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Viagra may harm fertility
Impotence drug makes mouse sperm peak too soon.1 April 2003
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Viagra may affect male fertility, warns new research from Queen’s University Belfast. The famous impotence drug appears to scupper sperm and reduce pregnancies in mice, in contrast to human studies, which have found no adverse effects on fertility.
Viagra is prescribed to treat the erectile problems often experienced by older men. But younger people are increasingly using it, sometimes as a recreational drug.
Reproductive biologist Sheena Lewis and her colleagues found that mice given the equivalent of a human Viagra dose fertilized roughly 30% fewer female eggs than normal, and many of the eggs fertilized grew more slowly. She presented her results at the meeting of the British Fertility Society in Cheltenham, UK.
The study raises questions about use of Viagra among men who are trying to conceive, Lewis says. Around 23 million men worldwide have taken Viagra since its launch in 1998. The exact proportion trying to conceive is not known, but according to the manufacturer, Pfizer, the majority of users are over 50 years old.
This fertility finding contrasts with human data from clinical trials: Viagra’s effect on sperm movement and function was extensively studied, says Larry Lipshultz of Baylor College of Medicine in Houston, Texas who has been involved in trials of Viagra and its competitors, Levitra and Cialis. "I've reviewed all the data and never seen a fertility problem," he says.
The new study prompts further study of Viagra's effects on humans, but Lipshultz believes the findings may only be applicable to mice. Lewis says she is testing this by studying the sperm of men after they have taken Viagra.
Premature regurgitation
Lewis's team have an idea of how Viagra might be hindering pregnancies in mice. After adding Viagra to mouse sperm in a test-tube, the sperm swam more vigorously but matured too early, regurgitating enzymes that normally help them invade the egg. "Those sperm are of no use," says Lewis.
Exactly how Viagra might upset the sperm's internal machinery is not known. The drug inhibits an enzyme called phosphodiesterase in human cells, which causes the accumulation of another molecule called cyclic GMP. This has a range of effects, including expanding the blood vessels that help men achieve an erection.
Researchers say they do not yet know if Viagra might have long-term effects on mouse fertility, or any effects at all on fertility in humans. It is also unclear whether the drug might affect the fertility of women, who sometimes take Viagra to increase their arousal.
Should the finding prove applicable to humans, it might be affecting the success rate of in vitro fertilisation, says Allan Pacey, who researches sperm biology at the University of Sheffield, UK. Men are sometimes given Viagra to help them produce a sperm sample in fertility clinics. If it actually reduces their ability to fertilise the egg, "you potentially defeat the point," Pacey says.
© Nature News Service / Macmillan Magazines Ltd 2003
