How Finasteride Works: The Complete Science of DHT Blocking for Hair Loss

If you've spent any time researching hair loss treatments, you've seen the phrase "finasteride blocks DHT" about a thousand times. That's technically true, but it's like saying a car "uses gasoline" — accurate, but it tells you almost nothing about what's actually happening under the hood.

Finasteride's mechanism is genuinely fascinating, and understanding it helps you make better decisions about dosing, expectations, and whether dutasteride might be a better fit. Let's break it down properly.

What Is 5-Alpha Reductase (And Why Does It Matter)?

Your body produces testosterone. That's not the problem. The problem — if you're genetically predisposed to male pattern baldness — is what happens next.

An enzyme called 5-alpha reductase (5AR) converts testosterone into dihydrotestosterone (DHT). DHT is about 2.5 to 10 times more potent than testosterone at binding to androgen receptors. In the scalp, DHT binds to receptors in hair follicles and, in genetically susceptible individuals, triggers follicular miniaturization — the follicle gradually produces thinner, shorter, lighter hairs until it eventually stops producing visible hair altogether.

Here's the critical nuance: there are three isoenzymes of 5-alpha reductase — Type I, Type II, and Type III. They're expressed differently across tissues:

Finasteride is a competitive, specific inhibitor of Type II and Type III 5-alpha reductase. It targets the enzymes most responsible for DHT in the scalp. But — and this becomes important later — it has minimal activity against Type I.

How Finasteride Actually Blocks DHT

This is where it gets interesting. Finasteride doesn't just temporarily block the enzyme and float away. It forms what pharmacologists call a stable enzyme-inhibitor complex — essentially locking onto the 5AR enzyme with a bond so tight that the enzyme is effectively taken out of commission.

The turnover half-life of this complex is approximately 30 days. Once finasteride binds to a 5-alpha reductase enzyme, that individual enzyme molecule is functionally disabled for about a month. Your body has to manufacture entirely new enzyme to replace it.

This is why finasteride works with daily dosing even though its plasma half-life is only 6–8 hours. The drug clears your bloodstream relatively quickly, but the enzymes it already bound remain inhibited for weeks. Each daily dose locks up a new batch of freshly produced enzyme.

The Numbers: How Much DHT Does Finasteride Reduce?

At the standard 1 mg daily dose for hair loss, finasteride reduces serum DHT by approximately 65–70% and scalp DHT by roughly 64%. These are substantial reductions, and for most men with androgenetic alopecia, they're enough to halt or reverse hair loss.

Meanwhile, testosterone levels rise by about 10–20% — a logical consequence of blocking the enzyme that converts testosterone to DHT. More testosterone stays as testosterone. This increase stays within the normal physiologic range and doesn't produce symptoms or clinical effects in the vast majority of men.

Why Finasteride Can't Block 100% of DHT

This is the part most articles skip, and it matters if you're evaluating whether finasteride is enough or whether dutasteride might be worth considering.

This is exactly where dutasteride enters the picture. Dutasteride inhibits all three 5AR isoenzymes, achieving 92–98% serum DHT reduction. If you've plateaued on finasteride, the reason may be that residual Type I activity, and dutasteride addresses that gap. (We cover this in depth in our finasteride vs dutasteride comparison.)

What Happens to Testosterone

A common concern — "if finasteride blocks the enzyme that converts testosterone, does my testosterone go up?" The answer is yes, slightly.

Testosterone rises approximately 10–20% on finasteride 1 mg/day. For a man with a baseline of 500 ng/dL, that might mean a level around 550–600 ng/dL. This stays comfortably within the normal reference range (typically 300–1,000 ng/dL) and doesn't produce symptoms or clinical effects in the vast majority of men.

Finasteride does not reduce testosterone. This is a persistent misconception worth addressing clearly. If anything, it modestly increases it — though the increase is not large enough to be used therapeutically for low testosterone.

What Happens When You Stop

Finasteride's effects are entirely reversible. Once you stop taking it:

DHT returns to baseline within 14 days. Despite the 30-day enzyme bond half-life, the rapid production of new 5AR enzyme means DHT levels normalize within about two weeks of the last dose.

Prostate volume returns to baseline within 3 months (relevant for men on the 5 mg dose for BPH).

Hair count reverses within 12 months. All gains made on finasteride are gradually lost as DHT-driven miniaturization resumes. This is not because finasteride is addictive — it's because the underlying genetic susceptibility never went away. Finasteride manages the condition; it doesn't cure it.

For men considering stopping due to side effects, there are intermediate options: reducing the dose to 0.5 mg daily, switching to every-other-day dosing, or transitioning to topical finasteride (which achieves meaningful scalp DHT reduction with significantly lower systemic exposure).

The Bottom Line