Infertile women who unsuccessfully take drugs to help them conceive may not only remain childless but end up with ovarian cancer, according to an analysis of 12 studies of the disease.

According to the findings, infertile white women who use fertility drugs were three times as likely to develop ovarian cancer as women who had either no fertility treatment or no history of infertility.

Infertile women who successfully conceived using drugs apparently "do not have a significantly increased risk", according to The Lancet (23 January 1993).

The studies have prompted the US Food and Drug Administration to urge manufacturers of fertility drugs using clomiphene citrate (such as Clomid) and menotropine (such as Pergonal) to add the risk of ovarian cancer to the list of potential side effects on their labels. Since fertility drugs were first marketed in the 1960s, some 12.5 million courses are estimated to have been prescribed.

Speaking of fertility problems, exposure to nitrous oxide or laughing gas can cause infertility in both men and women.

The gas, which is used as an anaesthetic for minor operations such as tooth extractions, appears to inactivate the vitamin B12 dependent enzyme methionine synthase, which is essential for normal cell division and DNA production.

According to a study of female dental assistants, women exposed to nitrous oxide for five or more hours a week have a three times greater risk of spontaneous abortion.