Oral Contraceptives and Cancer Risk

Oral contraceptives are hormone tablets taken by mouth to prevent pregnancy. They work by blocking ovulation and by keeping sperm from getting through the cervix.

Two kinds exist. The most common type in the United States is the combined pill. It holds synthetic versions of two natural hormones, estrogen and progesterone. The synthetic progesterone is called a progestin. The other kind is the progestin-only pill, often called the mini pill.

This page is about the combined pill, because that is what nearly all the cancer research measures.

Almost all of what is known here comes from observational studies. Some follow large groups of women forward for years. Others compare women who developed a cancer with women who did not.

NCI is careful about what that design can show. Observational data cannot prove that the pill causes or prevents a cancer. Women who take it may differ from women who do not in other ways, and those differences could explain part of the gap. Keep that limit in mind for every number below. The pattern across studies is consistent, which is why it is taken seriously, but consistency is not proof.

A pooled analysis brought together 54 studies and more than 150,000 women. Women who had ever used the pill had a 7 percent higher risk of breast cancer than women who never had. Women currently using it had a 24 percent higher risk. That current-use figure did not grow with longer use. Risk fell after stopping, and by 10 years after stopping no increase was visible.

The Nurses' Health Study followed more than 116,000 nurses who were aged 24 to 43 when they enrolled in 1989. It also found a slight rise. Nearly all of that rise sat with one formulation: the triphasic pill, in which the hormone dose steps up in three stages across the cycle.

A large Danish study published in 2017 looked at newer formulations. Current and recent users had about a 20 percent higher risk overall. Across specific products the increase ranged from 0 to 60 percent. In that study, longer use meant higher risk.

A percentage increase is not the same as a large number of cases. Breast cancer is uncommon in the age range when most women take the pill, so a 20 percent rise on a small baseline is still a small absolute change.

Here the pattern is different. Duration drives it. Women who have used the pill for 5 or more years have a higher risk of cervical cancer than women who never used it, and the longer the use, the greater the risk.

One study measured the steps:

• Under 5 years of use: about 10 percent higher risk.
• Five to 9 years of use: about 60 percent higher risk.
• Ten or more years of use: roughly double the risk.

Risk falls again over time after stopping. There is an important piece of context. Persistent infection with high-risk types of human papillomavirus causes virtually all cervical cancers. One proposed explanation is that the pill changes how vulnerable cervical cells are to that infection. That makes HPV and cervical screening the levers that matter most, whatever a woman decides about contraception.

Now the numbers move the other way, and they move further.

For endometrial cancer, women who have ever used the pill have at least a 30 percent lower risk, with more protection the longer they used it. The effect lasts many years after stopping. In the NIH-AARP Diet and Health Study, the risk reduction was especially strong among long-time users who smoked, had obesity, or rarely exercised.

For ovarian cancer, ever-users have a 30 to 50 percent lower risk. Protection rises with duration of use, and it has been found to persist for up to 30 years after stopping. It also appears in women who carry a harmful BRCA1 or BRCA2 variant.

That last point matters for a specific group of women. Anyone weighing this should talk it through with a genetics service; our page on genetic counseling explains what that visit involves.

Pill use is linked with 15 to 20 percent lower risk of colorectal cancer. One proposed mechanism is a drop in the level of bile acids in the blood.

Natural estrogen and progesterone drive the growth of some cancers, especially those whose cells carry receptors for these hormones. A pill containing synthetic versions of the same hormones could plausibly do the same. That is the raising side.

The lowering side has different mechanisms for different organs:

• In the uterus, the pill suppresses the growth of endometrial cells.
• In the ovary, it cuts the number of times a woman ovulates in a lifetime, which reduces exposure to her own hormones.
• In the colon, conjugated estrogens appear to lower blood bile acid levels.

Different tissue, different effect. That is why one drug can raise two risks and lower three.

IARC reviewed combined estrogen-progestogen contraceptives in Volume 100A, published in 2012. That volume revisited medicines already classified as carcinogenic to humans, which is IARC's Group 1.

The label looks strange next to the protective findings, and understanding why is the whole point. IARC answers one question: is there sufficient evidence that this agent can cause cancer in people? For the combined pill, the breast and cervical findings answer yes. IARC does not weigh that against the ovarian and endometrial benefit, or against pregnancy risk, or against the reasons a person takes the pill in the first place. That balancing is a clinical judgement, not a hazard classification. Our page on hazard versus risk unpacks the difference.

Three things are worth carrying into a conversation with a clinician.

First, the increases are concentrated during use and shortly after. The breast cancer signal was gone by 10 years after stopping. The cervical signal also declines after stopping.

Second, the decreases are large, durable and cumulative. Ovarian protection has been measured up to 30 years out. For most women the arithmetic across all five cancers is not obviously negative.

Third, the pill is not the main lever for either of the cancers it raises. Cervical cancer is prevented by HPV vaccination and screening. Breast cancer risk is shaped far more by age, family history, reproductive history and body weight. Our page on cervical cancer covers what screening actually catches.

None of this decides anything for one person. Family history, age, smoking, blood clot risk and why the pill was prescribed all belong in the conversation. Many women take it for heavy bleeding, endometriosis or severe acne rather than for contraception, and that changes what a switch would cost.

• NCI: oral contraceptives and cancer risk
• IARC volume 100A: pharmaceuticals
• NCI: cancer causing substances