2. Discovery: from prostate drug to hair treatment

Finasteride and dutasteride were originally developed for benign prostatic hyperplasia (BPH) – enlargement of the prostate in older men.

• Finasteride selectively inhibits the type II isoenzyme of 5α-reductase, which is expressed in the prostate and hair follicles.
• Dutasteride inhibits both type I and type II isoenzymes, resulting in a more pronounced DHT reduction in DHT.

During early clinical trials for BPH, researchers observed reduced prostate volume and improvement in urinary symptoms, as well as incidental changes in hair: some men noticed less hair loss and even regrowth, particularly at the vertex.

This prompted formal trials in men with androgenetic alopecia at lower doses (finasteride 1 mg rather than the 5 mg used for BPH). Those trials demonstrated that finasteride 1 mg:

• slows progression of male pattern hair loss
• increases hair counts in affected areas compared with placebo
• maintains benefits over several years in many men

Dutasteride followed as a “stronger” 5-ARI, with greater suppression of DHT and, as trials suggest, somewhat greater efficacy for hair in selected men.

3. The biology: 5α-reductase, DHT and hair

There are two main isoenzymes relevant to hair:

• Type I 5α-reductase – found in sebaceous glands, epidermis and liver
• Type II 5α-reductase – abundant in hair follicles and prostate

Together they convert testosterone to DHT, which has a higher affinity for androgen receptors than testosterone and a greater potency in driving androgen-responsive changes.

In androgenetic alopecia, susceptible follicles (in the front, top, and crown) express more androgen receptors, show higher 5α-reductase activity, and shrink with prolonged DHT exposure.

Meanwhile, occipital follicles (back and sides) are relatively resistant, explaining the “permanent” donor zone in hair transplant surgery.

By inhibiting 5α-reductase:

• Finasteride reduces serum DHT by about 60-70% at 1 mg/day.
• Dutasteride reduces serum DHT by more than 90% at 0.5 mg/day.

Lower DHT means less androgenic signalling at the follicle, slowing miniaturisation and allowing some follicles to produce thicker hairs again.

4. Efficacy in men

4.1 Finasteride 1 mg in men

A large two-year programme in over 1,500 men aged 18–41 with vertex thinning found that finasteride 1 mg daily:

• Increased hair counts in a 5.1 cm² target area by an average of around 100–140 hairs versus baseline, whereas men on placebo continued to lose hair over the same period.
• Improved patient and investigator assessments: a clear majority of men on finasteride were rated as “improved” or “no further loss”, compared with a minority on placebo.

A separate detailed analysis at 48 weeks found that, compared with placebo, finasteride produced:

• a net increase of roughly 17 hairs in the target area (about an 8% gain)
• a 26% increase in anagen hairs
• an improvement in the anagen:telogen ratio of nearly 50%

A meta-analysis pooling 12 randomised trials (nearly 4,000 men) concluded that:

• Men on finasteride were roughly 1.7–1.8 times more likely than those on placebo to rate their hair as improved over 1–2 years.
• Put differently, the “number needed to treat” for one extra man to perceive improvement was about 3–6, depending on time-frame and outcome measure.
• Mean hair counts increased by about 9% at 12 months and 24% at 24 months compared to placebo.

That does not mean everyone improves, but it does support finasteride as an effective disease-modifying therapy in a sizeable proportion of men.

4.2 Dutasteride vs finasteride

Dutasteride 0.5 mg daily, in a large 24-week trial comparing different doses vs finasteride 1 mg and placebo, demonstrated:

• Dose-dependent increases in hair count and hair shaft width with dutasteride.
• The 0.5 mg dose produced statistically greater improvements in hair count and panel photographic assessments at 24 weeks than finasteride 1 mg and placebo.

A subsequent meta-analysis comparing the two drugs reported:

• an average additional gain in total hair count of around 25–30 hairs (in standardised target areas) with dutasteride compared to finasteride over ~6 months
• consistently better global photographic and patient assessments with dutasteride
• similar rates of adverse events overall, including sexual side-effects, across the two drugs

A network meta-analysis that compared dutasteride, finasteride, minoxidil and various doses generally ranked:

• Dutasteride 0.5 mg/day at the top in terms of hair count gains,
• followed by higher-dose finasteride (5 mg),
• then finasteride 1 mg, oral minoxidil and topical treatments further down.

In practical terms, dutasteride appears more potent at halting and reversing miniaturisation in the short to medium term, with a safety profile broadly comparable to that of finasteride at the doses used for hair.

5. Use in women: narrow windows and necessary caution

5-ARIs in women are more complex.

5.1 Postmenopausal women

In postmenopausal women with female pattern hair loss:

• a large, high-quality trial of finasteride 1 mg daily showed no benefit over placebo after 12 months
• smaller studies using higher doses (2.5–5 mg of finasteride, or low-dose dutasteride) have reported improvements in some women
• a meta-analysis suggests that higher doses may help selected postmenopausal women, particularly when hyperandrogenism is present, but data are limited and heterogeneous (higly varied and subject to more bias)

5.2 Premenopausal women

In premenopausal women, finasteride and dutasteride are:

• teratogenic if taken in pregnancy (risk of feminisation of a male fetus)
• therefore only considered, if at all, in combination with reliable contraception and under specialist supervision
• not first-line; topical minoxidil, low-dose oral minoxidil and anti-androgens like spironolactone usually precede 5-ARIs

Because of these risks and the limited evidence, 5-ARIs in women sit firmly in the “case-by-case, specialist” category rather than routine therapy.

6. Topical finasteride: similar hair benefit, lower systemic exposure

Topical finasteride exists to reduce systemic DHT suppression and potential side effects.

In a large phase III trial (24 weeks), men were randomised to topical finasteride spray (0.25%), oral finasteride 1 mg, or placebo. The study reports:

• mean increase in target-area hair count at 24 weeks was about 20 extra hairs with topical finasteride compared with about 7 extra hairs with placebo
• oral finasteride produced a numerically similar increase to topical finasteride over the same period
• serum finasteride concentrations with the topical formulation were over 100-fold lower than with oral finasteride
• serum DHT fell by roughly 34% with topical finasteride versus around 56% with oral finasteride

The practical interpretation in the material is that topical finasteride can deliver similar short-term hair benefits to oral finasteride while causing less systemic DHT suppression. Systemic exposure is lower, but not zero.

Topical combination products (such as finasteride plus minoxidil) are also discussed as showing greater improvements than minoxidil alone in small studies, with the same contraception relevance in women of childbearing potential.

7. Dosing strategies and combinations

7.1 Finasteride

The most common regimen in men is 1 mg once daily (standard). Lower or intermittent dosing (for example, 0.5 mg daily or 1 mg several times per week) may occasionally be used in men concerned about side effects, although robust comparative data are limited.

7.2 Dutasteride

Common regimens in men (off-label):

• 0.5 mg once daily
• 0.5 mg two or three times per week in maintenance or in men wishing to reduce total exposure

Because dutasteride has a long half-life, alternate-day or a few-times-weekly dosing still maintains DHT suppression, though hair-specific data are less extensive.

7.3 Combining with minoxidil

In practice, 5-ARIs are very often combined with topical minoxidil or low-dose oral minoxidil because they target different aspects of follicle biology: minoxidil supports and prolongs anagen, while 5-ARIs reduce the androgenic pressure driving miniaturisation.

Combination therapy typically produces better stabilisation and regrowth than either alone.

8. Safety: what the trial-grade numbers look like

8.1 Sexual side effects: the clearest pooled estimate in the material

A systematic review and meta-analysis of 15 placebo-controlled trials of 4,495 men found:

• 5.85% of men on active drug reported at least one sexual adverse effect, compared with 3.77% on placebo
• Overall relative risk for “any sexual adverse effect” with 5-ARIs was 1.57

Translating that into more intuitive numbers:

• In large finasteride trials, around 3–8% of men on finasteride reported some sexual adverse effect (reduced libido, erectile difficulty, ejaculatory changes), compared with roughly 2–4% on placebo
• That equates to one or two extra men per hundred experiencing sexual side-effects attributable to the drug over 1–2 years of use
• Most events were mild to moderate and resolved either with continuation or after stopping the drug; discontinuation rates due to sexual side-effects were low and similar to placebo

Looking at the drugs separately:

• For finasteride 1 mg, 5.31% of treated men vs 3.05% on placebo experienced sexual adverse effects. Relative risk: 1.66 (95% CI 1.20–2.30)
• For dutasteride 0.5 mg, 8.27% of treated men vs 6.23% on placebo experienced sexual adverse effects. Relative risk: 1.37

A few important nuances:

• These trials were typically 6–24 months long; they do not address very long-term use
• They enrolled selected, relatively healthy men, so real-world rates may differ slightly
• Sexual side-effects were fewer in later years in long-term finasteride and dutasteride safety extensions; many cases appeared in year 1 and decreased thereafter

Taken together, the cleanest way to express this is: in high-quality trials, 5-ARIs increase the relative risk of sexual side effects, but because baseline risk is low, the absolute extra risk is on the order of a couple of extra men per 100 treated in the short to medium term. Most events are mild and resolve with time or after stopping, but not all.

8.2 Mood and suicidal thoughts: regulatory conclusions described in the material

The material states that following an EU-wide review, the European Medicines Agency concluded in 2025 that suicidal thoughts should be formally recognised as a side effect of 1 mg finasteride used for androgenetic alopecia, with frequency not reliably estimated, and that patient information should prompt users and clinicians to watch for mood changes and seek help promptly. Product information for dutasteride would be updated to include mood warnings as a precaution.

The practical approach described is:

• Screen for a history of depression or suicidal thoughts
• Advise stopping the drug and seeking review if significant mood changes arise
• Low threshold for discontinuation if a patient feels worse psychologically on therapy

8.3 Fertility and semen parameters

Data on fertility at hair-loss treatment doses are mixed but broadly reassuring. Some studies have shown small reductions in sperm count, semen volume, or motility in a subset of men taking finasteride or dutasteride, which were generally reversible after stopping the medication. Other studies have found no significant clinical impact.

A practical compromise used by many clinicians is:

• To reassure that gross, permanent infertility is unlikely in the absence of other problems
• To suggest pausing 5-ARIs for a few months in couples actively trying to conceive, if there are concerns about semen quality
• To frame the decision in conjunction with semen analysis and urological input, where needed

8.4 Prostate, PSA, and breast effects

5-ARIs:

• Reduce PSA by roughly 50%, which must be accounted for when screening for prostate cancer (by doubling measured PSA to approximate the true value)
• Shrink prostate volume and reduce urinary symptoms in benign prostatic hyperplasia
• Can sometimes cause gynaecomastia or breast tenderness in men; any new breast lump or nipple discharge should be evaluated

Long-term BPH trials show an overall reduction in prostate cancer incidence with dutasteride and finasteride, but a complex debate persists about increased detection of higher-grade cancers in some analyses. At the low doses used for hair loss, the relevance of these BPH data is unclear; however, they remind us that 5-ARIs are not trivial but true disease-modifying drugs.

9. Oral vs topical, finasteride vs dutasteride: how to choose?

In men with androgenetic alopecia, the choice runs along a few axes.

9.1 Finasteride vs dutasteride

• Finasteride 1 mg/day has robust long-term data, good efficacy, and a modest increase in sexual side-effects over placebo.
• Dutasteride 0.5 mg/day appears more effective on hair counts and photographic assessments over 6–12 months, with similar overall rates of sexual and other adverse events in the available RCTs.
• Dutasteride has a longer half-life and deeper DHT suppression, which is attractive in terms of hair but means side-effects may take longer to wash out.

In practice:

• Many clinicians start with finasteride 1 mg/day.
• Men with aggressive early loss or insufficient response sometimes escalate to dutasteride.
• Some men and physicians opt directly for dutasteride, accepting the deeper DHT suppression from the outset.

9.2 Oral vs topical finasteride

Topical finasteride 0.25% spray:

• offers similar short-term hair benefits to oral finasteride 1 mg in trials,
• reduces scalp DHT, but reduces serum DHT less than oral (about one-third versus over a half),
• yields blood finasteride levels more than 100 times lower than oral dosing.

It may appeal to men who are concerned about full systemic exposure, as well as men who have experienced mild systemic side effects with oral finasteride and want to see if a scalp-focused route is better tolerated.

The trade-off is that:

• long-term, real-world data are still limited compared with oral finasteride;
• topical requires consistent, daily application;
• systemic exposure is lower, not zero.

9.3 Combining with other treatments

Both finasteride and dutasteride are commonly combined with minoxidil (topical or low-dose oral) and, where appropriate, regenerative techniques such as PRP or LLLT.

Think of 5-ARIs as addressing the hormonal driver of miniaturisation, with other treatments working on follicular support and regeneration.

10. Conclusion: a grounded summary for someone considering a 5-ARI

The data tell us that:

• Finasteride and dutasteride meaningfully slow and often partly reverse androgenetic alopecia in many men, with dutasteride being the more potent of the two.
• Sexual side effects are uncommon but more frequent than on placebo; roughly one or two extra men per hundred treated will experience them in the short to medium term, and these often subside in either continuation or cessation of the drug.
• Severe or persistent side-effects are not the norm, but they appear to be real for the men who experience them.
• Mood and suicidality issues are rare but have been serious enough for regulators to mandate warnings and patient cards. Their exact incidence is unknown.

For someone considering a 5-ARI, useful questions are:

• How important is it to me to preserve or improve my hair, realistically, versus accepting further loss?
• How do I feel about taking a hormone-modifying drug long-term?
• If I did experience sexual or mood changes, what would I want my plan to be (dose reduction, switch, stop)?
• Would I prefer to start with a less potent option (finasteride, topical finasteride) and escalate only if needed?

The “right” answer is not the same for everyone. However, the aim is to ensure that whatever decision you make is well-informed and supported by evidence, rather than shaped solely by marketing promises or internet horror stories. Always consult a licensed professional before considering a 5-ARI.