Viagra may not work for men with low testosterone levels, says an expert.
Geoffrey Hackett, a consultant urologist, said men with erectile dysfunction could be "wasting hundreds of pounds on tablets" when their real issue is low testosterone.
He was speaking at the launch of new guidelines on the diagnosis and treatment of sexual disorders.
Hackett, of the Good Hope Hospital said the most common sexual problem men see their GP about is erectile dysfunction.
It affects 40 percent of men over 40 years old, and more than one in five men with erectile dysfunction have a testosterone deficiency.
"Low testosterone is linked to increased risk of mortality from diabetes and cardiovascular events so diagnosing it is very important in preventing those diseases," the BBC quoted Hackett as saying.
"Men for whom Viagra isn't working adequately need to have their cases reviewed. If low testosterone is the problem then Viagra won't be the answer on its own," he said.
In the new guidelines, the British Society for Sexual Medicine stresses the importance of doctors asking patients about their sex lives and any concerns they might have about sexual function.
Kevan Wylie, lead author of the guidelines and consultant in sexual medicine at the University of Sheffield, said: "The importance of sex life and sexual function to general health and well being is not often discussed or acknowledged in our society.
"During medical consultations, both patients and doctors shy away from discussing sexual symptoms."
But patients should be routinely asked by their GP if they have any sexual concerns, it said.
This is especially true of men at high risk, such as men with diabetes, osteoporosis, cardiovascular disease, erectile dysfunction or depression.
Women should also be asked about any sexual problems at routine GP appointments and at cervical screening, postnatal and menopausal assessments, the guidelines advise.
The findings were published in the journals Maturitas and Human Fertility.