The classification of hormonal birth control by international health organizations as an agent with the potential to cause cancer has led to significant public concern and confusion. Given the widespread use of these medications, women seek clarity on what this classification means for their health and safety. Understanding this designation requires examining the scientific context, which involves distinguishing between an inherent potential for harm (hazard) and the actual probability of that harm occurring (risk). This article provides an evidence-based perspective on both the increased and decreased cancer risks associated with the use of hormonal contraception.
Understanding the Group 1 Designation
The classification comes from the International Agency for Research on Cancer (IARC), the specialized cancer agency of the World Health Organization. IARC identifies environmental, lifestyle, and pharmaceutical agents that pose a cancer hazard to humans. The IARC Monographs program places agents into categories based on the strength of scientific evidence that they can cause cancer. The “Group 1” designation means the agent is “carcinogenic to humans,” indicating sufficient evidence that it causes cancer. This classification focuses purely on the hazard, which is the intrinsic capability of an agent to cause cancer under some circumstances, separate from risk. Risk measures the probability that cancer will occur, considering the level and conditions of exposure. Other Group 1 agents include tobacco smoke, processed meat, alcoholic beverages, and solar radiation. The IARC classification does not assess the magnitude of risk at a specific dose or exposure level.
Which Hormonal Contraceptives Are Classified
The combined oral contraceptives (COCs) are the specific medications classified as Group 1 carcinogens. These formulations contain both a synthetic estrogen, typically ethinylestradiol, and a progestin. The IARC found sufficient evidence that this combination causes cancer in humans. This classification also extends to other combined hormonal contraceptives, such as patches and vaginal rings, which deliver the same hormones. Progestin-only contraceptives (mini-pill, injections, or implants) are classified separately. Evidence for progestin-only methods is often placed in a lower category, such as Group 2A, reflecting a different and less conclusive risk profile.
Specific Cancer Risks Associated with Use
Combined hormonal contraceptives have been shown to increase the statistical risk for three specific cancer types: cervical, breast, and liver cancer. For breast cancer, women currently using or who have recently stopped using COCs have a modest increase in risk, with a relative risk that is often cited as being around 1.24 compared to never-users. This slightly elevated risk declines significantly after the medication is stopped, returning to the baseline risk of never-users within approximately five to ten years. The risk of cervical cancer is also increased, particularly with long-term use of five years or more, with the risk doubling for women who have used COCs for a decade or more. This increased risk also begins to drop soon after cessation, disappearing entirely after about ten years. The third cancer type is liver cancer, specifically hepatocellular carcinoma, although this risk is statistically small and mainly observed in long-term users. The increase in absolute numbers of cases from COCs remains low because the baseline incidence of liver cancer in women is rare. The overall conclusion is that while hormonal components can stimulate cell growth in these tissues, the risk is highly dependent on the duration of use and is largely reversible upon discontinuation.
Protective Effects Against Other Cancers
Hormonal contraception offers significant protective effects against other gynecological cancers. Combined oral contraceptives dramatically reduce the risk of both ovarian and endometrial (uterine lining) cancers. The protective effect against ovarian cancer is substantial, with the risk decreasing by about 20% for every five years of use. This protective benefit is long-lasting, with a significant reduction in ovarian cancer risk remaining for 15 to 20 years, and some studies suggesting benefits for up to 30 years after the last pill is taken. Similarly, the risk of endometrial cancer is reduced by about 50% in women who have ever used COCs, and this benefit also persists for decades after stopping the medication. The protection is thought to occur because the hormones suppress the monthly process of ovulation and stabilize the uterine lining. This risk reduction is so pronounced that for many women, the overall lifetime risk of cancer is not increased. The protective effect is strongest with longer duration of use.
Contextualizing Risk and Personalized Decision Making
Hormonal contraception presents a complex risk-benefit profile, simultaneously increasing the hazard for some cancers while providing a strong, long-lasting protective effect against others. The increased risks for breast and cervical cancer are generally small in absolute terms, especially when compared to the baseline risk for cancer in the general population. Furthermore, the protective effects against ovarian and endometrial cancers are substantial and endure for decades. Personalized decision-making requires a discussion with a healthcare provider to assess individual risk factors. Factors that modulate a woman’s individual risk include her age, the duration of planned use, family history of cancer, and lifestyle choices such as smoking. For instance, a woman with a strong family history of ovarian cancer may find the protective benefits of COCs particularly compelling. Conversely, women with a known genetic predisposition for breast cancer may be advised to consider non-hormonal or progestin-only methods. The consensus medical view is that for the majority of women, the benefits of effective pregnancy prevention and the significant reduction in ovarian and endometrial cancer risk tend to outweigh the small, reversible absolute risk increase for breast and cervical cancer. The classification as a Group 1 hazard should be viewed as a signal of a potential biological effect, not a measure of high individual risk.