A new study by Israeli researchers has found that drugs designed to help women conceive may increase the risk of cancer of womb.
Fertility drugs have been in use for more than 30 years and have been taken as part of IVF treatment to help them have children.
Women, who have trouble conceiving, are undergoing IVF, or who want to donate or sell there eggs are prescribed ovulation-inducing drugs.
Now, a study of more than 15,000 women - 30 years after they gave birth - has suggested they are at least three times more likely to develop cancer of the womb.
Although, the risk still remains low, the researchers who conducted the study believe that it is worth further investigation and that those who undergo the treatment should be carefully monitored.
Dr Ronit Calderon-Margalit at Hadassah-Hebrew University in Jerusalem and colleagues have examined the effects of these drugs by comparing cancer incidence in a group of 15,000 Israeli women 30 years after they gave birth.
The researchers found that of the 567 women who reported having been given ovulation-inducing fertility drugs, five developed uterine cancer - which is about three times the incidence in members of the group who had not been given these drugs.
For the 362 women who took clomiphene, which tricks the body into making extra eggs by blocking oestrogen receptors, the risk was over four times that of women who did not take the drugs.
Calderon-Margalit has admitted that the numbers are small, but said they carry extra weight because they make "biological sense" as tamoxifen, a breast cancer treatment which, like clomiphene, reduces sensitivity to oestrogen, was known to increase the risk of womb cancer.
However, Richard Kennedy, a consultant at the Centre for Reproductive Medicine at the University Hospital Coventry and a spokesman for the British Fertility Society, sought to reassure patients.
"There have been a high number of studies that have failed to find a conclusive link. It is important to remain vigilant about these things but the broad message must be reassurance," the Telegraph quoted him, as saying.