Men who take Viagra when they are hoping to start a family could be affecting their fertility.
Viagra affects sperm
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.
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 significantly fewer eggs are fertilised - and fewer of the resulting embryos continue to develop.
Researcher Dr Sheena Lewis said the acrosome reaction involved the channelling of charged calcium atoms, or ions.
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 reproduction."
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."
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 environment.
"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."