hair loss and treated them with finasteride 2.5 mg daily alongside an oral contraceptive containing drospirenone and ethinyl estradiol [4]. At 12 months, 23 of 37 were rated improved on global photography, 13 showed no improvement and one worsened; no adverse reactions to the drug were reported [4]. The authors noted it was unclear whether the result came from the higher 2.5 mg dose or from the antiandrogenic contraceptive [4]. No skin endpoints were measured.

Finasteride with minoxidil

A 2002 open randomized study of 100 men with androgenetic alopecia (Hamilton grades II to IV) compared four regimens over one year: finasteride 1 mg alone (30 men), finasteride plus topical 2% minoxidil (36), topical 2% minoxidil alone (24), and finasteride plus 2% ketoconazole shampoo (10) [5]. Hair growth occurred in every group, was best with finasteride plus minoxidil, and the finasteride-containing groups improved significantly more than minoxidil alone (p<0.05) [5]. No skin outcomes were assessed.

Topical finasteride

No topical finasteride product is FDA-approved [2]. A 2022 phase III double-blind, double-dummy trial randomized 458 men with androgenetic alopecia at 45 European sites to a topical finasteride spray or placebo for 24 weeks; 446 were evaluated for safety and 323 completed the study [6].

Change in target-area hair count at week 24 was 20.2 hairs with topical finasteride versus 6.7 with placebo (P<0.001), numerically similar to oral finasteride [6]. Mean serum DHT fell 34.5% with topical finasteride versus 55.6% with the oral tablet, and adverse events did not differ meaningfully from placebo [6].

What happens when you stop

The label states that withdrawal of treatment leads to reversal of effect within 12 months, and that continued use is recommended to sustain benefit, with periodic re-evaluation [2]. Finasteride's plasma half-life is 5 to 6 hours, but the type II enzyme complex it binds turns over with a half-life of about 30 days [2].

Safety the label lists

In three 12-month controlled trials (945 men on finasteride, 934 on placebo, aged 18 to 41), drug-related decreased libido was 1.8% versus 1.3%, erectile dysfunction 1.3% versus 0.7%, and ejaculation disorder 1.2% versus 0.7% [2]. Overall, 3.8% of treated men reported one or more of these versus 2.1% on placebo (p=0.04), discontinuation for drug-related sexual events was 1.2% versus 0.9%, and the incidence of each fell to 0.3% or lower by year five [2]. A self-administered sexual function questionnaire at month 12 favored placebo in three of four domains (sexual interest, erections and perception of sexual problems), with no significant difference in overall satisfaction with sex life [2].

There is no boxed warning on Propecia [2]. The postmarketing section lists sexual dysfunction that continued after discontinuation, including erectile dysfunction, libido, ejaculation and orgasm disorders, and lists depression, suicidal ideation and behavior; the label states these voluntary reports come from a population of uncertain size and that a causal relationship cannot reliably be established [2]. The suicidal ideation wording came from a 2022 safety labeling change order [7].

Pooled data point the same direction. A 2019 meta-analysis of 15 randomized placebo-controlled trials in 4,495 men with androgenetic alopecia found a 1.57-fold relative risk of sexual dysfunction with 5-alpha reductase inhibitors (95% CI 1.19 to 2.08); for finasteride alone the relative risk was 1.66 (1.20 to 2.30) [8]. A 2015 review of 34 finasteride hair-loss trials found that none had adequate safety reporting and that 76% evaluated safety for one year or less [9].

Breast tenderness and enlargement were not reported more often than placebo in the Propecia trials [2], while the postmarketing section lists breast tenderness, breast enlargement and male breast cancer, with the relationship to long-term use unknown [2]. The label reports that Propecia lowered mean PSA from 0.7 to 0.5 ng/mL at month 12 in men aged 18 to 41, and that a confirmed increase from the lowest PSA value on treatment may signal prostate cancer [2].