Treatment Science · 5α-Reductase Inhibition
Topical Finasteride 0.005%
What the Dose Reveals
Finasteride's DHT blockade saturates at a tiny dose. From 0.2 mg upward, more drug barely lowers DHT any further; it only adds systemic exposure, which is the part tied to the side effects. The case for 0.005% topical is to deliver that tiny effective dose to the scalp and keep it out of your blood.
- The blockade plateaus at 0.2 mg: Going from 0.2 mg to 5 mg oral, a 25-fold dose jump, moves blood DHT by about 3 points (Drake 1999).
- Route changes everything: The same finasteride applied to your scalp barely moves your blood DHT, because most of it never reaches your bloodstream.
- 0.005% is the lowest-exposure version of the idea: In the one published human study (single-blind, n=52) it reported hair regrowth over 16 months with no detectable change in blood testosterone or DHT (Mazzarella 1997). It is the most finasteride anyone has shown to grow hair without a detectable effect on systemic DHT — one old, unreplicated study, not proof.
Evidence synthesis · oral dose-ranging, topical PK, and the 2025 FDA topical-finasteride alert · Last reviewed June 2026
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.
| Oral dose | Serum DHT reduction | Note |
|---|---|---|
| 0.05 mg | 49.5% | steep early climb |
| 0.2 mg | 68.6% | blockade essentially maxed |
| 1 mg | 71.4% | the standard pill, +3 points over 0.2 mg |
| 5 mg | 72.2% | 25× the dose, ~1 point more |
Source: Drake 1999 (JAAD), serum DHT reduction, 42 days. Scalp-DHT percentages are noisy and not cross-comparable across assays, so serum is used throughout.
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.
| Concentration | Applied / day | Reaches blood (~2%) | Predicted serum DHT reduction |
|---|---|---|---|
| 0.005% | 0.10 mg | ~0.002 mg | ~6% (within assay noise, none detected, Mazzarella) |
| 0.05% | 1.0 mg | ~0.02 mg | ~33% |
| 0.1% | 2.0 mg | ~0.04 mg | ~46% |
| 0.25% | 5.0 mg | ~0.10 mg | ~59% (studies actually report 5 to 75%) |
| 1% | 20 mg | ~0.40 mg | ~69% |
Drag the dot, type above, or use arrow keys. Predicted serum DHT reduction vs topical concentration.
Model: applied = concentration × 10 × volume; systemic = applied × bioavailability; serum DHT = an Emax curve fit to Drake 1999 (Emax 73%, ED50 0.024 mg/day). Shaded band = 1.4 to 2.5% bioavailability. A first-order estimate, not measured data.
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.
| Study | Applied per day | Serum DHT | Design |
|---|---|---|---|
| Caserini 2014 | 1 mL twice daily (4.55 mg) | 68 to 75% | 24 healthy men, 1 wk, open-label |
| Caserini 2016, b.i.d. | ~4.55 mg | 60 to 70% | same lab, P-3074 vehicle |
| Caserini 2016, 100–200 µL o.d. | 0.23 to 0.46 mg | 24 to 26% | same lab, lower volume |
| Piraccini 2022 | 50–200 µL o.d. (0.11 to 0.46 mg) | 34.6% | AGA patients, phase 3 |
| Suchonwanit 2018 | 0.25% in 3% minoxidil, b.i.d. | about 5% | independent, double-blind, 24 wk |
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.
| Oral dose | Serum DHT reduction (Drake) | Scalp hair count, month 12 (Roberts) |
|---|---|---|
| 0.2 mg | 68.6% | 65 |
| 1 mg | 71.4% | 85 |
| 5 mg | 72.2% | 93 |
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.
| Drug | Enzyme blocked | Serum DHT reduction |
|---|---|---|
| Finasteride 5 mg | type 2 | ~71% |
| Dutasteride 0.5 mg | type 1 + type 2 | ~95% |
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.
The 0.005% evidence is a single old study. Mazzarella 1997 is one single-blind, placebo-controlled trial from 1997. Over 16 months it reported slow, modest hair regrowth and no detectable systemic DHT change, but it is single-blind, has not been replicated, and is not a modern controlled hair-count efficacy trial.
Topical 0.25% serum-DHT data is contradictory and partly conflicted. The high reductions come from short, manufacturer-funded, open-label trials; the low reduction comes from the one independent double-blind study. Treat any single number with caution.
Topical finasteride is not risk-free. The 2025 FDA alert documents real systemic sexual and mood effects, most persisting after stopping. Lower and localized is the goal, not zero risk.
DHT suppression does not equal hair outcome. DHT plateaus at 0.2 mg, but hair-count efficacy keeps climbing to 1 mg oral. Localized dosing is a systemic-exposure argument, not a claim of maximal regrowth.
References
Sources
Primary literature behind every dose, curve, and threshold above — plus the 2025 FDA topical-finasteride alert.
- 1Drake L, Hordinsky M, Fiedler V, et al. The effects of finasteride on scalp skin and serum androgen levels in men with androgenetic alopecia.
- 2Roberts JL, Fiedler V, Imperato-McGinley J, et al. Clinical dose ranging studies with finasteride in men with male pattern hair loss.
- 3Gisleskog PO, Hermann D, Hammarlund-Udenaes M, Karlsson MO. A model for the turnover of dihydrotestosterone in the presence of the irreversible 5-alpha-reductase inhibitors finasteride and dutasteride.
- 4Caserini M, Radicioni M, Leuratti C, et al. A novel finasteride 0.25% topical solution for androgenetic alopecia: pharmacokinetics and effects on plasma androgen levels.
- 5Caserini M, Radicioni M, Leuratti C, et al. Effects of a novel finasteride 0.25% topical solution on scalp and serum dihydrotestosterone in healthy men.
- 6Piraccini BM, Blume-Peytavi U, Scarci F, et al. Efficacy and safety of topical finasteride spray solution for male androgenetic alopecia: a phase III randomized controlled trial.
- 7Suchonwanit P, Srisuwanwattana P, Chalermroj N, Khunkhet S. A randomized, double-blind controlled study of the efficacy and safety of topical solution of 0.25% finasteride admixed with 3% minoxidil vs. 3% minoxidil solution in the treatment of male androgenetic alopecia.
- 8Mazzarella GF, Loconsole GF, Cammisa GA, et al. Topical finasteride in the treatment of androgenic alopecia: preliminary evaluations after a 16-month therapy course.
- 9US Food and Drug Administration. FDA alert: systemic side effects reported with compounded topical finasteride products.
At Anagen
Two ways to put this at the scalp
Finasteride 0.005% is the lowest-exposure expression of the idea on this page. Mr MiddleGround pairs a sub-systemic finasteride dose with minoxidil and T3 for users who want more than 5α-reductase inhibition alone. A licensed clinician decides whether either is right for you.
