What Is Finasteride? Dosage, Half-Life & How to Track It

What finasteride is and how it works

Finasteride is a Type II 5-alpha-reductase inhibitor — it blocks the enzyme that converts testosterone into dihydrotestosterone (DHT), a more potent androgen responsible for androgenic alopecia (pattern hair loss) at the scalp and for prostate tissue growth. Lowering DHT specifically, without lowering testosterone itself, is the entire mechanism.

It is FDA-approved at two doses for two different indications: 1 mg (Propecia) for male pattern hair loss, and 5 mg (Proscar) for benign prostatic hyperplasia (BPH) — an enlarged prostate. Both use the same drug and mechanism; the dose difference reflects the different tissue targets.

In the TRT/AAS context specifically, finasteride shows up as a way to blunt DHT-driven scalp hair loss or prostate growth from testosterone use — but because DHT itself contributes meaningfully to androgenic effects like libido and a sense of general drive, lowering it is not a free action; it is a real tradeoff most people only fully notice once they are on it.

Typical dosing & half-life

At a roughly 5–6 hour half-life, finasteride is taken once daily, and — unlike many drugs — its clinical effect takes far longer to show up or reverse than its plasma half-life suggests: DHT suppression is essentially complete within days, but the visible effect on hair (or measured effect on prostate size) unfolds over months, since it is working on a slow biological process, not an acute symptom.

The 1 mg and 5 mg tablets are not simply interchangeable by splitting one to match the other in practice — they are marketed and typically prescribed for their specific indication, though the underlying drug is identical.

Effects on hair or prostate size build over months, not days — plasma half-life describes how fast the drug clears, not how fast its downstream effect appears or fades.

DHT, sexual side effects, and what "post-finasteride syndrome" refers to

DHT is the more potent of the two major androgens and plays a real role in libido, and reported sexual side effects on finasteride — reduced libido, erectile difficulty, and less commonly reduced ejaculate volume — are documented in the prescribing information, not just anecdotal. For most users these resolve after stopping.

A smaller subset of users report symptoms — sexual, cognitive, or mood-related — that persist after stopping the drug, a cluster sometimes referred to as post-finasteride syndrome (PFS). It is a genuinely contested area: the phenomenon is documented in case reports and the FDA added persistent-symptom language to the label, but the underlying mechanism and how common it actually is remain unsettled in the research. Worth taking seriously as a possibility, not as either a certainty or a fringe claim.

What to monitor & the real risks

The one lab quirk that catches people off guard: finasteride roughly halves measured PSA (prostate-specific antigen) levels within about 6 months of continuous use. That is expected and not itself a red flag — but it means a PSA result while on finasteride needs to be doubled for an accurate comparison to normal screening ranges, and anyone getting routine prostate-cancer screening needs their prescriber and lab aware they are on it.

A small increased risk of gynecomastia has also been reported. Mood changes, including reports of depression, are listed in prescribing information and are worth tracking honestly rather than dismissing, particularly for anyone already managing mood alongside a TRT or AAS protocol.

Frequently asked questions