The Food and Drug Administration said it is investigating the possible increased risk of stroke, heart attack and death in men who are taking testosterone-boosting products for the condition widely called "low-T."

The drugs are approved by the agency for men with low levels of the male hormone for medical reasons such as genetic problems or chemotherapy. But they are widely prescribed for "off-label" use (or uses not specifically approved by the agency), mostly for older men seeking to boost their flagging sex drives and low energy levels.

Among the products are AndroGel from AbbVie Inc. and Axiron from Eli Lilly & Co. Axiron advertising uses the term low-T, a term that has caught on widely in the general public.

Lilly said Axiron is a prescription drug and that it "does not condone the use of our medicine for off-label purposes." It noted that the FDA hasn't concluded there is an increased risk caused by the drugs.

AbbVie couldn't immediately be reached for comment on the FDA statement, which was issued late Friday.

The FDA said it is monitoring the risk from the testosterone drugs and that it has decided to "reassess this safety issue based on the recent publication of two separate studies that each suggested an increased risk" among men taking the products.

The agency stressed that it hasn't concluded that, in fact, the cardiovascular risk is increased with these products, and advised men not to stop taking them without consulting their doctors. The products take the form of gels, transdermal patches, material applied to the upper gum or inner cheek, and injection.

A November 2013 study published in the Journal of the American Medical Association found an approximately 30% increased risk of death, heart attack or ischemic stroke—those caused by artery blockage—in men treated with testosterone compared with those who didn't receive it. Dr. Michael Ho of the Department of Veterans Affairs in Colorado called the link "an association" that is "not causal," but said that it presented evidence of some risk.

The most recent study, published in the journal PLoS One and funded by the National Institutes of Health, followed about 56,000 older and middle-aged men. It found that those over 65 had double the rate of heart attacks as did those not taking the testosterone drugs, and that there was a similar increased risk among men under 65 with previous heart disease.

The most recent research compared the testosterone group to men taking the "erectile dysfunction" drugs Viagra and Cialis, who didn't experience increased cardiac adverse events.

Write to Thomas M. Burton at