Despite concerns and some evidence that fertility treatments increase a woman’s chances of developing breast cancer, a large, long-term study finds no added risk.

Nearly 10,000 women who received drugs to stimulate ovulation were no more likely to develop breast cancer during 30 years of follow-up than those who never used the drugs, researchers report.

Lead author Louise Brinton, chief of the Hormonal and Reproductive Epidemiology Branch at the National Cancer Institute in Bethesda, Md., said the findings are reassuring.

Previous studies have raised concerns over the drugs because women are exposed to higher levels of estrogen during fertility treatments. Extended exposure to extra estrogen could increase the risk of breast cancer.

Overall, women who were exposed to either type of fertility drug were no more likely to develop breast cancer than those who didn’t use the medicines to stimulate ovulation.