Finasteride: What to Expect Week by Week in Your First Month

What Actually Changes in Month One

Finasteride is a competitive inhibitor of type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone (DHT) [1]. Type II produces about two-thirds of circulating DHT and is the isozyme found in hair follicles, and finasteride is roughly 100-fold more selective for it than for type I [1]. One 1 mg tablet suppresses serum DHT by about 65% within 24 hours [1].

The drug itself clears quickly: peak plasma levels arrive 1 to 2 hours after a dose, with a terminal half-life of about 5 to 6 hours in men aged 18 to 60 and about 8 hours in men over 70 [1]. The enzyme complex it inhibits turns over slowly, with a half-life of roughly 30 days, which is why the DHT effect outlasts the plasma half-life [1]. Bioavailability is about 65% and food does not change it [1].

Finasteride 1 mg is prescription-only [1]. HealthRX.com offers finasteride 1 mg by prescription.

Week 1: The Enzyme Block Is Already in Place

By the end of day one the DHT drop has happened [1]. Nothing visible follows from it in the mirror: the label says daily use for three months or more is generally necessary before benefit is observed, and continued use is recommended to sustain it [1].

The Propecia label reports mean PSA falling from 0.7 to 0.5 ng/mL at month 12 in men aged 18 to 41 [1]. The Proscar label for the 5 mg dose says PSA falls about 50% within six months, that a new baseline should be established at least six months after starting, and that an isolated value in a man treated for six months or more should be doubled for comparison with normal ranges [4]. The Propecia label says a confirmed increase from the lowest PSA value during treatment may signal prostate cancer and calls for evaluation [1].

Weeks 2 and 3: What the Trials Report

The 12-month trials report full-year figures, not weekly ones, for the three drug-related events the label tabulates: decreased libido 1.8% vs 1.3% on placebo, erectile dysfunction 1.3% vs 0.7%, and ejaculation disorder 1.2% vs 0.7%. Overall, 3.8% of men on finasteride reported at least one of these compared with 2.1% on placebo (p=0.04), and 1.2% vs 0.9% discontinued because of them [1].

Does Finasteride Cause an Early Shed?

The Propecia label does not describe an early shedding phase, and its year-one adverse reaction table lists only the three sexual events at 1% or more [1]. Hair counts in the label's trials were significantly increased at 6 and 12 months in a 5.1 cm2 circle of vertex scalp, while placebo-treated men lost hair progressively [1][3]. The label gives no rate for increased shedding in the first weeks.

Side Effects: What the Trials and the Label Show

Across 15 randomized placebo-controlled trials of 5-alpha reductase inhibitors in 4,495 men with androgenetic alopecia, the relative risk of sexual dysfunction was 1.57 (95% CI 1.19 to 2.08); for finasteride 1 mg alone it was 1.66 (95% CI 1.20 to 2.30) [5]. In the Propecia trials the incidence of each listed event fell to 0.3% or lower by the fifth year of treatment [1].

The Propecia label's postmarketing section lists sexual dysfunction that continued after discontinuation, including erectile dysfunction, libido disorders, ejaculation disorders and orgasm disorders, plus male infertility and poor seminal quality, with normalization or improvement in seminal quality reported after stopping [1]. It also lists depression, suicidal ideation and behavior, and notes that these reports are voluntary, come from a population of uncertain size, and cannot reliably establish a causal relationship [1]. There is no boxed warning for depression, suicide or sexual dysfunction on the Propecia label; these appear in the postmarketing experience section, and the supplement that added the current wording was approved 08/01/2022 [1][6].

Other postmarketing reports include hypersensitivity reactions (rash, itching, hives, angioedema), testicular pain, hematospermia, breast tenderness and enlargement, and male breast cancer, with the relationship to long-term use unknown [1]. In the 12-month Propecia trials, breast tenderness and enlargement, hypersensitivity reactions and testicular pain occurred no more often than on placebo [1].

Observational data come from older men. In an Ontario cohort of 93,197 men aged 66 or older (median age 75) starting a 5-alpha reductase inhibitor for prostatic enlargement, depression risk was higher in the first 18 months (HR 1.94, 95% CI 1.73 to 2.16) with no increase in suicide (HR 0.88, 95% CI 0.53 to 1.45) [7].

Week 4 and Day 30

The label states that continued use is recommended to sustain benefit and that withdrawal of treatment leads to reversal of effect within 12 months [1].

What Month One Cannot Tell You

The endpoints that matter were measured later. In the two 1-year vertex trials of 1,553 men aged 18 to 41, hair count in a 1-inch (5.1 cm2) circle of balding vertex scalp, starting from a baseline of 876 hairs, was 107 hairs higher than placebo at one year and 138 higher at two years (P<0.001) [3]. The label's long-term extension reports a 277-hair difference at five years (n=219 on finasteride vs n=15 on placebo) [1]. The label also states that efficacy in bitemporal recession has not been established, and the anterior mid-scalp hair counts excluded the bitemporal recession and anterior hairline [1].