WASHINGTON (AP) -- The Food and Drug Administration is warning doctors against the overuse of testosterone-boosting drugs for men, saying the popular treatments have never been established as safe or effective for treating common signs of aging like low libido and fatigue.

The agency says drugmakers must clarify that their drugs, currently taken by millions of U.S. men, are only approved to treat low testosterone levels caused by disease or injury, not general aging. Additionally, the FDA warned that the drugs may increase the risk of heart attack. Drugmakers must add information about that potential risk to their warning labels and conduct a long-term study to address the issue, the FDA said.

(Sorapong Chaipanya, ThinkStock)

The federal rebuke follows years of industry marketing for new pills, patches, gels and injections that promise relief from low testosterone or "Low-T." Promotions from AbbVie and Eli Lilly link the condition to a variety of ailments common in aging men, including sexual dysfunction and low mood.

The advertising blitz for drugs like AbbVie's Androgel and Lilly's Axiron has pushed sales of testosterone drugs to over $2 billion. Roughly 2.3 million U.S. patients received a prescription for testosterone in 2013, up 77 percent from 2010, according to FDA figures.

Testosterone injections were first approved in the 1950s for men who had been diagnosed with hypogonadism, a form of abnormally low testosterone caused by disorders of the testicles, brain or other organs.

But current labeling on the drugs is vague enough that companies have been able to promote their drugs to millions of otherwise healthy men who simply have lower-than-normal levels of testosterone.

"The benefits and safety of this use have not been established," the FDA said in a statement Tuesday.

Companies sell prescription testosterone in a variety of forms. The market leader, Androgel, is a gel applied to the shoulders and arms. Endo Pharmaceuticals sells a long-acting injectable testosterone called Aveed.

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