The use of testosterone-replacement therapy has grown from 1.3 million patients in 2009 to 2.3 million in 2013, according to the U.S. Food and Drug Administration.

What's going on? Is everyone who undergoes this therapy really benefiting from it? The US National Institute on Aging decided to find out, and so funded what have come to be known as the Testosterone Trials. They involved 790 men aged 65 and older with low levels of testosterone caused by aging, in addition to symptoms that could be related to low testosterone such as sexual problems, fatigue, muscle weakness, or impaired memory and thinking. These studies have just wrapped up, and the results, as they say, are in.

Previously released data from the first three of the trials taught us that testosterone could improve our sexual desire and function, but really did not offer much to improve our overall physical function or vitality. The final four tests were focused upon testosterone therapy's effects – if any – on men's bones, cardiovascular system, red blood cell count, and mental function. From these, we learned:

• Both bone mineral density and estimated bone strength increase in men after a year of testosterone therapy. The spine, in particular, benefits.
• Arterial plaque increases meaningfully over the course of a year of T-therapy. That is, testosterone causes our arteries to become “clogged.”
• T-therapy causes clinically significant increases in the red blood cell count among men who had been anemic prior to the trial.
• Testosterone therapy has zero effect on our visual memory, verbal memory, or problem solving skills.

The scientists viewed the added benefit to bone health and anemia as “happy extras” of testosterone therapy, and not as worthwhile replacements for the specific therapies and medicine already available to address these areas of care.

But T-therapy's effect on cardiovascular health is a real cause for concern. 

"If you have a heart attack, it's hard to come back from that. If you have a stroke, it's hard to come back from that," said American Heart Association President Stephen Houser, who is also senior associate dean of research at Temple University in Philadelphia. "I want to be young again, too, but I don't think there's enough evidence out there that I would consider taking testosterone."

The FDA currently mandates that testosterone therapy carry a boxed warning regarding potential risks.

The results of the last four Testosterone Trials were published in the Journal of the American Medical Association.