Researchers say a cheap, generic pill called finasteride prevents almost 40 percent of low-grade prostate cancers without increasing the risk of dying from more aggressive tumors.

New evidence points to the drug as a potentially safer way to deal with prostate cancers that now get more intense treatment. Many prostate cancers that aren't destined to cause men serious health problems are often treated with surgery or radiation.

The safety of finasteride sold as a prostate cancer prevention pill has been a bone of contention for the past decade, after the $73 million federally sponsored Prostate Cancer Prevention Trial showed that the drug substantially lowered the overall risk of prostate cancer. But there was a big hitch.

"It was immediately apparent that the main reduction was in the lower-grade cancers and there was a slight increase of the higher-grade cancers," says Dr. Dr. Ian Thompson, of the University of Texas Health Sciences Center in San Antonio, who led the project.

The more serious, or high-grade, prostate cancers have a so-called Gleason score of 7 and up. They grow faster and are more lethal.

Doctors have debated for years whether finasteride caused high-grade cancers or somehow just made them more likely to be detected. (Finasteride was sold by Merck under the brand-name Proscar for treatment of enlarged prostates and Propecia for baldness.)

Whatever the reason, the 2003 finding prevented finasteride and its cousin dutasteride from being marketed as prostate cancer prevention pills. Both are approved to treat the urinary symptoms of benign prostate enlargement, although the drugs can cause sexual side effects. They work by blocking the conversion of testosterone into a form that is active in the prostate gland.

In 2011, the Food and Drug Administration warned men taking these drugs about the increased risk of aggressive tumors.

The latest study, published in this week's New England Journal of Medicine, updates the fates of the nearly 19,000 men who enrolled in the Prostate Cancer Prevention Trial. Researchers drew on an additional seven years of experience since the study stopped in 2004.

Study authors say finasteride, which costs about 40 cents a pill, has turned out to be even better at preventing prostate cancer than they originally thought.

Initially the men who took finasteride had a 25 percent lower risk of a prostate cancer diagnosis. The new analysis shows a 30 percent overall reduced risk –-- and an impressive 38 percent lower risk of getting a low-grade prostate cancer.

"A 30 percent reduction is 50,000 to 80,000 fewer cancers per year, if everyone took it," Thompson says.

Like the earlier study, the new one also found men who took finasteride had a somewhat higher risk of high-grade cancers — 17 percent higher. But importantly, the update finds no evidence that men who took finasteride were more likely to die of prostate cancer.

In fact, 18 years after men began enrolling in the Prostate Cancer Prevention Trial, those who took finasteride and those who got a placebo had exactly the same survival rate.

"Even if there is a higher risk of high-grade cancer, it doesn't appear to have an impact on how long a man lives, and that's reassuring," Thompson says.

It's not clear why finasteride-users who got higher-grade cancers didn't die at higher rates. Study authors note that men whose higher-grade cancers were discovered through PSA screening tests generally have a good prognosis, because their malignancies tend to be found at an earlier stage.

But even if finasteride doesn't make men live longer, preventing them from getting diagnosed with low-grade, nonlethal cancers is still a big deal.

That's because the vast majority of men who get a diagnosis of prostate cancer ofany kind get treated for it — even if it wasn't destined to kill them.

"Many of the men have surgery," Thompson says. "Many of the men have radiation. And those treatments have urinary and sexual side-effects that are actually far more common than people think. So eliminating all those cases would be a profound public health benefit."

Thompson says the results should revive the idea of finasteride as a prostate cancer prevention pill.

Dipen Parekh, chief of urology at the University of Miami, agrees. Parekh says that many doctors abandoned the notion that finasteride and dutasteride could prevent prostate cancer after the FDA warning in 2011.

"That was a major, major deterrent to use these medications for prostate cancer prevention," Parekh tells Shots. "Now, with the most recent results, I think this should reopen the doors for a lot of us who believe we can use preventive agents for prostate cancer."

He thinks the findings could also rejuvenate enthusiasm for the use of PSA to screen men for prostate cancer. The big rap on PSA screening is that it reveals many low-grade cancers that really don't need to be treated. But finasteride prevents many of those.

And because it shrinks the prostate, the drug increases doctors' ability to find cancers worth worrying about, either through digital rectal exams or biopsies.

"That has the potential to revolutionize the way we think about prostate cancer," Parekh says.

But not all are so enthusiastic. Dr. Michael LeFevre of the University of Missouri, one of the leaders of the U.S. Preventive Services Task Force, acknowledges in editorial accompanying the study that men "may make a rational decision to take [finasteride] to reduce the harm of [PSA] screening. Of course, another way to reduce the harm of screening is to choose not to be screened."

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