The report's lead researcher said testosterone is typically prescribed to men who have both low testosterone levels and symptoms such as little interest in sex or low bone and muscle mass.

But, "there's a tremendous amount of clinical judgment" that goes into that, said Dr. Matthew Spitzer, from the Boston University School of Medicine. "People are certainly being prescribed and using these medications at increasing amounts."

According to Spitzer, studies have suggested that about one-quarter to one-third of men with erectile dysfunction, or ED, also have low testosterone. There's a range in part because doctors and researchers don't all agree on where the cutoff should be for low levels of the male sex hormone.

Spitzer and his colleagues found that a starting course of sildenafil citrate, marketed as Viagra, helped improve sexual functioning for men with both conditions. But adding testosterone on top of that didn't provide any added sexual benefits.

The study included 140 men, aged 40 to 70. All were prescribed Viagra at 50 or 100 milligrams, which they took as needed before sex. After three to seven weeks, half of the men were randomly assigned to also use a daily testosterone gel, Testim, and the other half used a drug-free placebo gel.

During the Viagra-only portion of the study, men's erectile function scores improved. On the sexual functioning scale, a score of 11-16 is considered "moderate" erectile dysfunction and 17-21 is "mild to moderate" dysfunction. The highest possible score, signaling no erectile problems, is a 30. On average, men's scores increased from 12.1 to 19.8 with Viagra.

The men's testosterone levels also rose on Viagra, according to the findings published Monday in the Annals of Internal Medicine.

For men who were then given the testosterone gel, testosterone levels increased significantly again. But neither those men nor the ones who used the placebo gel had any further change in their erectile function over the next three months.

There was also no difference between the two groups on measures of sexual desire, orgasm and frequency of intercourse.

The testosterone gel used in the study was provided by Auxilium Pharmaceuticals and the Viagra by Pfizer.

Spitzer told Reuters Health his team didn't look at the effects of testosterone without Viagra, and it's possible the gel would boost sexual functioning compared to no treatment.

In addition, testosterone may have other beneficial health effects, such as on strength and body composition, he pointed out.

"It doesn't mean that if the individual has either symptoms of androgen deficiency or hypogonadism (low hormone production from the testes), that those wouldn't get better with testosterone," said Dr. Alvin Matsumoto, a geriatrician from the University of Washington School of Medicine and the Seattle VA Puget Sound Health Care System who wasn't involved in the new study.

In addition, he told Reuters Health, "What you don't know is if you don't respond significantly to sildenafil and you have low testosterone, whether testosterone wouldn't help in addition."

Not all men see improvement with Viagra-type drugs, he said.

Viagra costs $5 to $10 per pill. Testim runs about $350 for a month's supply, depending on the daily dose. Side effects of testosterone gel include headache, skin irritation and high blood pressure.

"Androgen deficiencies are complex situations. Individuals always need to talk with their doctor about their own medical problems and figure out what is the best therapy for their set of individual problems," Spitzer said.

The new study, he said, "doesn't inform us about how to treat one patient."

SOURCE: bit.ly/MnBiCA Annals of Internal Medicine, online November 19, 2012.

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