Understudied Hormonal Birth Control Pill Linked to Cancer — But It’s Complicated

As much as they may be a small nuisance to take every day, oral contraceptives — that’s the pill — are here to stay (at least until better, longer-term, and more convenient birth control options come along). The best thing to do then, is continue to understand them as they evolve. And since becoming mainstream medication in the 1960s in the United States, they’ve evolved quite a bit.

In the last 60 years, researchers have made great strides in safely and effectively preventing pregnancy using medication. But the effects of long-term hormonal contraceptive use are less well understood.

Most of the research we do have is on the combination pill, which contains the hormones estrogen and progesterone and regulates the menstrual cycle and ovulation. Some people can’t take this pill, however, because they can’t tolerate estrogen.

The progestogen-only pill — which uses a synthetic version of the hormone — can present a safer and more inclusive option. But there’s a gap in the scientific knowledge to do with how progestogen-only pills affect health in the long run.

Epidemiologists at Oxford University are closing that gap. Published today in the journal PLOS Medicine, their new study investigates how progestogen-only birth control relates to risk for a form of breast cancer.

Get with the times

The study analyzes data from 9,498 people who use or have used hormonal birth control between the ages of 20 and 50 who developed invasive breast cancer between 1996 and 2017, compared against a control group of over 18,000 people without breast cancer. The data were gathered from the Clinical Practice Research Datalink, a primary care database in the United Kingdom.

On average, 44 percent of those with breast cancer and 39 percent of those from the control group used hormonal contraceptives; about half those prescriptions were progestogen-only. The researchers note that the odds of breast cancer for both groups increased by between 20 to 30 percent.

Before the alarm bells ring, we need to consider the difference between relative and absolute risk. Relative risk compares two specific groups’ rate of developing a condition. Absolute risk looks at an individual’s risk of contracting that condition. The absolute risk of developing breast cancer as a result of taking progestogen-only contraceptives is low and comparable to combination pills — which many have been using for decades.

Absolute risk for users of both combination and progestogen-only birth control between ages 25 and 29, they had a 0.57 percent increase in risk, compared to a 0.5 percent increase in nonusers of the same age — which amounts to a 0.07 percent increase between the two. For those between ages 35 and 39, users had a risk of 2.2 percent compared nonusers at 2 percent.

by age 30 You've got in the nonusers, a risk of breast cancer of about point naught apes and and this becomes point naught 9% in the uses of oral contraceptives. Now, obviously, as women age, their background risk of breast cancer increases. And so you might expect that any excess risk associated with oral contraceptive use would also increase. But again, it doesn't amount to much of an increase in either case. So if you look at women who use it from 25 to 29, we're now talking about over that 15 year period after stopped after starting HR starting OC use. You've got point 5% risk in nonusers and point five 7% risk in users. So only point naught 7% increase. And lastly for women taking hormone hormonal contraceptives from age 35 to 39. You've got over that 15 year period, an increase in risk of about point 2% from 2% in nonusers to 2.2%. In oral contraceptive users.

“The risks are very low, and you really have to balance them against the many benefits,” co-author Gillian Reeves, a professor of statistical epidemiology at the University of Oxford, told reporters during a press briefing on Monday, March 20. Some of those benefits, Reeves says, include decreased risk of ovarian and endometrial cancer.

Progesterone in the picture

Progestogen-only birth control methods may be safer options for some users. It’s preferable if a patient is at high risk for thrombosis or cardiovascular events, perhaps because they smoke. Reeves also noted that progestogen-only prescriptions have increased in the United Kingdom such that they almost mirror combination prescriptions.

Progestogen isn’t just for oral contraceptives. It can be used in effective intrauterine devices (I.U.D.s) too, because, Reeves says, “they have better efficacy in preventing pregnancy” and are “a lot easier than needing to take a pill every day.”

As progestogen-only methods become more common, the need to understand their long-term effects also grows. “The main purpose of doing this research was really to fill a gap in our knowledge,” Reeves says.

Balancing risks and benefits

The research outlines that this risk increase persists only as long as contraceptive use. Once someone stops using hormonal birth control, the risk drops off relatively quickly.

Reeves and her team are quick to underscore the good that long-term hormonal contraceptive use can do besides preventing pregnancy. Using these pills and other hormonal birth control methods, according to Reeves, offers protection against other reproductive cancers “into middle age, maybe 20 years after stopping.” The risk for breast cancer, on the other hand, is much lower and transient, returning to typical risk level after after stopping birth control for about five years.

“The benefits that you get for those cancers will be outweigh the risks of breast cancer,” Reeves says.

It’s also worth considering that there’s no risk-free contraceptive. “Every option has potential adverse events,” Malcolm Munro, a gynecologist at the David Geffen School of Medicine who was not involved with the study, tells Inverse. “Condoms and other barrier methods can be positioned to have enhanced risks associated with the higher risk of pregnancy; those not using hormonal contraception have greater risks of ovarian cysts and their sequela landing them in hospital, including admissions for surgical management. Non-hormonal I.U.D.s have higher pregnancy rates.”

In other words, don’t panic. And talk to your healthcare provider if you have questions about contraception.

“I don’t really see that there’s any indication here to say that women need to necessarily change what they’re doing,” Reeves says.