Boston University scientists announced today that the anti-miscarriage drug diethylstilboestrol (DES) - commonly prescribed from 1940 to 1975 - could double the normal risk of breast cancer in exposed women.

In the study, disease rates were tested in around 5,000 women who were exposed to DES in the womb, and then compared to disease rates of about 2,000 women who were not. Seventy-six cases of breast cancer were identified in the DES group, while 26 were identified in the control group.

“This is really unwelcome news because so many women worldwide were prenatally exposed to DES, and they are just now approaching the age at which breast cancer becomes more common,” said lead researcher Julie Palmer.

The study found that the cancer risk did not seem to change for women under the age of 40, but women over 40 were twice as likely to develop the condition. Cancer risk for participants over the age of 50 was found to be even higher.

“This study suggests that daughters of mothers who took the drug DES during their pregnancies may have a higher risk of breast cancer as they get older,” said Ed Yong, a senior cancer information officer at Cancer Research UK. “But this does not mean that they will definitely develop the disease.

“Early detection of breast cancer is important and all women should be aware of any unusual changes to their breasts and report them promptly. Women should be sure to attend their breast screening appointments when invited,” he added.

The use of DES was stopped in 1975 after researchers discovered evidence that exposed daughters had an increased risk of developing pregnancy complications, as well as rare cancers of the vagina and cervix. Sons of DES users were also found to have a lower-than-average sperm count and an increased risk for testicular cancer.

DES Action UK spokesperson Heather Justice said, “People should be asking their mothers whether they can remember being prescribed (DES) during their pregnancy, then insisting on proper screening and care.”

“This is yet another example of the hidden dangers of FDA-approved prescription drugs,” said Mike Adams, a consumer health advocate. “It also shows that the true dangers of drugs may not be understood until decades later.”