Fertility Drugs and Breast Cancer Risk

Use of fertility drugs did not increase breast cancer risk over 30 years, except in women who were exposed to a dozen or more cycles of clomiphene citrate (Clomid), according to recently published research.

The study results, published in the American Association for Cancer Research’s Cancer Epidemiology, Biomarkers & Prevention journal, should be seen as a reassuring sign that fertility drugs are safe to use, the authors said. 

Pertinent Points

- Overall use of the fertility drug comiphene citrate (Clomid) was not associated with an increased risk of breast cancer, researchers report.

- The study evaluated the 30-year risk of breast cancer.

- A small subset of women were at increased risk for cancer when exposed to extensive use of fertility drugs, but it is unknown whether this risk results from the drug or factors related to infertility.

"Given the high doses of drugs received by our study participants and the lack of large increases in breast cancer risk many years after exposure, women previously exposed to such drugs should be reassured by these findings," said Louise Brinton, PhD, MPH, chief of the Hormonal and Reproductive Epidemiology Branch at the National Cancer Institute in Bethesda, Md. "However, the women in our study who developed breast cancer were on average only 53 years old, which is still young in terms of when we usually expect breast cancers to develop.

Brinton cautioned that the increase in risk for breast cancer occurred in a small subset of women and could be related to persistent infertility rather than being caused by the medications.

"Nevertheless, these findings stress the importance of continued monitoring of women who are exposed to fertility drugs," he said.

The study involved women who took either clomiphene or gonadotropins. Researchers examined data from 9,892 women who were evaluated for infertility between 1965 and 1988 at five different clinics in the United States that maintained records until 2010. Of the women included in the study, 749 developed breast cancer. The researchers obtained medical documentation for 696 of those who developed cancer, and were able to validate 536 as having invasive breast cancers.

Ever-use of clomiphene among 38.1% of patients was not associated with risk of breast cancer (HR, 1.05; 95% CI, 0.90–1.22 vs never-use), the researchers reported. However, patients who received multiple cycles had a little more than 1.5 times the risk of developing breast cancer (HR, 1.69; 95% CI, 1.17–2.46).

Gonadotropins, which were used by 9.6% of patients and mostly in conjunction with clomiphene, showed inconsistent associations with risk, the authors reported.