Finasteride Missed-Dose Protocol: What to Do When You Skip a Pill

What the two labels say

The Propecia dosing section is short: one tablet (1 mg) once daily, with or without meals; daily use for three months or more is usually necessary before benefit is observed; and continued use is recommended to sustain benefit, with withdrawal of treatment leading to reversal of effect within 12 months [1]. The Proscar section is shorter still: one tablet (5 mg) taken once a day, with or without meals [2]. The Propecia patient information adds that if you forget a tablet, you should not take an extra tablet and should just take the next one as usual, and that taking it more than once a day will not work faster or better [1]. The dosing sections themselves give no separate missed-dose rule [1][2].

Why plasma half-life and DHT effect differ

Finasteride is a competitive, specific inhibitor of type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone; type II accounts for about two-thirds of circulating DHT, and finasteride is roughly 100-fold selective for it over type I [1]. A 1 mg tablet produces 65% suppression of serum DHT within 24 hours [1]. In a 24-week randomized trial of 399 men with benign prostatic hyperplasia (randomized across dutasteride doses, finasteride 5 mg and placebo), the finasteride 5 mg arm had a 70.8% mean decrease in serum DHT [4].

The drug clears faster than the enzyme effect does. Mean terminal half-life is about 5 to 6 hours in men aged 18 to 60 and about 8 hours in men over 70, with peak levels at 1 to 2 hours and bioavailability of about 65%, unaffected by food [1]. The label attributes the persistence of DHT suppression to slow turnover of the enzyme complex: a half-life of about 30 days for type II and 14 days for type I [1].

The hair-loss data come from daily dosing

Two 1-year placebo-controlled trials enrolled 1,553 men aged 18 to 41 with male pattern hair loss who took finasteride 1 mg daily or placebo, and 1,215 of them continued into blinded second-year extensions [3]. Hair count in a 1-inch (5.1 cm2) circle of vertex scalp was 107 hairs higher than placebo at 1 year and 138 higher at 2 years (P<0.001), while the placebo group kept losing hair [3].

Missing a tablet and stopping treatment

Propecia's label recommends continued use to sustain benefit and states that withdrawal of treatment leads to reversal of effect within 12 months [1]. That statement is about withdrawal of treatment, not about a single missed tablet.

PSA results and consistency of dosing

The labels tie one laboratory value to consistent dosing. The Propecia label reports mean serum PSA falling from 0.7 ng/mL to 0.5 ng/mL at month 12 in men aged 18 to 41 [1]. Proscar lowers PSA by approximately 50% within six months; its label says a new PSA baseline should be established at least six months after starting, that any confirmed increase from the lowest PSA value on treatment should be evaluated for prostate cancer, and that an isolated PSA value in a man treated for six months or more should be doubled for comparison with normal ranges in untreated men [2]. Percent free PSA is unchanged, so no adjustment to it appears necessary [2]. Both labels note that non-compliance with therapy may affect PSA test results [1][2].