The finding, by Queen’s University, Belfast, also casts doubt on the use of the anti-impotence drug by IVF clinics.

The researchers will tell a British Fertility Society meeting that the drug does enhance sperm movement.

However, it also seems to undermine the timing of a chemical process needed to fertilise the egg.

The message we want to get across is that caution should be taken when using drugs if you are hoping to start a family Dr Sheena Lewis

This process, known as the acrosome reaction, releases digestive enzymes that break down the egg’s protective outer layer, allowing the sperm to penetrate more easily.

Viagra seems to speed up the acrosome reaction, so that by the time the sperm reaches the egg it has no digestive enzymes left to penetrate the outer layer. Sperm that have undergone this process are known as fully “reacted”.

The researchers tested 45 samples of semen. They found that up to 79% more sperm were fully “reacted” in samples treated with Viagra.

The findings echo previous work on mice showing that in the presence of Viagra fewer eggs are fertilised - and fewer of the resulting embryos continue to develop.

Recreational use

Researcher Dr Sheena Lewis said the acrosome reaction involved the channelling of charged calcium atoms, or ions.

It would be a terrible shame if an unnecessarily alarmist headline put people off using a treatment which may actually help them Dr John Dean

This was known to influence numerous cellular mechanisms - and could effect early embryonic development.

Dr Lewis said: “When Viagra came out in 1998 it was aimed at men with impotence problems, primarily older men not interested in having children. Now it has become a very popular drug for sexual enhancement.

“The message we want to get across is that caution should be taken when using recreational drugs if you are hoping to start a family.”

Dr David Glenn, who also worked on the study, said: “Nearly half of licensed fertility units in the UK currently use Viagra to assist patient semen production.

“Our study raises questions about the drug’s use in assisted .”

Sheena Young, from the support group Infertility Network UK, said it was important that people who used Viagra recreationally were fully aware of its full effect.

“When Viagra was introduced it was never meant for this purpose.”

Caution urged

However, Dr John Dean, secretary general of the European Society for Sexual Medicine, told BBC News Online that it was difficult to draw firm conclusions from the study.

He said lab results often did not reflect what happened in the human body, and sperm was known to be highly sensitive when removed from its natural .

“Childless couples - and the general population - should be aware that in the five years that Viagra has been around no overall detrimental effect on fertility has been observed,” he said.

“It would be a terrible shame if an unnecessarily alarmist headline put people off using a treatment which may actually help them.”

And some information of .

Dr Nic Wilson from the University of London tested Internet-sold samples using a new technique that accurately spots the of tablets.

She told the British Conference in Manchester how many authentic-looking tablets were fakes.

Drug giant Pfizer, which manufactures Viagra, is conducting its own investigations into the fake copies.

Counterfeits

Dr Wilson used a technique called near infrared (NIR) microscopy which provided a detailed picture of what was in each tablet.

This technology is similar to the older method called NIR spectroscopy but gives much more information.

There is a high probability that the tablets have no clinical effect Researcher Dr Nic Wilson

Dr Wilson explained: “A counterfeit tablet may contain lactose as an ingredient in the bulk tablet, whereas the authentic tablet does not.

“NIR spectroscopy could only show that the tablet is different, while NIR microscopy could actually identify the likely presence of lactose.”

Many of the samples tested contained less of the active ingredient sildenafil than authentic Viagra.

They also contained different components from the bona fide Viagra.

Dr Wilson said: “We don’t know that ‘wrong’ components will be harmful, but the user runs the risk of poor quality and possible toxicity, not to mention the fact that there is a high probability that the tablets have no clinical effect.”

She said NIR microscopy should help regulatory authorities monitor the movement of counterfeit tablets.

Crackdown

It is the job of the Medicines Healthcare products Regulatory Authority to investigate any reports it receives of websites under its jurisdiction which appear to be in breach of regarding , or sale and supply of medicines.

In 2003, counterfeit Viagra with an estimated value in excess of 2.35m was seized.

Pfizer said it welcomed the research and was also analysing the samples.

A spokesman said: “We do not recommend that anyone obtains any prescription-only medicine online without seeing a doctor.

“Without knowing what ingredient is in a counterfeit medicine a patient could be putting their health at risk.

“It is important for men to see their doctor if they have erectile dysfunction as it may be an indicator of a more serious condition.

“Viagra may not be suitable for all men and there are also some men for whom sexual activity is not considered safe or for whom an oral treatment may not be appropriate.”

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Read more on The rise and rise of Viagra has created a 1.5bn worldwide market in pills. Now rivals are fighting for a share of the spoils and it is becoming a drug of choice for some in the party generation. Last week, Pfizer&39;s authorities to clamp down on the copycats. New research Pfizer, the world&39;s top Urological Surgeons, based in Bristol. He had spent his whole career trying to treat and improve the lives of thousands of men suffering from impotence. In those days, commonly used treatments included the fitting of implants directly into the penis, a vacuum pump and self injection. Most sufferers were thoroughly put off and consigned themselves to a life without sex. Viagra arrives Mr Gingell ran a new series of trials, and the results impressed him. Pfizer chief executive McKinnell says copycats pose a threat He describes Viagra as “a wonder drug”. “The thought of having a pill that would cure impotence was amazing to me,” he says. “I never thought I would see it in my lifetime.” “There has been a kind of Holy Grail idea associated with curing impotence,” Pfizer&39;s share price doubled. It was apparent that there was a huge previously untapped market out there. Doctors claim that half of all men over 40 become impotent at some point in their lives. That is more than 150 million worldwide, with two million sufferers in Britain alone, so the potential market for drugs like Viagra is colossal. Overnight Viagra made Pfizer famous. “We discovered the mass production of penicillin, yet it was Viagra that put Pfizer on the map,” says Ms Caprino. Embarrassing subject Nevertheless, despite the highly successful launch, the company faced a huge potential problem in selling Viagra. Men were simply not willing to talk about impotence, they were ashamed. If they were not prepared to discuss their impotence, how could they be persuaded to ask their doctor for a prescription? Ray Reynolds, who suffered from impotence for 30 years, had simply given up hope of ever being able to have sex again. “I thought well, I&39;s top spectator sport

Firstly, they asked the Vatican, and other world religious leaders, for their blessing. This headed off possible moral and religious objections.

Secondly, they employed big name celebrities to encourage men to seek treatment for impotence.

Pele, the legendary footballer, headed a men&39;s courage in coming forward.

“When I saw it on TV, I admired him for it,” he says.

“You might say he was my idol.”

Withdrawal of campaign

Pfizer decided not to use the term “impotence” in the advertisements, instead replacing it with a more bland technical term “erectile dysfunction”.

Pfizer&39;s aggressive marketing campaign has recently run into trouble.

A recent television advertisement has been criticized in the United States for suggesting that Viagra might be better and more effective for patients than the clinical experience suggests.

The Food and Drug ordered its withdrawal.

Efficient sex

There are potential problems, too, in the increasing use of Viagra as a recreational drug.

Half of all men over 40 become impotent at some point

“For a lot of gay people it is just a normal way of life,” says Gary Mercado, who runs the Elysium Resort, the largest gay hotel in Fort Lauderdale, Florida.

When Viagra is taken with amphetamines, “you forget about having protective sex, so there are huge capabilities of all sorts of sexual diseases”, he says.

Pfizer says that a very small percentage of people abuse Viagra, but accepts there is great potential in developing the market for sexual pharmaceuticals.

Meika Loe, author of the book The Rise of Viagra, agrees: “In the Viagra era, sexuality is subject to the cult of efficiency. It&39;s-ised. Serve it up fast and hot.”

The Money Programme: Viagra: The Hard Sell was broadcast at 2200 GMT on Wednesday, 9 February on BBC Two .

And some information of .

at the University of Minnesota Medical School in the US identified seven men who developed vision problems after taking Viagra.

The team, writing in the Journal of Neuro-ophthalmology,said it brought the total number of reported cases to 14.

But Pfizer, the makers of the drug which has been used by more than 20m men since its launch in 1998, said the cases were a coincidence.

The seven men, aged between 50 and 69 years old, had all suffered from a swelling of the optic nerve within 36 hours of taking Viagra for erectile dysfunction.

Six vision loss within 24 hours, although only one had problems with both eyes.

The condition, known as nonarteritic anterior ischemic optic neuropathy (NAION), causes a rapid reduction of vision and can, in the most serious cases, lead to blindness.

Report co-author Dr Howard Pomeranz said the drug had long been linked to sight problems.

“For years, we have known some men who take Viagra will experience temporary colour changes in their vision and see things as blue or green.

“NAION is a much more serious condition because it can lead to permanent vision loss.”

But a Pfizer said: “Pfizer takes these reports very seriously, as we do anything concerning the safety of patients.

“However, there have been no reports of NAION in the 103 clinical trials of Viagra.”

She said she believed the cases were just a coincidence, but said the company would continue reports.

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The move comes after an audit found 198 convicts in New York state had been reimbursed by Medicaid for the drug between January and March 2000.

Their crimes included offences against children as young as two.

The Medicaid programme, whose cost is shared by states and the federal government, provides health care for the poor.

The federal Centres for Medicare and Medicaid Services said they should not pay for erectile drugs for sex offenders.

Spokesman Gary Karr said “states already have the power to determine if a drug is not medically appropriate for a certain patient or certain class of patients”, the Press news agency reported.

Public risk

The New York audit, conducted by Comptroller Alan Hevesi, did not cover other states, but Mr Hevesi said states are required by law to include Viagra in Medicaid programmes covering prescription drugs when medically necessary.

He said the policy raised “serious policy considerations and has the potential to place the public at risk” and asked the government to take administrative action or amend the Medicaid law.

On Monday, Florida Attorney General Charlie Crist said that Medicaid had paid $93,000 to provide Viagra to 218 sex offenders in that state over the last four years, AP reported.

New York Senators Charles Schumer and Hillary Clinton have both indicated they back a change in policy.

Sen Schumer said: “It is just mind-boggling to think that Level 3 sexual offenders can get Viagra, which may indeed help them other horrible crimes.

“Giving convicted sex offenders government-funded Viagra is like giving convicted murderers an assault rifle when they get out of jail,” Schumer said.