Objectives. To examine the effect of finasteride on serum testosterone in men with benign prostatic hyperplasia (BPH). Methods. The Proscar Long-Term Efficacy and Safety Study (PLESS) was a 4-year trial comparing the safety and efficacy of finasteride 5 mg with placebo in 3040 men with moderate to severe symptomatic BPH and enlarged prostates. PLESS included the prospective measurement of annual serum testosterone in a randomly selected subset of patients comprising approximately 10% of the randomized population (n = 301). Results. Finasteride treatment led to a modest, but significant (P <0.001), increase relative to placebo in serum testosterone, with this increase greatest in patients who had low baseline testosterone levels. The larger testosterone increases seen in finasteride-treated patients in the lower baseline testosterone tertiles were associated with significant mean reductions relative to placebo at year 4 in body mass index (BMI), ranging from 0.6 to 0.8 kg/m 2. No statistically significant between-group difference was found in BMI in the upper testosterone tertile. The sexual adverse experience profiles for finasteride and placebo were similar across the baseline testosterone cohorts examined. Conclusions. Finasteride treatment led to a generally modest increase relative to placebo in serum testosterone, with the greatest increases occurring in men with low baseline testosterone levels. The physiologic significance of these changes in men with low baseline testosterone levels is unclear, but the associated reduction in BMI is intriguing and may be related, because BMI is known to be negatively correlated with serum testosterone levels in men.
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