As many as half of the Viagra anti-impotence pills sold on the Internet could be counterfeit, British scientists said on Tuesday.

They analyzed samples of Viagra sold on Web sites and found that some of the little blue pills contained different components or less of the active ingredient than the 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, along with hair-loss and weight treatments. All of these types of drug are 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 older than 40 experience some degree of ED, which increases with age. About 95 percent of cases can be successfully treated.

The bogus drugs were branded and labeled Viagra and came in identical packaging to the real thing.

“What we are talking about is somebody selling something as Viagra which is clearly not made by Pfizer,” Wilson’s colleague Professor Tony Moffat said in a telephone interview.

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.

“Part of the side reaction of the sildenafil is increased heart pressure, so people could get heart attacks,” Moffat added.

“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.”

Wilson and Moffat used 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.

The researchers tested the technique on known counterfeit Viagra before using it on pills they bought on the Internet. Pills arrived from a variety of countries including Thailand, India and Malta.

Eventually they believe the technology will be able to track the counterfeit products across the world.