Finasteride

Finasteride is a daily pill that lowers DHT, the more potent androgen that drives male pattern hair loss and benign prostate enlargement. It blocks the enzyme that converts testosterone to DHT, so circulating and scalp DHT drop by roughly 60-70% on the 1 mg dose used for hair, and over 90% on the 5 mg dose used for the prostate. The hair follicles that were being slowly miniaturised by DHT stop shrinking and, in most users, partially recover.

Most people take it because they're losing their hair, they want to keep it, and finasteride is the single intervention with the strongest long-term evidence in men. It is the closest thing to a "keep your hair" pill that exists. The trade-off is that it works systemically, lowering DHT everywhere in the body, which is where the well-known concerns about libido, mood, and sexual function come from. The vast majority of users have no significant side effects, but a meaningful minority do, and for a smaller subset those side effects persist after stopping.

The last point has a name, post-finasteride syndrome (PFS), and it's the reason this drug needs more thought than the average prescription. A small fraction of users report sexual, mood, and cognitive symptoms that continue for months or years after stopping the drug. The medical community is split on how to characterise it, the absolute risk is low (well under 1% in cohort data), and there's no reliable predictor of who develops it. It doesn't make finasteride a bad drug, but it does make it one you should understand before you start, not after.

Deep-dive

Androgenetic alopecia is driven by DHT binding androgen receptors in genetically susceptible scalp follicles, mostly at the temples, mid-scalp, and crown. The follicles progressively shrink, the growth (anagen) phase shortens, and over years the hairs become finer, shorter, and eventually invisible. DHT is produced from testosterone by the enzyme 5-alpha-reductase, which exists in two main isoforms. Type II dominates the scalp follicles, prostate, and external genitalia. Type I sits in the skin, sebaceous glands, and parts of the brain.

Finasteride selectively inhibits Type II 5-alpha-reductase. The 1 mg/day dose used for hair drops scalp DHT by around 60-65% and serum DHT by a similar amount, while serum testosterone rises slightly (around 10-15%) and oestradiol is largely unchanged. Dutasteride, the other 5-AR inhibitor on the market, inhibits both isoforms and drops DHT by over 90%, which makes it more potent but also more impactful on systemic side effects.

Efficacy, the core trial data. The original Kaufman et al. 1998 trial randomised 1,553 men aged 18-41 with vertex hair loss to finasteride 1 mg or placebo for 24 months. Hair counts in a 1-inch circle of balding scalp rose by 107 hairs at year 1 and 138 hairs at year 2 versus continued loss on placebo. Investigator photo review confirmed improvement in two-thirds of finasteride users versus 7% of placebo. The 5-year extension showed continued improvement over placebo through year 2, then maintenance, with placebo users continuing to lose. The 10-year Japanese cohort in 523 men showed that Norwood classification improved by roughly one grade over a decade of daily use, with low adverse event rates. The 93% reduction in further visible hair loss over 5 years is the single most consequential efficacy number in the literature.

Where it works, where it doesn't. Finasteride works best on the crown and mid-scalp, less reliably on the frontal hairline, and barely at all on a mature receded hairline where follicles have already been gone for years. The earlier you start, the more hair you keep. Starting at the first signs of recession in your twenties gives a different result than starting at Norwood 5 in your forties. The drug doesn't regrow follicles that have already died, it protects follicles that are still alive but miniaturising.

Non-responders. About 10-20% of users either don't respond or respond inadequately at 1 mg. Some of this is genetic variation in androgen receptor sensitivity, some is scalp pharmacokinetics, some is just patience (you need 6-12 months to judge). A subset of non-responders on finasteride respond well to dutasteride, which is a reasonable next step if you've been compliant for a year and you're still progressing.

Prostate effects, the original use. Finasteride 5 mg (Proscar) is approved for symptomatic benign prostatic hyperplasia and shrinks prostate volume by about 25% over 6-12 months, reducing urinary symptoms and the need for surgery. The 1 mg dose used for hair has a smaller but real effect on prostate size and on PSA (it suppresses PSA by roughly 50%, so you have to double the measured value when interpreting cancer screening tests).

Prostate cancer signal, what the data actually says. The Prostate Cancer Prevention Trial randomised 18,880 men to finasteride 5 mg or placebo for 7 years. Finasteride cut overall prostate cancer incidence by about 25%, but there was an apparent increase in high-grade (Gleason 7+) cancer. Subsequent analyses showed this was largely a detection artefact, finasteride shrinks the prostate, which makes biopsies more accurate at finding the higher-grade tumours that were already there. The 18-year follow-up confirmed there was no increase in prostate cancer mortality and overall survival was identical between groups (78% vs 78.2%). The net read: finasteride doesn't cause aggressive prostate cancer, and the early scare was a statistical artefact. This matters because finasteride is now considered safe from a prostate cancer standpoint, even at the higher 5 mg dose.

Sexual side effects, the on-drug picture. In the original Phase III trials, sexual side effects (reduced libido, erectile dysfunction, ejaculation problems) occurred in around 3.8% of finasteride users versus 2.1% of placebo, an absolute increase of about 1.5-2 percentage points. Most of these resolved either with continued use or after stopping. In real-world reporting and post-marketing surveillance the rates appear higher than the trials suggested, partly because the trials were not designed to capture sexual side effects well and partly because there's a genuine signal. A systematic review of trial adverse event reporting found the original trials underreported these events. Realistically, expect 5-15% of users to notice some sexual side effect on the drug, with most being mild and reversible.

Post-finasteride syndrome (PFS), the off-drug concern. A subset of users report persistent sexual, mood, cognitive, and physical symptoms (low libido, erectile dysfunction, anhedonia, brain fog, depression, suicidal ideation) that continue after stopping the drug, sometimes for years. The medical community is split on whether this represents a distinct biological syndrome or a constellation of overlapping phenomena (psychogenic, nocebo, underlying depression, separate sexual dysfunction). The most recent comprehensive review acknowledges that contrasting data exist but concludes that a real subset of men develop persistent, non-negligible symptoms. The proposed mechanism centres on neurosteroids: 5-alpha-reductase is the enzyme that makes allopregnanolone and related neuroactive steroids in the brain, and chronic suppression may disturb GABA-A signalling and androgen receptor expression in ways that don't immediately normalise after stopping. The honest summary: PFS is real for some users, the absolute risk is low (under 1% in the largest cohort analyses), the predisposing factors aren't well characterised, and there's no reliably effective treatment. This is the most important risk to understand before starting.

Depression and suicide. Pharmacovigilance studies have detected signals for depression and suicidal ideation in finasteride users, particularly in men under 45 on the hair loss dose. The JAMA Dermatology VigiBase analysis found a 1.63-fold disproportionality signal for suicidality and a 4.33-fold signal for psychological adverse events. Larger cohort and Mendelian randomisation studies have failed to confirm a causal link, including a recent UK Biobank analysis finding no significant association. The signal is real in spontaneous reporting but doesn't survive every controlled analysis, which is consistent with either a small absolute risk in a susceptible subset or a reporting bias amplified by online communities. Either way, if you have a history of depression or current mood symptoms, this is a meaningful caution.

Topical finasteride, the half-step option. A 0.25% topical solution applied to the scalp reduces scalp DHT by 50-70%, comparable to oral, while reducing serum DHT only by about 34% versus 56% with oral. The Phase III trial (458 men, 24 weeks) showed topical finasteride produced hair count gains numerically similar to oral with markedly lower systemic exposure. It's a reasonable compromise for men who want the benefit but worry about systemic side effects, though scalp irritation and the inconvenience of daily application are real downsides, and long-term data is thinner than for oral.

Women. This is the area where the picture genuinely differs. The Price et al. trial of finasteride 1 mg in postmenopausal women was negative, no benefit over placebo on hair count after 12 months. That trial is the reason finasteride was historically considered ineffective in women. Subsequent work has changed the picture. Higher oral doses (2.5-5 mg/day) show benefit in premenopausal women with normal androgens and in a retrospective cohort of 112 women on 2.5 mg/day with measurable improvements in hair density and clinical photographs. Topical finasteride 0.5% works in postmenopausal women, with the Rossi et al. retrospective study of 119 women showing it outperformed topical 17α-estradiol when both were combined with topical minoxidil. The 2025 JAAD scoping review of oral 5-AR inhibitors in women concluded the evidence now supports their use for female pattern hair loss and frontal fibrosing alopecia despite remaining off-label. The major caveat is that finasteride is teratogenic, it can disrupt the development of male fetal genitalia, so any woman of reproductive age using it must be on reliable contraception and ideally not handle crushed tablets without protection. For premenopausal women without contraception or with pregnancy plans, finasteride is off the table and topical minoxidil, spironolactone, or topical anti-androgens like RU58841 (no published female-specific data but mechanistically plausible) are the alternatives.

Stacking. The standard combination is finasteride plus topical or oral minoxidil. The two work through different mechanisms (DHT blockade vs hair growth phase stimulation) and additively produce better outcomes than either alone. Red light therapy is a reasonable third layer. Some users add ketoconazole shampoo for local anti-androgen and anti-inflammatory effect. Stacking finasteride with RU58841 gives systemic plus local androgen blockade but reintroduces the systemic side effect risk that some users took RU to avoid, so the case for layering is weaker.

Limitations of the evidence. Most efficacy trials were funded by Merck and run on men aged 18-41 with vertex loss. Frontal loss, older men, women, and non-white populations are less well studied. Long-term safety data beyond 10 years exists but mostly in observational cohorts. The PFS literature is dominated by self-reported case series and patient registries with strong selection bias, which is part of why the medical community remains divided. None of this means the drug doesn't work, the efficacy data is among the cleanest in dermatology, but it does mean the risk side of the equation has more uncertainty than the marketing suggests.

Dosage:

Standard hair loss dose: 1 mg/day, oral, once daily, at any time of day with or without food. This is the dose with the strongest evidence and the one most users land on. Half-life is short (6-8 hours) but the enzyme inhibition lasts much longer, which is why once-daily dosing works

Microdosing (0.25-0.5 mg/day, or 1 mg every other day): Reasonable alternative for users who want most of the benefit with potentially fewer side effects. The dose-response curve plateaus relatively quickly, 0.2 mg/day reduces scalp DHT around 50% versus the 60-65% at 1 mg/day. Real-world experience suggests microdosing maintains most of the hair benefit, but the long-term controlled trial data is thin

BPH dose: 5 mg/day, oral. This is the dose for prostate enlargement and is overkill for hair loss. It doesn't produce meaningfully more hair benefit than 1 mg, but it does increase systemic side effect risk

Topical finasteride 0.25% solution: 100-200 µL applied to the scalp once daily, delivering roughly 0.25-0.5 mg of finasteride directly to the scalp. Produces scalp DHT reduction comparable to oral with around 60-70% lower serum DHT impact. Apply to clean, dry scalp, let it absorb for 10 minutes before lying down or applying minoxidil. Reasonable choice for users who want to minimise systemic exposure

Female dosing: 2.5-5 mg/day oral for premenopausal women (off-label, requires reliable contraception); 1 mg/day showed no benefit in postmenopausal women in the only large trial, but 2.5-5 mg has shown signal in observational cohorts. Topical 0.5% solution daily is a reasonable choice in postmenopausal women specifically. Do not use in pregnancy or while trying to conceive, this is non-negotiable

Timing: No clinically meaningful difference between morning and evening dosing. Pick the time you're most likely to take it consistently, that's the variable that matters

Onset and assessment window: Expect no visible change in the first 3 months. By month 6 hair shedding usually slows and existing hair starts to thicken. By month 12 photo comparisons typically show stabilisation or modest regrowth. Don't judge before 12 months. If after 12-18 months of consistent daily use there's no improvement on photos, you're a non-responder and switching to dutasteride is a reasonable next step

Discontinuation: Whatever hair you've kept or regrown will be lost within 6-12 months of stopping, the underlying genetics take over again. This is a long-term commitment by nature of the disease, not the drug. If you're not prepared to take a daily pill for years, the cost-benefit doesn't favour starting

Stacks: Pairs naturally with topical or low-dose oral minoxidil, the two mechanisms are complementary and the combination consistently outperforms monotherapy. Red light therapy is a reasonable third addition. Ketoconazole 2% shampoo a few times per week adds a small local anti-androgen and anti-inflammatory effect. Stacking with RU58841 is overkill for most users

Here's what you can expect:

The first 3-4 months are usually uneventful or feature an initial shed as follicles cycle from telogen back into anagen. Don't quit during this phase, it resolves and is a positive sign that follicles are responding. By month 6, hair shedding noticeably slows and existing hair feels thicker. By month 12, photo comparison under consistent lighting typically shows clear stabilisation and partial regrowth in the crown and mid-scalp. Frontal hairline gains are usually more modest. By year 2-3, the benefit plateaus and the rest of the work is maintenance.

Subjective experience for most users is unremarkable: no on-drug sensation, no day-to-day cognitive or sexual change, just the gradual realisation a year in that you're not seeing more scalp through your hair in photos. A minority will notice sexual side effects in the first weeks to months (lower libido, softer erections, smaller ejaculation volume). These resolve in most cases either with continued use or after stopping. A smaller minority will notice mood changes, low motivation, or cognitive flatness. Don't push through these, stop the drug, reassess at 4-8 weeks off, and make the decision again with that information.

If you stop, expect to lose all of the regrowth and most of the maintained hair within 12 months. This isn't the drug withdrawing, it's the underlying genetics expressing themselves now that the brake is off.

Side effects & risks:

Sexual side effects are the most discussed. Reduced libido, erectile dysfunction, reduced ejaculation volume, occasionally orgasmic anhedonia. Trial-reported absolute incidence is around 1.5-2% above placebo; real-world rates are probably higher, in the 5-15% range. Most are mild and reversible. If they appear, stop the drug, give it 4-8 weeks, and reassess

Post-finasteride syndrome (PFS) is the rare but serious off-drug concern. Persistent sexual, mood, cognitive, and physical symptoms that continue after stopping. Absolute risk is low (under 1% in cohort analyses) but the consequences for those affected can be severe and long-lasting. No reliable predictor of who develops it. There is no proven treatment. If you're risk-averse or have a history of depression or mood instability, topical finasteride or minoxidil monotherapy is a more conservative starting point

Mood, depression, anxiety, suicidal ideation. Pharmacovigilance signals exist; controlled cohort data is mixed. The plausible mechanism is reduced allopregnanolone in the brain, since 5-alpha-reductase makes neuroactive steroids. If you have a history of depression, take this seriously and consider topical or skip the drug. If mood changes appear after starting, stop and reassess

Gynecomastia (breast tissue growth) in around 0.5% of users, reflecting the shift in testosterone-to-oestradiol ratio. Usually reverses on stopping if caught early. Persistent gynecomastia may require surgical correction

Reduced semen volume and altered sperm parameters. Around 1-2% of users notice lower ejaculation volume. Effects on fertility are real but usually modest, finasteride can reduce sperm count and motility in a subset of users. If you're actively trying to conceive, consider stopping a few months before

PSA suppression by roughly 50%. This matters because it can mask early prostate cancer detection. Anyone over 40 or 45 starting finasteride should have a baseline PSA, and any subsequent value should be doubled when interpreting against standard cutoffs. Make sure any new GP or urologist knows you're on it

Pregnancy and breastfeeding: absolute contraindication in women who are or might become pregnant. Finasteride is teratogenic for male fetuses and can interfere with normal genital development. Premenopausal women taking it must use reliable contraception. Crushed or broken tablets shouldn't be handled by pregnant women. Topical solutions should be applied carefully and not transferred to partners

Skin and scalp effects. Topical finasteride can cause local irritation, redness, and contact dermatitis, mostly from the propylene glycol or ethanol vehicle. Oral finasteride occasionally produces mild skin reactions

Liver. Finasteride is metabolised hepatically. Liver enzyme elevation is uncommon but possible, especially in users with pre-existing liver disease

Drug interactions are minimal at hair loss doses. The drug doesn't significantly affect CYP450 enzymes and there are few clinically meaningful interactions. Combination with other anti-androgens (RU58841, spironolactone, bicalutamide, dutasteride) compounds anti-androgenic effects and reintroduces systemic side effect risk that any single agent's design tried to minimise

Prostate cancer. Long debated, now reasonably settled. Finasteride does not increase the risk of dying from prostate cancer; the early high-grade signal was largely a detection artefact from reduced prostate volume making biopsies more accurate. Long-term survival in the PCPT trial was identical between finasteride and placebo groups

Long-term safety data for hair loss dosing exists out to 10 years and is reassuring. Decades-long use data is sparser but the available signal is consistent with the short-term picture

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Blood markers

Total testosterone, free testosterone, DHT, oestradiol, LH, FSH, SHBG, baseline before starting and at 3-6 months. On 1 mg/day expect DHT to drop 50-70% and testosterone to rise 10-15%. If DHT isn't suppressed, you're a non-absorber or non-compliant. Oestradiol typically rises slightly; large increases warrant attention.

PSA, baseline for any man over 40-45 before starting, then annually thereafter. Finasteride halves PSA values, so any measured PSA on the drug needs to be doubled when interpreting against population cutoffs. Make sure your GP knows you're on finasteride before any prostate workup.

Liver enzymes (ALT, AST), baseline and at 6 months. No specific signal of hepatotoxicity at hair loss doses but worth a baseline since the drug is hepatically metabolised.

Lipid panel, baseline and at 6 months. Anti-androgen exposure can subtly shift lipid metabolism, particularly HDL. The effect is usually small but worth tracking if you already have lipid concerns.

Sperm parameters (count, motility, morphology), if you're actively trying to conceive or planning to within 12 months. Finasteride can reduce sperm count in a subset of users and most labs offer a standard semen analysis.

For women specifically: cycle-day-3 FSH, LH, oestradiol, total and free testosterone, DHEA-S, and SHBG at baseline, plus prolactin if there's any unexplained hair shedding pattern. Pregnancy test before starting for any premenopausal woman, and confirmation of reliable contraception.

For most men on 1 mg/day, a baseline panel plus a 6-month recheck is sufficient. Once the hormone shift has stabilised, annual PSA (after 40) and a periodic lipid and liver check is enough. Men using topical only or microdosing need less monitoring than full-dose oral users. Anyone with new sexual, mood, or cognitive symptoms should recheck the full hormone panel before deciding what to do next.

Finasteride is a prescription medication in most countries (Propecia 1 mg for hair loss, Proscar 5 mg for BPH). Sold OTC in a few jurisdictions and widely available through telehealth services.