Topical Finasteride 0.005%
What the Dose Reveals

Overview

The dose almost nobody questions

Finasteride is a competitive, selective inhibitor of type 2 5α-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT). The standard pill is 1 mg. The off-label prostate dose is 5 mg. The compounded topicals are usually 0.25%. Almost no one asks the obvious question: how much finasteride do you actually need to block DHT?

The answer, from the drug's own dose-ranging trial, is “very little.” DHT suppression saturates at a fraction of the standard dose, and everything above that adds systemic exposure without adding blockade. That single fact reframes both the dose and the route.

Dose-response

The dose ladder: DHT suppression by dose

In the definitive dose-ranging study, Drake 1999 gave 249 men 0.01 to 5 mg of finasteride daily for 42 days and measured serum and scalp DHT. The serum numbers tell a clean, monotonic story, and they plateau almost immediately.

A 25-fold increase in dose, from 0.2 mg to 5 mg, lowers blood DHT by roughly 3 percentage points. In the authors' words, doses as low as 0.2 mg per day “maximally decreased both scalp skin and serum DHT levels.” The standard 1 mg pill sits well past the point of diminishing returns.

Pharmacokinetics

How much finasteride reaches your blood

Only about 1.4 to 2% of topically applied finasteride crosses normal scalp into the circulation (Caserini 2014 back-calculates to roughly this range; topical 0.25% delivered about 11% of an oral 1 mg tablet's plasma exposure despite applying several times more drug). That is the entire premise of going topical: deliver the drug to the follicle, not the bloodstream.

But the premise only holds if the formulation actually keeps systemic exposure low, and that is exactly where standard 0.25% topical gets murky. The published serum-DHT numbers for the same 0.25% concentration disagree wildly, which means “topical, so low-systemic” is a claim you cannot take at face value. The good news is that the systemic side is simple enough to model.

The model

A model: from concentration to systemic dose

The systemic dose from a topical is just three numbers: the concentration, the volume you apply, and how much of it crosses your skin. Concentration times ten gives milligrams per millilitre (0.005% is 0.05 mg/mL, 0.1% is 1.0 mg/mL). Multiply by daily volume to get the applied dose. Multiply by roughly 2% bioavailability to get what actually reaches your blood. Then map that systemic dose onto finasteride's known dose-response (an Emax curve fit to Drake 1999) to predict the serum DHT change.

Worked at 2 mL per day (1 mL twice daily), the model reproduces what we actually observe: 0.005% lands at a predicted ~6%, inside assay noise, which is exactly the “no detectable change” Mazzarella reported; and 0.25% lands near the middle of the real-world 5 to 75% disagreement. The concentration where systemic DHT starts to clearly move is around 0.1%, where the systemic dose (~0.04 mg/day) approaches the oral 0.05 mg threshold.

Reading the data honestly

The 0.25% disagreement is a dose-response, not noise

Standard 0.25% topical is marketed as low-systemic, and the published serum-DHT reductions range from about 5% to 75%. That looks like chaos. It is not. The studies applied a roughly 40-fold different amount of finasteride, and the systemic DHT drop tracks the applied dose almost exactly.

Caserini 2016 settles it inside a single study: 0.25% applied at 100 microlitres once daily reduced serum DHT 24%, while 1 millilitre twice daily reduced it 60 to 70%. Same drug, same lab, only the applied amount changed. The high end (Caserini 2014, 68 to 75%) applied 1 mL twice daily, 4.55 mg per day, in healthy men over one week. The low end (Suchonwanit 2018, about 5%) dissolved the finasteride into a minoxidil solution, a poorer-penetrating vehicle, in an independent double-blind trial. A “0.25% topical” can mean a 40-fold range of actual drug.

There is a sharp corollary. A vehicle engineered to push more finasteride through the skin raises efficacy and systemic exposure at the same time. “Topical, so low-systemic” is a claim about a specific formulation and applied dose, not about the concentration on the label. And it is not risk-free either way: in 2025 the FDA warned about systemic sexual and mood side effects from topical finasteride, most persisting after discontinuation.

The nuance

DHT suppression saturates faster than hair regrowth

Honesty matters here. DHT suppression and hair-count efficacy do not plateau at the same dose. Serum DHT is essentially maxed at 0.2 mg (Drake 1999). Hair-count benefit keeps climbing toward 1 mg, which is why Roberts 1999 identified 1 mg as the optimal oral dose.

So at 0.2 mg you have captured roughly 96% of the DHT blockade but only about 76% of the 1 mg hair benefit. The point is not that more dose is useless; it is that once DHT is maxed, the extra hair gain from a higher oral dose comes almost entirely with extra systemic drug. Topical breaks that trade by localizing the dose to the scalp.

The case for low concentration

Why 0.005%, not 0.25%

If the DHT blockade is saturated at a tiny dose, the goal is to deliver that tiny dose where hair grows and keep it out of the blood. 0.005% is the lowest-exposure expression of that idea.

At 0.005%, the only published human study (Mazzarella 1997, 1 mL twice daily for 16 months, n=52, single-blind, placebo-controlled) found no detectable change in plasma testosterone or DHT. It also reported, over the 16-month course, slow, partial hair regrowth: increased hair density at the balding margins and a partial return of hair in previously bare areas, with the response emerging only after the first few months. The effect was modest and incomplete, the trial is old and only single-blind, and it has never been replicated, so this is suggestive, not proof. Still, it is the cleanest version of one principle: put the DHT blockade at the scalp, at the lowest, most predictable systemic cost. And that is the entire point of this page: 0.005% is the most finasteride anyone has shown to grow hair without a detectable effect on systemic DHT.

Mechanism

What finasteride actually does

Finasteride selectively inhibits type 2 5α-reductase, the isoenzyme that produces the majority of circulating DHT (Gisleskog 1998 modeled the type 2 contribution at roughly 80% of plasma DHT). Less DHT means less of the androgen signal that miniaturizes genetically susceptible scalp follicles.

Because the enzyme is the rate-limiting step, blocking it saturates quickly. That is the molecular reason the dose-response plateaus: once you have inhibited most of the available type 2 enzyme, additional drug has little left to block.

Benchmark

The dutasteride benchmark

Dutasteride blocks both type 1 and type 2 5α-reductase, so it suppresses DHT more deeply than finasteride and, in head-to-head trials, regrows measurably more hair. It is the upper bound of 5α-reductase inhibition for hair.

That matters for context: finasteride is not the most powerful DHT blocker available. The argument for low-dose topical finasteride is not maximum suppression. It is favorable suppression-to-systemic-exposure, the opposite end of the trade from oral dutasteride.

Caveats

Limitations

This page summarizes published clinical literature and is for informational purposes only. It is not medical advice. Topical and oral finasteride are prescription medicines; finasteride for hair loss in women, and any compounded or off-label use, should be decided with a clinician.