Fertility drug use and breast cancer risk

Women with a history of unsuccessful use of ovulation-stimulating fertility drugs were at a statistically significantly reduced risk of young-onset breast cancer compared with nonusers; however, women using the drugs who had a successful stimulated 10+ week pregnancy had a statistically increased risk of breast cancer compared with unsuccessfully treated women, but a comparable risk to nonusers, according to a study published July 6 in the Journal of The National Cancer Institute.

“The widespread use of ovulation-stimulating fertility drugs has raised concern about possible implications for breast cancer. Some, but not all, studies report increased risk following infertility treatment,” wrote the researchers from the National Institute of Environmental Health Sciences.

To examine the possible consequences of the high hormonal exposures experienced in early pregnancy by women who conceive with ovarian hyperstimulation, Chunyuan Fei, PhD, and colleagues, conducted a sister-matched case-control study, called the Two Sister Study that was, in part, funded by Susan Komen for the Cure. The investigators enrolled 1,422 women diagnosed with breast cancer under aged 50 years and their 1,669 cancer-free control sisters. The investigators’ focus was whether treatment had resulted in a pregnancy lasting at least 10 weeks.

Participants reported their use of clomiphene citrate (CC) and follicle-stimulating hormone (FSH) and whether any treatment had resulted in a pregnancy lasting 10 or more (10+) weeks. Women who reported use of CC alone (n=193 women) or both CC and FSH (n=66 women) showed a non-statistically significantly reduced odds of young-onset breast cancer compared with nonusers of these drugs, the authors reported.

After adjusting for exposure to a 10+ week stimulated pregnancy, women who had taken CC only or both CC and FSH, but without a pregnancy that lasted ≥10 weeks, were less likely to be diagnosed with young-onset breast cancer, compared with nonusers (CC only, OR=0.61; 95% CI, 0.41–0.90; CC and FSH, OR=0.53; 95% CI, 0.29–0.96). FSH only (n=29 women: 17 case sisters and 12 control sisters) without success was not associated with risk (OR=1.03; 95% CI, 0.44–2.40).

“Use of fertility drugs, particularly CC, may have pharmacological effects and could (more plausibly) serve as a marker for a different hormonal milieu in women with categories of infertility for which CC is used. For those who achieve conception through treatment, the elevated production of ovarian hormones early in a stimulated pregnancy might increase the risk of breast cancer by modifying pregnancy-related remodeling of breast tissue,” the authors concluded.