A study involving mice found that mice given equal amounts of human Viagra fertilized approximately 30 percent fewer female eggs than normal and many of the fertilized eggs were shown to grow at a slower pace.

Since Viagra was launched in 1998, about 23 million men worldwide have taken the impotence drug. The average age of the Viagra user was 50 years old.

It has yet to be determined how Viagra might disrupt the sperm’s internal functions.

The drug works by inhibiting an enzyme called phosphodiesterase into the human cells, which then results in the accumulation of another molecule called cyclic GMP. One of the effects of this process includes the expansion of the blood vessels that help men achieve an erection.

Researchers claimed they were unable to prove if the effects of Viagra would be long-term on mouse fertility and if there were any adverse fertility effects in humans. It was also unclear whether or not the fertility of women, who took Viagra for arousal purposes, was affected. If the findings prove that human’s fertility are negatively affected by Viagra, then the success rate of in vitro fertilization will be under scrutiny, considering men at the fertility clinics are frequently given Viagra to assist them with giving a sperm sample.

Nature April 1, 2004