But that doesn't mean testosterone is of no use to any man with erectile dysfunction, said Matsumoto, an endocrinologist who studies the effects of testosterone throughout the body.
There are men who are diagnosed with hypogonadism, a deficiency in testosterone. That's based on more than the level of testosterone in a man's blood, which, Matsumoto said, should be measured more than once, since a "low" will often turn out to be normal on a second test. The diagnosis is also based on possible signs and symptoms -- not just erectile dysfunction, but also problems like low bone mass, waning muscle, fatigue and depression.
The men in this study, whose ages ranged from 40 to 70, were assessed only for erectile dysfunction and not other symptoms, Spitzer said. So it's not clear whether the findings would apply to men with hypogonadism.
"If you have low testosterone, but [erectile dysfunction] is the only thing that's going on, it's reasonable to try sildenafil alone," Matsumoto said.
Another question, Matsumoto said, is whether adding testosterone therapy can help some men with low testosterone who fail to get a good result from Viagra or other erectile dysfunction drugs.
All of the men in this study first went through a few weeks on Viagra alone. If they failed to respond to the drug, the researchers upped their dose, to a maximum of 100 milligrams a day. Spitzer's team then randomly assigned 70 men to use the testosterone gel every day for 14 weeks; the other half of the group used the placebo gel.
In the end, men in both groups essentially maintained the improvement they'd had when they were on Viagra only.
Matsumoto said that speaks to the benefit of "optimizing" a man's Viagra dose, and giving the drug some time to work.
But, he added,
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