Data from the U.K. Biobank
To examine the time-dependent effects between long-term oral contraceptive use and cancer risk, the researchers examined data from 256,661 women from the U.K. Biobank who were born between 1939 and 1970. The researchers identified cancer diagnoses using information from national registers and self-reported data until March 2019.
Of the women included in the study, 82% had used or still were using oral contraceptives, whereas 18% had never used oral contraceptives. Overall, ever users were younger, more frequently smokers, and had a lower body mass index, compared with never users. Most women started using oral contraceptives between 1969 and 1978. Last use of oral contraceptives occurred on average 10.7 years after starting.
The researchers adjusted for covariates and used logistic regression analyses to measure the cumulative risk of cancer. They used Cox regression analysis to examine instantaneous risk, measured using hazard ratios.
In all, there were 17,739 cases of breast cancer (6.9%), 1,966 cases of ovarian cancer (0.76%), and 2,462 cases of endometrial cancer (0.96%).
Among ever users, the likelihood of ovarian cancer (OR, 0.72) and endometrial cancer (OR, 0.68) was lower, compared with never users. “However, we did not see a significant association between oral contraceptive use and breast cancer” for the study period as a whole, the researchers reported. When the researchers limited follow-up to age 50 years, however, the odds ratio for breast cancer was increased (OR, 1.09).
Dr. Åsa Johansson
“Surprisingly, we only found a small increased risk of breast cancer among oral contraceptive users, and the increased risk disappeared within a few years after discontinuation,” Åsa Johansson, PhD, a researcher in the department of immunology, genetics, and pathology at Uppsala University and one of the study authors, said in a news release. “Our results suggest that the lifetime risk of breast cancer might not differ between ever and never users, even if there is an increased short-term risk.”
Oral contraceptives today typically use lower doses of estrogen and other types of progesterone, compared with formulas commonly used when participants in the study started taking them, so the results may not directly apply to patients currently taking oral contraceptives, the researchers noted.
The study was supported by the Swedish Research Council, the Swedish Cancer Society, and the Kjell and Märta Beijers, the Marcus Borgström, the Åke Wiberg, and the A and M Rudbergs foundations. The authors, Dr. Keating, and Dr. Badalian had no conflicts of interest.