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The agency reviewed testosterone replacement therapy in 2010 after a clinical trial of the hormone was halted when patients taking it started having heart attacks, and in 2015, the F.D.A. required the makers of testosterone to conduct a clinical trial to evaluate the risk.

Testosterone replacement therapy is safe for men with “low T” who have heart disease or are at high risk for it, a new study suggests.

It can also take months to see the effects of testosterone replacement therapy—two weeks is still early.

Mixed martial arts once frequently showcased fighters semi-openly using steroids and testosterone replacement therapy, among other performance enhancements.

Some men are buying sperm counters to use at home, and some are turning to testosterone replacement therapy, convinced that modernity has feminized them.