5-alpha reductase inhibitors for hair loss, explained

Evidence tier: A - finasteride and dutasteride are the best-evidenced way to lower DHT for androgenetic alopecia.

If you read anything about male pattern hair loss for more than five minutes, you hit three letters over and over: DHT. And the drugs that get talked about most - finasteride, dutasteride - are all after the same target. They're 5-alpha reductase inhibitors. Let me explain what that actually means, because once it clicks, half the hair-loss internet suddenly makes sense.

What 5-alpha reductase inhibitors are

5-alpha reductase is an enzyme. Its job is to convert testosterone into a more potent androgen called dihydrotestosterone (DHT). In men genetically prone to hair loss, DHT binds to receptors in the follicles on top of the scalp and slowly miniaturizes them - each growth cycle produces a thinner, shorter hair, until the follicle effectively gives up.

A 5-alpha reductase inhibitor (5-ARI) does exactly what the name says: it blocks that enzyme, so less testosterone gets converted to DHT. Less DHT, less miniaturization. That's the whole idea.

Two drugs dominate:

• Finasteride - blocks mainly the type II enzyme.
• Dutasteride - blocks both type I and type II ("dual inhibition"), so it drops DHT further.

Type I vs type II (why dutasteride hits harder)

Your body makes two versions of the enzyme, and they live in different places:

Finasteride knocks out type II and lowers serum DHT by roughly 70%. Dutasteride knocks out both and lowers it by ~90%+. [1] That extra suppression is why dutasteride tends to edge out finasteride on hair-count in head-to-head data - but it's also why the side-effect conversation gets more cautious. More on that trade-off in dutasteride vs finasteride.

Where 5-ARIs sit in the bigger picture

Androgenetic alopecia gets attacked from a few different angles, and 5-ARIs own one of them:

1. Reduce DHT production - this is the 5-ARI lane (finasteride, dutasteride).
2. Block the androgen receptor locally - the newer experimental route, e.g. clascoterone.
3. Stimulate growth independent of DHT - minoxidil, microneedling.

That's why a common evidence-based regimen pairs a 5-ARI (angle 1) with minoxidil (angle 3): you're turning down the cause and pushing on growth at the same time. You can see how these angles stack on the Treatment Evidence Map.

What the research actually shows

• Finasteride 1 mg/day halts progression in the large majority of men and produces a modest hair-count increase over 12 months, with benefit maintained on continued use. It's FDA-approved for male pattern hair loss and sits at Tier A. [1][2]
• Dutasteride 0.5 mg/day ranks at or near the top of network meta-analyses of AGA monotherapies for efficacy. It's approved for hair loss in a few countries (e.g. South Korea, Japan) and used off-label for it in most others. [1]
• They reverse when you stop. This is the part people underestimate: a 5-ARI suppresses DHT while it's in your system. Stop, and DHT returns, and the follicles resume miniaturizing. Like every effective AGA therapy, it works while you use it.

Read the efficacy numbers the way we always push here: judge them by absolute hair-count and photos over 6–12 months, not by an impressive-sounding relative percentage.

The side-effect trade-off (said straight)

You cannot talk about 5-ARIs honestly without this section. A systematic review and meta-analysis found finasteride and dutasteride raise the risk of sexual side effects - reduced libido, erectile or ejaculatory issues - with a relative risk around 1.66 versus placebo. [3] In plain terms: most men don't get them, but the risk is real and above baseline, and it's higher-profile with dutasteride's deeper DHT suppression.

A smaller number of men report symptoms that they say persisted after stopping - the contested territory of post-finasteride syndrome, which we cover on its own page without hand-waving it away or catastrophizing it.

None of this is a verdict for or against the drugs. It's the exact trade-off - strong, proven DHT reduction versus a real but uncommon side-effect risk - that you should be weighing with a clinician, not a forum.

How I'd track it (my two cents)

Here's the practical problem with any DHT-lowering drug: the benefit is mostly prevented loss, which is invisible. You don't feel hairs that didn't fall out. So people quietly wonder if it's doing anything and drift off it - which is the one guaranteed way to lose the benefit.

The fix is measurement. Before I started a 5-ARI, I'd take baseline crown and hairline photos - same angle, distance, and light - and repeat them monthly. Over 6–12 months, "holding steady or slightly better" is the win for a DHT blocker, and photos are the only way to see it honestly. I built the progress tracker to ghost your baseline over the camera so every shot lines up. Measure it; don't rely on the mirror and your mood.

FAQ

What do 5-alpha reductase inhibitors do? They block the enzyme that converts testosterone into DHT, the hormone that shrinks scalp follicles in male pattern hair loss. Lower DHT means less miniaturization. Finasteride and dutasteride are the two used for hair loss.

Which is stronger, finasteride or dutasteride? Dutasteride lowers DHT more (it blocks both enzyme types, ~90%+ vs ~70%) and tends to rank higher for efficacy - but with a correspondingly cautious side-effect profile. See dutasteride vs finasteride. "Stronger" isn't automatically "better for you."

Do 5-ARIs regrow hair or just stop loss? Both, to a degree: they reliably slow or halt loss and produce modest regrowth over 6–12 months. The bigger, more reliable effect is prevention. Benefit ends when you stop.

Are the side effects permanent? For most men, side effects (if they occur) resolve on stopping. A minority report persistent symptoms - see post-finasteride syndrome. This is a real, if uncommon and debated, concern to discuss with a doctor before starting.

Can I use a 5-ARI with minoxidil? Combining a DHT reducer with a growth stimulator like minoxidil is a common evidence-based approach because they work through different mechanisms - but any regimen should be set with a clinician who knows your health.

Sources

1. Gupta AK, et al. Relative efficacy of minoxidil and the 5-alpha reductase inhibitors in androgenetic alopecia - a network meta-analysis. JAMA Dermatology, 2022. https://pmc.ncbi.nlm.nih.gov/articles/PMC8811710/
2. U.S. FDA - finasteride 1 mg (Propecia) prescribing information (approved indication: male pattern hair loss).
3. Sexual dysfunction associated with 5-alpha reductase inhibitors - systematic review and meta-analysis. Acta Dermato-Venereologica, 2018. https://pubmed.ncbi.nlm.nih.gov/30206635/

Educational information, not medical advice. 5-ARIs are prescription (or off-label) drugs - decisions belong with a qualified clinician. See our editorial standards.