How Finasteride Works: Mechanism of Action Explained

Finasteride works by blocking an enzyme called 5-alpha-reductase Type II, which converts testosterone into dihydrotestosterone (DHT). DHT is the androgen primarily responsible for both male pattern hair loss and prostate growth. By reducing DHT production by approximately 70% in the blood and over 60% in the scalp, finasteride removes the hormonal driver of these conditions without significantly affecting testosterone levels.

The Androgen Pathway: Testosterone and DHT

Testosterone is the primary male sex hormone, produced mainly by the testes (with small amounts from the adrenal glands). Most testosterone circulates bound to proteins (SHBG and albumin); only a small free fraction is biologically active. In certain tissues — including the scalp follicle, prostate, liver, and skin — testosterone undergoes local conversion to DHT by the enzyme 5-alpha-reductase.

DHT is significantly more potent than testosterone: it binds to the androgen receptor with approximately 3–5 times higher affinity and a slower dissociation rate, meaning it produces a stronger, more prolonged androgenic signal in tissues that express the androgen receptor. In the scalp follicle, this amplified androgenic signal progressively miniaturizes genetically susceptible hair follicles. In the prostate, DHT drives the hyperplastic growth that causes BPH.

The Two Isoforms of 5-Alpha-Reductase

	Type I (SRD5A1)	Type II (SRD5A2)
Primary locations	Sebaceous glands, skin, liver, non-genital follicles	Prostate, scalp follicles, genital skin, liver, seminal vesicles
Inhibited by finasteride?	Weak inhibition (minor effect)	Strong selective inhibition (~90% inhibition)
Inhibited by dutasteride?	Yes (dual inhibitor)	Yes
Clinical relevance	Less relevant in AGA and BPH	Primary driver of scalp DHT and prostate DHT

Finasteride is a selective Type II 5-alpha-reductase inhibitor. This selectivity is clinically significant: Type II is the dominant isoform in the scalp and prostate — the exact tissues where finasteride exerts its therapeutic effects. It explains why finasteride achieves ~70% serum DHT reduction while testosterone levels remain unchanged or slightly increased.

How DHT Causes Hair Loss (Follicular Miniaturization)

In genetically susceptible individuals (determined by polymorphisms in the androgen receptor gene, primarily on the X chromosome), scalp follicles in the vertex and mid-scalp regions are hypersensitive to DHT. Repeated DHT binding to the androgen receptor in these follicles triggers a signaling cascade that:

1. Progressively shortens the anagen (growth) phase of the hair cycle, from years to weeks or months
2. Reduces follicle size (miniaturization) — producing successively finer, shorter, lighter vellus-type hairs
3. Eventually silences follicles into a prolonged telogen (resting) phase before they disappear entirely

The process is gradual and happens over years to decades, following the Norwood–Hamilton scale of progression. The critical insight is that miniaturization is reversible in the early and middle stages — as long as the follicle’s dermal papilla remains intact. Once the dermal papilla is lost (end-stage balding), no drug can regenerate the follicle.

Finasteride interrupts this process at the hormonal step — by removing the DHT stimulus. With DHT reduced, the miniaturization process halts; follicles in early-to-mid miniaturization stages recover and re-enter normal anagen cycles. This is why early treatment produces better results: more reversibly miniaturized follicles remain.

Pharmacokinetics: What Happens After You Take Finasteride

Parameter	Finasteride 1 mg
Bioavailability (oral)	~65% (not affected by food)
Time to peak plasma concentration (Tmax)	1–2 hours
Protein binding	~93%
Plasma half-life	~6 hours (5–8 h range)
Enzyme inhibition duration	24+ hours (exceeds plasma half-life; enzyme recovery is slow)
Serum DHT suppression at steady state	~65–70%
Scalp DHT suppression at steady state	~64%
Effect on serum testosterone	+10–15% increase (within normal range)
Time to steady-state enzyme suppression	~3–7 days
Metabolism	Hepatic (CYP3A4); renal + fecal excretion

The plasma half-life of finasteride (~6 hours) is considerably shorter than its duration of pharmacodynamic effect. This is because finasteride forms a stable enzyme-inhibitor complex with 5-alpha-reductase Type II — the enzyme slowly regenerates over 24+ hours. This is why once-daily dosing achieves sustained 24-hour DHT suppression despite the short plasma half-life, and why occasional missed doses do not significantly diminish the drug’s long-term effect.

Why 1 mg for Hair Loss and 5 mg for BPH?

Both doses produce similar degrees of serum and scalp DHT suppression (~65–70%). The dose separation reflects the different tissue targets and approval pathways, not a meaningful pharmacological distinction:

• The 5 mg dose (Proscar) was developed first, approved for BPH in 1992, and shows the same degree of DHT suppression as 1 mg
• Dose-ranging studies found that doses as low as 0.2 mg/day produce near-maximal DHT suppression; 1 mg is at the plateau of the dose-response curve
• The 1 mg dose was developed specifically for AGA to minimize unnecessary drug exposure while maintaining equivalent efficacy

This is also why some men use generic finasteride 5 mg tablets cut into quarters (effectively 1.25 mg/tablet) as a cost-reduction strategy — though this is off-label and the tablets are not scored for cutting.

Frequently Asked Questions

Does finasteride affect testosterone levels?

Minimally and within normal range. When DHT production is blocked, there is a small compensatory increase in testosterone (typically 10–15%). This increase remains within the normal physiological range and does not cause symptoms of testosterone excess in most men. Finasteride does not raise testosterone to supraphysiological levels; it does not function as a testosterone booster.

Why does finasteride work for some areas of the scalp but not others?

The response of scalp follicles to DHT is genetically programmed and varies by scalp region. Vertex (crown) and mid-scalp follicles are most DHT-sensitive and respond best to finasteride. Frontal hairline follicles in advanced AGA are typically more resistant to DHT suppression — possibly because they have already undergone more complete miniaturization or have androgen-independent loss pathways. Finasteride shows the strongest evidence for vertex AGA (grades II–V vertex on the Norwood–Hamilton scale); frontal hairline responses are more variable and generally more modest.

How is finasteride different from dutasteride?

Dutasteride inhibits both Type I and Type II 5-alpha-reductase (a dual inhibitor), producing more complete DHT suppression (~90% vs. ~70% for finasteride). In head-to-head trials for AGA, dutasteride 0.5 mg shows somewhat greater hair growth outcomes than finasteride 1 mg, with a higher side effect burden due to more complete androgenic suppression. Dutasteride is FDA-approved only for BPH; its use for AGA is off-label in the US (approved for AGA in Japan and South Korea). Full finasteride vs. dutasteride comparison

Sources

• Dallob AL, et al. “The effect of finasteride, a 5 alpha-reductase inhibitor, on scalp skin testosterone and dihydrotestosterone concentrations.” J Clin Endocrinol Metab. 1994;79(3):703–706.
• Drake L, et al. “The effects of finasteride on scalp skin and serum androgen levels.” J Am Acad Dermatol. 1999;41(4):550–554.
• Imperato-McGinley J, et al. “Androgens and the evolution of male-gender identity among male pseudohermaphrodites with 5α-reductase deficiency.” N Engl J Med. 1979;300(22):1233–1237.
• Merck & Co. Propecia (finasteride 1 mg) US Prescribing Information. Revised 2022.