Men buying Viagra over the internet could be putting themselves at increased risk of heart attacks or other serious side-effects, British scientists warn.

A group of scientists have analysed samples of Viagra sold on the web and found some of the anti-impotence pills contained different components or less of the active ingredient than the authentic top-selling drug made by pharmaceutical giant Pfizer Inc.

"On our initial estimate, around half of those Viagra samples could be counterfeit," Dr Nic Wilson, of the University of London, told the British Pharmaceutical Conference.

Viagra, which works by allowing more blood-flow to the penis during sexual arousal, is a lifestyle drug which is widely available on the internet and major targets for counterfeiters. With impotence, or erectile dysfunction (ED), affecting about 152 million men worldwide, there is a huge market for the treatment. It is estimated that half of all men over 40 experience some degree of ED, which increases with age.

The bogus drugs were branded and labelled Viagra and came in identical packaging to the real thing. The scientists are not sure whether wrong components in the bogus pills are harmful, but at the very least it is highly probable the fakes will not work. If the counterfeiters get the dose wrong and the bogus pills contain too much of the active ingredient, sildenafil, it could be dangerous.

The scientists behind the study had the following comment:

"If you go to a site that looks a bit wonky, they are selling it cheap and you've got no address or idea where they are based, you are chancing it."

A technique called near infrared (NIR) microscopy which provides a more detailed picture of what is in a tablet and its active ingredients to separate the fakes from the real thing.

There are a few UK sites which are legitimate and these include The Online Clinic, FirstMed and Health Express. Any site that offers you the medication prior to an online consultation with a qualified medical practitioner should be avoided.