More than half the Viagra prescribed to men is not working, says an expert in sexual medicine.
Dr Geoffrey Hackett, a consultant urologist, says 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.
The guidelines are published in the journals Maturitas and Human Fertility.
Dr Hackett, sexual health specialist at Good Hope Hospital in Birmingham and former chairman of the British Society for Sexual Medicine, says the most common sexual problem men see their GP about is erectile dysfunction.
It affects 40% of men over 40 years old, and more than one in five men with erectile dysfunction have a testosterone deficiency.
Testosterone levels in men peak in their mid-20s then slowly decline throughout life, but a fall can also be a sign of underlying health problems.
"Low testosterone is linked to increased risk of mortality from diabetes and cardiovascular events so diagnosing it is very important in preventing those diseases," Dr Hackett said.
"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."
When testosterone replacement therapy is prescribed, "it can change the lives of patients", he says.
Having worked in this area for more than 20 years, Dr Hackett describes how the partners of the patients he has helped often thank him "for giving me back the man that I married", 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.
Professor 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 says.
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.
Sexual problems are thought to affect 44% of women, but long-term problems are less common than in men.
'Good to talk'
Women who have gone through a particularly early menopause or who have had their ovaries removed are more likely to experience sexual problems.
Common concerns are lack of desire, lack of arousal, problems reaching orgasm or pain during intercourse.
Broaching the subject of sexual disorders is not easy but sex is part of our daily lives, says Prof Wylie, and "clinicians should give patients permission to talk about it".
Sexual problems can be very distressing.
"Women complain it's like a light going out," said Professor Margaret Rees, consultant in medical gynaecology at the University of Oxford, when describing how women sufferers feel when they come to see her.
She emphasises that there are unlikely to be any quick fixes, because most women just need to talk about their problems.
Depression and relationship breakdown, for example, could be the result of sexual problems.
"It's vital to work out if the problem is medical, to do with the relationship they're in or something else entirely," Prof Rees said.
Working out the root causes of health problems could save the NHS money in the long term and mean that money may not have to be spent in other areas, she said.