Finasteride Bone Health and Density Impact

What the finasteride labels say about bone

Both labels address bone mineral density in their pharmacology sections. The Propecia label states that finasteride did not alter plasma lipids or bone mineral density, and the Proscar label states that no clinically meaningful effect was observed on the plasma lipid profile or bone mineral density [1][2].

The adverse reactions the labels tabulate are not bone effects. The Propecia label's clinical adverse reactions come from three 12-month controlled trials in 945 men on finasteride and 934 on placebo, and they are sexual: decreased libido 1.8% vs 1.3%, erectile dysfunction 1.3% vs 0.7%, and ejaculation disorder 1.2% vs 0.7% [1]. The postmarketing list adds sexual dysfunction that continued after stopping, depression and suicidal ideation, hypersensitivity reactions, testicular pain, hematospermia, breast tenderness and enlargement, and male breast cancer [1][7]. The Proscar label's four-year safety table follows the same pattern: impotence, decreased libido, decreased ejaculate volume, ejaculation disorder, breast enlargement, breast tenderness and rash [2]. Neither product carries a boxed warning [1][2].

What the bone density studies found

A substudy of the 4-year PLESS trial in men with BPH took lumbar spine DXA scans in 157 men aged 46 to 76 randomized to finasteride 5 mg or placebo. Among 117 men with a baseline and at least one later measurement, mean bone mineral density at baseline was 1.12 g/cm2 on finasteride and 1.10 on placebo; after 4 years it was 1.14 and 1.13, not different between groups [8].

A randomized, double-blind trial in 99 healthy men aged 18 to 55 gave dutasteride 0.5 mg (33), finasteride 5 mg (34) or placebo (32) daily for 1 year and measured bone mineral density at baseline, at 1 year and 6 months after stopping. Marked suppression of circulating DHT did not significantly affect bone mineral density or bone metabolism markers [9]. Both studies used 5 mg; the label statements cover both strengths [1][2].

How finasteride changes androgen levels

Finasteride is a competitive, specific inhibitor of Type II 5-alpha-reductase, the enzyme that converts testosterone to dihydrotestosterone [1]. Type II is the isozyme found mainly in prostate, seminal vesicles, epididymides and hair follicles, and it produces about two-thirds of circulating DHT [1]. A 1 mg tablet suppresses serum DHT by 65% within 24 hours [1].

Mean circulating testosterone and estradiol rise by about 15% but stay within the normal range, and finasteride has no affinity for the androgen receptor [1]. The enzyme complex turns over slowly, roughly 30 days for Type II, so the DHT effect outlasts the drug's plasma half-life of about 5 to 6 hours in men aged 18 to 60 [1].

Monitoring in the labels

The monitoring the labels describe is PSA-based, and neither label calls for bone-density testing [1][2]. Proscar lowers PSA by about 50% within six months, so the label says an isolated value after six months of treatment is doubled for comparison with normal ranges [2]. Propecia lowered mean PSA from 0.7 to 0.5 ng/mL at month 12 in men aged 18 to 41 [1].