A 62-year-old man is evaluated during a routine follow-up visit. He expresses concern about developing prostate cancer because his father was diagnosed with the disease at age 55 years. He has read that the 5α-reductase inhibitor finasteride may prevent prostate cancer and asks whether he would be an appropriate candidate for treatment with this drug.
The physician reviews a study to determine whether finasteride prevents prostate cancer. In the study, investigators randomized 2000 patients equally to treatment with finasteride or placebo. Seven years later, all patients underwent a prostate biopsy. Results showed that prostate cancer occurred in 200 of 1000 (20%) patients taking finasteride compared with 300 of 1000 (30%) patients taking placebo.
Based on this study, how many patients need to be treated (number needed to treat) with finasteride for 7 years to prevent one case of prostate cancer?