If you believe the ads from drugmakers such as Solvay Pharmaceuticals, you might well think that getting a testosterone prescription is the key to save a faltering sex life in middle age.

But a new study adds to evidence that men’s levels of the hormone can vary quite a bit without causing sexual problems like impotence and decreased libido.

“Testosterone replacement therapy has become a very common thing,” said Dr. Michael Marberger, who heads the urology department at the University of Vienna Medical School in Austria, and led the new study.

“It is like putting more gas in the car to make it go faster,” he told Reuters Health.

There is no doubt that very low testosterone levels affect the body and dampen sex drive. But where to set the threshold is still an open question, whose resolution is not made easier by natural variation in the male sex hormone.

Its level dwindles with age, for instance, and fluctuates between countries, races and individuals - even from one hour to the next.

In the new study, funded by GlaxoSmithKline, researchers used data from an earlier study of more than 8,000 middle-aged and older men at risk for developing prostate cancer.

The men filled out questionnaires about their sexual function - whether they could keep an erection, whether they lacked sex drive - and had blood drawn. Various other measures were taken, too.

One in five had testosterone levels below 300 nanograms per deciliter of blood, the threshold for what Solvay and others call “Low T.”

And about two in five men had at least “a small problem” in one or more areas related to their sex life.

Of the measures taken - age, urinary symptoms, body mass index (BMI) and diabetes - testosterone showed the weakest link to the men’s sexual function.

“We need a lot less testosterone for sexual function than people used to think,” said Marberger, whose findings appear in the British Journal of Urology International. “Obesity impacts sexual function much more.”

For instance, earlier research has shown that erectile dysfunction can disappear by itself over time, especially in men who aren’t obese.

Marberger, a paid consultant for GlaxoSmithKline, which makes diabetes and weight loss drugs, added that doctors are often too quick to give patients testosterone, when high blood pressure or obesity might be the real culprit.

Every year, millions of testosterone prescriptions are written to U.S. men, and some products aren’t cheap. Solvay’s AndroGel, for example, costs about $280 per month. It can cause swelling around the joints and in the breasts, but Marberger said most side effects aren’t serious.

Dr. Frederick Wu, a hormone expert at the University of Manchester, was critical of the some of the study’s methods, but agreed with the overall message.

“Most people with erectile problems do not have hormone problems,” he told Reuters Health, but often have blockages in the arteries in their penis.

He stressed, however, that a lot of research has found that once testosterone dips below a certain threshold, it does impact sexual function.

But the exact level is controversial, and the picture gets extra murky because many of the symptoms that accompany very low testosterone - fatigue, depression, loss of muscle strength and bone density - are commonly found in seniors.

“Low testosterone and sexual problems quite often coexist,” said Wu, “but the causal relationship isn’t clear.”

SOURCE:  British Journal of Urology International, online October 18, 2010.