Next to skin cancer , prostate cancer is the second most common cancer among men in the U.S., and is the second leading cause of cancer deaths in men (after lung cancer ). The number of new cases of prostate cancer—estimated to be more than 220,900 per year—is expected to rise to more than 380,000 by 2025 because of the aging male population.

Although treatments for prostate cancer are available, they frequently do more harm than good. Surgical removal of the prostate gland ( radical prostatectomy ), for example, produces impotence and urinary incontinence—a high price to pay when you consider that most men with prostate cancer will not die from it. While one man in six will get prostate cancer during his lifetime, only one in 32 will die of the disease.

Methods to lower prostate cancer risk, therefore, are urgently needed. Since male sex hormones (androgens like testosterone) normally stimulate prostate cells to grow, researchers would like to know whether reducing the prostate’s exposure to androgens would lower the risk of prostate cancer.

Results from the Prostate Cancer Prevention Trial (PCPT)—the first major trial of chemoprevention for prostate cancer—published in the July 17, 2003 issue of The New England Journal of Medicine , indicate that the drug finasteride , which is used to treat benign prostatic hyperplasia (BPH, or prostate enlargement) and fight baldness (in a much lower dose), may lower the risk of prostate cancer. But it also appears to produce sexual side effects and increase the risk of an aggressive form of prostate cancer.

About the Study

Between January 1994 and May 1997, researchers randomly assigned 18,882 men 55 years of age or older with a low risk of prostate cancer to 5 milligrams per day (mg/d) of finasteride (which inhibits the conversion of testosterone to dihydrotestosterone, the primary androgen in the prostate) or placebo for seven years. Men were eligible if they had no clinical indication of prostate cancer.

Participants were carefully followed for signs of prostate cancer over the study period. They underwent annual digital rectal exams (DREs) and measurements of prostate-specific antigen (PSA). Twice a year they were seen for reissuing of medication, counts of pills, and recording of clinically significant medical conditions and side effects. Every three months, the men were contacted by telephone to ascertain whether they had experienced any interim medical events.

Those subjects who were still alive and had not received a diagnosis of prostate cancer at the end of the study were advised to have a biopsy of the prostate to see if any hidden cancer had developed.

The Findings

Prostate cancer was detected in 18.4% of the men in the finasteride group and 24.4% of the men in the placebo group, amounting to a significant cancer risk reduction of 24.8% for men treated with the drug. However, subjects who had cancer in the finasteride group were slightly more likely to have a more aggressive form (according to the Gleason scoring system , the accepted standard for grading prostate cancer) than those with cancer in the placebo group.

The researchers noted that 36.8% of men in the finasteride group and 28.9% in the placebo group temporarily discontinued treatment at some time during the study for reasons other than death or diagnosis of prostate cancer. Side effects were reported in both groups: erectile dysfunction , loss of libido, and gynecomastia (temporary enlargement of breasts) were common in the finasteride group, while urinary urgency, prostatitis (prostate inflammation), and urinary tract infection (UTI) were common among men in the placebo group.

Because the results were so striking and the study’s objectives were met (and the conclusions were unlikely to change), researchers ended the trial 15 months early.

How Does This Affect You?

The findings from this study suggest that finasteride may be able to prevent the growth of microscopic prostate cancer in its early stages by preventing the gland from responding to stimulation from testosterone. While this is encouraging, there are still many unanswered questions.

For one, the cancer detection rate was high in this study, suggesting that insignificant cancers (those that would never have caused harm) were more likely to be diagnosed in study subjects than in the general population. A more meaningful finding, thus, would be if finasteride not only reduced the risk of developing prostate cancer but also reduced the risk of dying from it, something this study did not address.

A second surprising finding was the increase in the rate of aggressive cancers in the finasteride group. One explanation is that the drug affects the appearance of prostate cancer cells, leading pathologists to conclude incorrectly that the tumor is a more aggressive type. Another possibility is that finasteride actually causes more aggressive tumors to develop—either by preventing only low-grade tumors, or by making the prostate gland more favorable to aggressive tumors.

Finally, there is the matter of side effects. The incidence of adverse effects on sexual function was higher with finasteride, but the group also had a lower incidence of urinary symptoms and complications compared to the placebo group.

As with most prevention strategies, there are trade-offs. On balance, however, finasteride does not appear to be an attractive option. Men will have to weigh their individual cancer risk against the potential for the acceleration of high-grade cancers and troublesome side effects. At this point, however, there is no compelling reason for men currently taking finasteride for BPH to discontinue it. Talk to your doctor, though, if you have any concerns about your risk of prostate cancer.