Objectives: Finasteride reduced the risk of prostate cancer by 24.8% in the Prostate Cancer Prevention Trial (PCPT). Whether this represents treatment or prevention and who is most likely to benefit are unknown. We sought to clarify these issues by this investigation. Methods: We fit a logistic regression model to men in the placebo group of the PCPT using risk factors for prostate cancer at entry to predict prostate cancer during the subsequent 7 years of study. Men in the two treatment groups were categorized into quintiles of risk of prostate cancer based on the predictive logistic model. A second model was fit evaluating finasteride's effect on prostate cancer for each subgroup defined by quartiles of baseline prostate-specific antigen (PSA) . The magnitude of the prevention effect of finasteride on prostate cancer was then evaluated across risk and PSA strata. Results: Finasteride significantly reduced prostate cancer risk for all risk quintiles. For quintiles 1 through 5, odds ratios were 0.72, 0.52, 0.64, 0.66, and 0.71, respectively (all P ≤0.05). For quartiles of risk of entry PSA (less than 0.7 ng/mL, 0.7 to 1.1 ng/mL, 1.1 to 1.7 ng/mL, and 1.8 to 3.0 ng/mL), odds ratios increased (smaller treatment effect) as PSA increased: 0.60, 0.62, 0.66, and 0.69, respectively, but remained significant for all strata (each P <0.001). Conclusions: Finasteride significantly reduced prostate cancer risk regardless of the level of this risk, estimated either by multivariable risk or by PSA stratum; this suggests that finasteride exerts both treatment and preventive effects. All men undergoing PSA screening should be informed of the potential for finasteride to reduce their risk of prostate cancer.
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