An ageing woman's libido, on the other hand, is harder to define because it is influenced by not only physical issues brought on by menopause but emotional and psychological ones as well.

The New England Journal Of Medicine published a study of sexuality among older adults in the US last year.

Dividing the roughly 3,000 subjects into three age groups (57 to 64, 64 to 74 and 75 to 85), it found that women were significantly less likely than men at all ages to report sexual activity.

'At the risk of generalisation, men are more coitus- and penile-centric. Women look at other more encompassing aspects, including love and commitment,' says Dr Fones.

Indeed, evolutionary theory suggests that men are by nature more polygamous than women, and more accepting of casual sex. They lust after physicality and novelty, and are biologically driven to sow their seed.

Women, on the other hand, are more sexually sedate because they want a provider, a man for whom they can bear children, one who will provide for them and their brood.

That may explain the lengths men go to to explore ways to stay virile - from imbibing potions brewed from animal penises and so-called aphrodisiac herbs to using pumps to draw blood to their sexual organs.

Some ask doctors for injections to deliver instant erections or for surgically inserted inflatable implants which enable even paraplegics to engage in sex.

Little blue wonder

AND then came Viagra - the first oral treatment for impotence.

It was discovered accidentally by scientists who found that sildenafil nitrate - a drug being tested to treat angina and other cardiovascular problems - also gave the subjects erections.

In 1998, pharmaceutical giant Pfizer launched the little blue pill, putting an end to the woes caused by ED.

Since then, an estimated 1.8 billion Viagra pills have been dispensed to 35million men in more than 120 countries around the globe.

Viagra made its debut in Singapore a year later. More than 3.5 million pills have since been sold here.

Cialis and Levitra, two other oral drugs used to treat impotence, followed five years later.

Viagra revolutionised sex for men.

Dr Gan Tek Kah, 51, from Singapore Men's Health Clinic, says: 'In the past, treatments for ED included injections, pumps and urethra suppositories which were either invasive or difficult to use.'

With Viagra, you just pop a pill.

More important, the drug also removed the stigma of ED and brought discussion of men's sexual issues out in the open.

Prof Lim says: 'Inadequate sexual performance now has a medical solution. And when a problem can be medicalised, it also becomes respectable, not one you need to hide.'

Dr Li agrees: 'It's like saying you can't play golf because you have back pain. So you just fix your back pain, and you can play again.

'And you can even see your GP about it. In the past, you would have to consult a specialist.'

Dr Gan, who has been in practice for more than 20 years, says that where he once saw just one ED case a month, he easily gets between five and 10 cases a day now.

Freer discussion of sexual dysfunction issues is a good thing, he says.

'ED is a warning sign of other health problems like diabetes, hypertension and cholesterol. With more men coming forward, it can help us pick up these conditions earlier,' he says.

Viagra has been a godsend for MrJohn Foo, 53, who had ED problems because of hypertension and excessively high cholesterol levels.

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