(CBS News) When restaurant owner Luca Marcato started feeling tired, he asked his doctor to check his testosterone level.
"I did not have energy right in the morning. You know in my business, you have to go and go and go," Marcato said.
His levels were low and his doctor prescribed a gel containing testosterone.
"Now I feel the way I should be feeling," he said.
Low testosterone levels are linked to problems like fatigue, muscle and bone loss and decreased libido.
While testosterone replacement therapy is approved by the Food and Drug Administration for some conditions, it's widely marketed as a way to improve health and stay young. But there's little data on its long-term effects.
A major study published Tuesday in the Journal of the American Medical Association looked at older men, most with heart disease, in the Veterans Affairs system who had low testosterone levels. There was a 29 percent increased risk of heart attack, stroke or death in those given testosterone.
Cardiologist Dr. Steve Nissen of the Cleveland Clinic says we need better studies to assess the benefits and risks of testosterone therapy.
"Just like women go through menopause, men go through man-opause," Nisen said. "That's probably a normal part of male aging, and treating it as a disease may get us in a lot of trouble.
Urologist Dr. Elizabeth Kavaler of Lenox Hill Hospital in New York City says the decision to prescribe testosterone is complicated.
"There's risks that are associated with having low T and there are risks associated with T replacement,," she said.
Nissen said, "Before you start treating millions of Americans with testosterone, you probably ought to think twice. "
There are several possible ways testosterone replacement might raise the risk of cardiovascular disease. Research suggests it mighty increase the likelihood of clotting within arteries. Further study is clearly needed.