A controversial drug that can prevent prostate cancer appears to be even more beneficial than previously thought. Two studies to be published in the Journal of the National Cancer Institute found the drug finasteride not only shrinks enlarged prostate glands and reduces the risk of prostate cancer but also improves detection of the most lethal tumors. Finasteride is one of only a handful of drugs that have actually been shown to prevent cancer. (Tamoxifen, which cuts breast cancer risk in half, is another.) But doctors have been reluctant to prescribe finasteride because it was believed to promote the growth of high-grade, or aggressive, tumors. A 2003 study published in the New England Journal of Medicine showed that m…