By Toni Clarke

WASHINGTON, Jan 31 (Reuters) - The U.S. Food and Drug Administration said on Friday it is investigating the risk of stroke, heart attack and death in men taking prescription testosterone drugs.

The move is based on the recent publication of two studies that suggest an increased risk of heart attacks in men who take testosterone, a hormone produced in the testicles that is responsible for maintaining muscle bulk, bone growth and sexual function, the FDA said.

Symptoms of low testosterone, or "Low T" as it is sometimes called in advertisements, can include loss of libido, depression, decreased muscle mass and fatigue.

The FDA said it has not concluded that testosterone increases the risk of cardiac events but said it is issuing an alert "while we continue to evaluate the information from these studies and other available data."

The FDA's action comes two days after the publication of a new study in the journal PLOS ONE, showing that men over the age of 65 had a two-fold increase in the risk of heart attack within 90 days of filling an initial prescription for a testosterone drug.

Among younger men under the age of 65 with a history of heart disease, there was a two- to three-fold increased risk of heart attack, though there was no increased risk in younger men without a history of heart disease. In older men the increased risk was seen whether or not they had a previous heart disease history.

The study, which analyzed 55,593 patients in a large health-care database, echoed the results of a much smaller study published in November in the Journal of the American Medical Association which found that older men, many with underlying heart disease, had a 30 percent increased chance of death, heart attack and stroke after taking testosterone therapy.

The market for testosterone treatments currently include skin patches, short-acting injections and topical gels. AbbVie Inc's AndroGel, a treatment for hypogonadism and the market leader, generated sales in 2012 of $1.2 billion. The company did not immediately respond to a request for comment.

Testosterone therapy is only approved by the FDA for men who lack or have low testosterone in conjunction with an associated medical condition, such as a genetic failure of the testicles to produce testosterone, or as a result of chemotherapy.

"Health care professionals should consider whether the benefits of FDA-approved testosterone treatment is likely to exceed the potential risks of treatment," the agency said in its announcement.

The FDA's move comes as it prepares to decide whether to approve a long-acting testosterone product made by Endo Health Solutions Inc. An advisory panel to the FDA split 9-9 at a meeting last year on whether the drug could safely be prescribed for men with low testosterone.

Some panel members said the drug, Aveed, offers a valuable alternative to existing products, especially since it is already sold under the brand name Nebido outside the United States. Others were unconvinced of the drug's safety.

Aveed has twice before been rejected by the FDA based on concerns about injection-site reactions and the risk that it could cause blockages in blood vessels in the lungs.

Dr. Sidney Wolfe, senior adviser to the consumer watchdog Public Citizen, said in an interview that he thinks the FDA should delay its decision on Aveed until the cardiac risks have been assessed. He also said testosterone drugs should carry cardiac risk warnings.