A new study has reignited a debate about whether men should use a baldness drug to prevent prostate cancer.

The hormone-blocking drug finasteride is a generic drug widely used by men to shrink enlarged prostates, and it has been used to treat male-pattern baldness. In 2003, a study of 18,000 men showed that the drug also lowered a man’s risk for prostate cancer by 30 percent.

But the Food and Drug Administration (FDA) never approved finasteride to prevent prostate cancer because of a troubling finding. Although the drug clearly reduced the overall risk for prostate cancer, slightly more men who used it developed fast-growing tumors compared with men who took a placebo.

On Wednesday, a follow-up study in The New England Journal of Medicine reopened the debate about the potential benefits of the drug. It showed that 14 to 17 years after the men first enrolled in the study, survival was the same among men who used finasteride as those who took a placebo. That finding, based on a review of Social Security death records, suggests that the drug was not causing the aggressive tumors. Instead, it is more likely that by reducing the size of a man’s prostate, the drug makes it easier to find aggressive tumors.

The results should reassure men that the drug can safely be used to treat an enlarged prostate. However, it also raises questions about whether men should consider using the drug to prevent prostate cancer. The study’s authors estimate widespread use could save 70,000 men a year from the emotional and physical trauma of a prostate-cancer diagnosis and treatment. Using the drug will not prolong their lives or protect them from aggressive cancer, but it will reduce the chances that doctors will find and treat a cancer that was unlikely to ever cause harm.

Dr. Ian Thompson, director of the cancer therapy and research center at the University of Texas Health Science Center in San Antonio and the study’s lead author, said that because large numbers of men with low-grade tumors are treated unnecessarily, often with treatments that render them impotent or incontinent, there is a benefit to preventing low-grade, nondeadly cancers.

“It doesn’t reduce the risk of cancers that take men’s lives, but low-grade cancers lead to huge amounts of follow-up testing and treatment,’’ Thompson said. “With the drug, you just don’t find as many cancers, and that’s a good thing.’’

Dr. Peter Scardino, head of the prostate-cancer program at Memorial Sloan-Kettering Cancer Center in New York, said he doesn’t expect the finding to prompt doctors to prescribe the drug to prevent cancer. However, it may prompt some men at very high risk of prostate cancer, such as those with a family history who undergo regular screening, to take the drug. It also could pave the way for more research into prostate-cancer prevention and treatments for men with early-stage prostate cancer.

The drug is sold as Proscar by Merck. It’s also sold in a lower dose as Propecia to treat hair loss.

Another new prostate-cancer study suggests that a tumor’s aggressiveness is fixed at the time of its appearance, although diet, lifestyle and environmental factors may trigger progression of the disease in low-level cases.

The findings, published Wednesday in the journal Cancer Research, add to mounting evidence that many small, slow-growing prostate tumors can be left in the body and carefully monitored instead of being treated with surgery, radiation, hormone therapy or drugs.

Material from The Associated Press and the Los Angeles Times is included in this report.