John Bancroft, a psychiatrist with the Medical Research Council reproductive biology unit in Edinburgh, said drugs research at his unit was showing that male sexuality was extremely complex.
The growing popularity of 'hormone replacement' therapy for men, which involves doses of the male hormone testosterone, had lead to increasing interest in other treatments.
Dr Bancroft said that too little was known about giving testosterone to normal men and its links with prostate disease including cancer.
He was cautious about the use of testosterone as a prop to flagging male sexuality, although Italian research has shown that male hormone replacement in young men abnormally deficient in it can be effective.
'First of all, ageing in men is different to ageing in women in an important way. There is no equivalent in the male to the female menopause, although in older men you do see some decline in the production of testosterone,' he said. 'But this is very variable and tends to happen much later in life. However testosterone therapy tends to be taken by men in their forties and fifties when you are not seeing any decline in their testosterone levels. They are too young to be replacing testosterone.'
Dr Bancroft said it was normal for older men to experience changes in their sexual responses, including a less rapid erection, a less full one and difficulty in regaining an erection if it was lost. The number of erections during sleep also declined. But he said these symptoms were 'much more likely to be related to stress'.
He said that more investigation into the effects of testosterone was needed. 'Before you start to use a hormone like this you must be able to show from carefully controlled trials that it is superior to a placebo. We do not really have this information yet.'Reuse content