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Men looking for improved energy and memory won't find it in testosterone treatment, which should be used in older men only for sexual dysfunction, according to researchers and a new guideline published Monday.

The only scientifically proven use for the hormone in older men with age-related low testosterone levels is for sexual dysfunction, according to a new recommendation from the American College of Physicians published in the Annals of Internal Medicine.

The new recommendation was decided after researchers reviewed 38 randomized, controlled testosterone treatment trials, which ranged from 10 to 798 male participants from the U.S., Europe, Asia and Australia between 1997 and 2018. The studies tracked men from less than 12 months up to three years. Twenty four of the studies reported at least partial industry sponsorship.

In the studies, men were given testosterone via intramuscular shots or through the skin with a hormone patch or gel. That analysis was published Monday, along with the new guideline.

Millions of men seek a quick health fix with testosterone replacement, both from prescriptions and over-the-counter supplements. The new analysis showed that there's no evidence that long-term testosterone supplementation improves — or damages — health, said Dr. Robert McLean, president of the APA and a rheumatologist at Yale New Haven Health's Northeast Medical Group. The only benefit is a short-term treatment for men with sexual issues.

Other issues, such as "general vitality and cognition do not seem to be symptoms that improve with testosterone replacement," McLean said. There's nothing in the data, he added, "that shows that testosterone is a fountain of youth."

And that means, doctors shouldn't even bother to check testosterone levels unless their older male patient "says my libido is in the tank," McLean said.

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The new recommendation applies only to men in their 60s who have been diagnosed with a decline in testosterone.

In that case, testosterone levels can be tested and if they are low, "I can say, 'If I give you a little extra testosterone, it might help your libido and sexual functioning,'" McLean said.

Younger men, those in their 30s, 40s and 50s, don't normally have low testosterone unless they have a rare medical condition — or if they are have certain lifestyle issues, said Dr. Harry Fisch, a professor of urology and reproductive medicine at Weill Cornell Medical College.

According to Fisch, men can experience drops in testosterone levels if they:

Overexercise.

Don't get enough sleep. Sleep is when the body generates testosterone.

Are overweight. Fat cells can cause testosterone to turn into estrogen.

Use marijuana frequently. Marijuana affects men's estrogen levels, which is linked to sexual dysfunction and infertility.

The solution is to make lifestyle changes rather than to add testosterone, because supplementation can ruin fertility, Fisch said.

"In young men, a side effect of testosterone replacement is low sperm count," he added. "And the longer you're on testosterone, the more issues you might have getting the sperm back."

The guidelines also recommended that men getting testosterone supplements take it as an injection rather than as a skin patch or gel due to differences in cost. That difference, according to the guidelines was $2,135 for the patch versus $156.24 for self-administered injections per year.

How safe is testosterone treatment?

There is no long-term safety data on testosterone supplementation, the report found. Many of the studies used in the pooled analysis were short term.

There was enough data back in 2014, however, for the Food and Drug Administration to issue a warning that testosterone replacement for men with age-related declines could raise the risk of heart attack and stroke, said Fisch.

Doctors should look for possible underlying causes for declining testosterone levels and sexual dysfunction such as untreated diabetes, the beginnings of heart disease, a sedentary lifestyle and being overweight, Fisch said. Sexual dysfunction should be seen as a marker for other health issues.

"The penis is the dipstick of the body's health," Fisch said.

Fisch doesn't believe the new guideline will do much to stem the steady stream of customers heading into testosterone clinics hoping the hormone will improve their sense of vitality.

"They come in and say, 'I'm tired.' They forget about the fact that they are 50 pounds overweight," Fisch said. "Unfortunately, there's no money to be made telling people to sleep longer and get a better lifestyle."

Even though the pooled analysis found some benefit from testosterone replacement therapy in men with sexual dysfunction, "that was mild and everybody knows sexual function is complicated," said Dr. Adrian Dobs, a professor of medicine and oncology at the Johns Hopkins University School of Medicine.