A new study 30 years in the making finds that fertility drugs do not raise most women’s risk for breast cancer.

Women who have trouble getting pregnant are often given drugs containing estrogen to stimulate ovulation either as a stand-alone fertility treatment or as part of in vitro fertilization. Experts have been concerned that prolonged exposure to estrogen as well as increased ovulation could put women at higher risk for breast cancer. The new study, published in Cancer Epidemiology, Biomarkers & Prevention, should put at least some of those fears to rest.

Researchers at the National Cancer Institute analyzed the medical records of 9,892 women in the United States who had been evaluated for infertility between 1965 and 1988. About 38 percent of the women in the study used the fertility drug clomiphene, marketed under the name Clomid, and about 10 percent used another class of drugs known as gonadotropins, though often in conjunction with Clomid. Over the decades that followed, 792 cases of breast cancer were diagnosed among the women in the study, but analysis found that women exposed to these drugs were no more likely to be diagnosed than women who had not taken either class of medication.

There were some exceptions to the results, however. Women who were exposed to 12 or more cycles of Clomid had a 70 percent higher risk of developing breast cancer compared to women not exposed to the drug at all. However, these women got much more of the drug then women taking it today. The current normal dose of Clomid is half that of what it used to be, and guidelines now limit women to three to six cycles of the drug.

Another small subset of women were also at increased risk. Women who took gonadotrophins together with Clomid and were still never able to become pregnant were about twice as likely to develop breast cancer as women who had not taken either drug. However, the researchers suspect that for these women, some of the increased risk is likely tied to the underlying problem that caused their persistent infertility.

The researchers say that their results are “generally reassuring” but caution that they need to keep watching these women, as risk of the breast cancer goes up with age. Louise Brinton, chief of the Hormonal and Reproductive Epidemiology Branch at the National Cancer Institute and lead author of the study, told the Plain Dealer, “Even though we’ve had 30 years of follow-up, it’s still pretty early in the game. The women are still quite young.”

Still, some experts were pleased with the results. Dr. Kurt Barnhart, president of the Society for Reproductive Endocrinology and Infertility, told Reuters Health, “It’s reassuring that if women desire pregnancy and unfortunately have infertility that they can undergo treatment without modification of their overall risk for cancer later.”