What the Research Shows About Hormone Therapy and Cancer Risk

The short answer: hormone therapy increases risk for some cancers, but the size of that increase depends on which hormones you take, how long you take them, and your age when you start

Hormone replacement therapy (HRT) — estrogen alone or estrogen plus progestin — does raise the risk of certain cancers. The most studied are breast cancer, endometrial cancer, and ovarian cancer. The increased risk is real and measurable, but it is not automatic. Most people who take HRT do not develop cancer. The risk goes up with duration of use and comes down after you stop.

The evidence comes from large studies that followed thousands of people over years, most notably the Women's Health Initiative (WHI), which began in 1993 and produced results that changed how HRT is prescribed. Understanding what the research actually shows — rather than what headlines claim — helps you have an informed conversation with your doctor about whether HRT makes sense for your situation.

Key Takeaways

Estrogen plus progestin (combined HRT) raises breast cancer risk by roughly 20 to 30 percent after five years of use, while estrogen alone does not significantly increase breast cancer risk.
Estrogen alone raises endometrial cancer risk substantially, which is why progestin is added for people with a uterus.
The increased cancer risk begins to decline after you stop taking HRT and returns closer to baseline within a few years.
Your personal risk depends on your age, family history, weight, alcohol use, and how long you take hormones — not just on the hormones themselves.
Discussing your individual risk factors with your doctor is the only way to weigh whether symptom relief outweighs cancer risk in your case.

What the Women's Health Initiative found about breast cancer

The Women's Health Initiative was a landmark study that tracked over 16,000 postmenopausal people taking either combined HRT (estrogen plus progestin) or a placebo. After about five years, researchers found that the combined HRT group had a higher rate of breast cancer diagnosis. The increase was approximately 26 percent higher than the placebo group — meaning if breast cancer risk in the placebo group was 30 per 10,000 people per year, it rose to about 38 per 10,000 in the HRT group.

That same study found that estrogen alone (given to people who had a hysterectomy) did not significantly increase breast cancer risk. In fact, estrogen-only HRT showed a slight decrease in breast cancer risk, though this finding was not statistically strong enough to call it protective. This distinction matters: the increased breast cancer risk appears tied to the combination of estrogen and progestin, not to estrogen by itself.

The breast cancers found in the HRT group tended to be detected at earlier stages and were slightly less aggressive than cancers in the placebo group. This does not eliminate the risk, but it is part of the full picture.

Endometrial cancer risk with estrogen alone

Estrogen without progestin significantly raises the risk of endometrial cancer (cancer of the uterine lining). This is why doctors do not prescribe estrogen alone to people who still have a uterus. When progestin is added, it protects the endometrium and brings endometrial cancer risk back down to near baseline levels.

If you have had a hysterectomy, your uterus is gone, so endometrial cancer cannot develop. That is why estrogen-only HRT is an option for people without a uterus and why it carries a different risk profile than combined HRT.

Ovarian cancer and other cancers

Long-term use of HRT — particularly combined HRT — is associated with a modest increase in ovarian cancer risk. The absolute risk remains small: ovarian cancer is rare, and the increase in risk from HRT is measured in a few additional cases per 10,000 people per year. The risk appears to rise with duration of use and decreases after you stop taking hormones.

HRT does not significantly increase the risk of colorectal cancer, and some research suggests it may slightly lower colorectal cancer risk. Lung cancer risk is not increased by HRT. Skin cancer risk is not increased. The cancers of concern are breast, endometrial, and ovarian.

How risk changes based on how long you take HRT

Duration matters. The increased breast cancer risk from combined HRT is small in the first few years and grows the longer you take it. After five years of use, the risk is noticeably higher. After ten years, it is higher still. This is why current medical guidance emphasizes using HRT at the lowest effective dose for the shortest time needed to manage symptoms.

The good news is that the increased risk does not persist forever after you stop. Studies show that breast cancer risk begins to decline within one to two years of stopping HRT and returns much closer to baseline within five years. This means the risk is not cumulative in a way that locks in permanently.

If you take HRT for two years to manage severe hot flashes and then stop, your cancer risk profile is different from someone who takes it for ten years. Your doctor can help you think through what duration makes sense for your symptoms and your risk tolerance.

Personal risk factors that matter alongside HRT

Your baseline cancer risk — before considering HRT — depends on factors you cannot change and some you can. Age, family history of breast cancer, personal history of breast cancer or precancerous lesions, weight, alcohol use, physical activity level, and reproductive history all affect your risk. HRT does not erase these factors; it adds to them.

Someone with a strong family history of breast cancer, obesity, and heavy alcohol use starts from a higher baseline risk. For that person, adding HRT may shift the calculation differently than for someone younger with no family history and lower baseline risk. This is why a one-size-fits-all answer does not exist and why your conversation with your doctor needs to include your full picture, not just the HRT question alone.

What happens to cancer risk after you stop HRT

If you decide to take HRT and then stop, your cancer risk does not stay elevated indefinitely. Research shows that breast cancer risk declines fairly quickly after stopping combined HRT. Within two to three years, the excess risk drops noticeably. Within five years, it approaches the level of someone who never took HRT, though it may not return completely to baseline immediately.

This pattern is important because it means HRT is not a permanent decision with permanent consequences. If you take it for a defined period — say, two to five years to manage menopausal symptoms — and then stop, your long-term cancer risk profile is different from someone who takes it for fifteen years. You can revisit the decision as your symptoms change and as you age.

Frequently Asked Questions

Does HRT cause cancer or just increase the risk?

HRT increases the risk of developing certain cancers; it does not cause cancer directly. Most people who take HRT do not develop cancer. The increased risk is real and measurable but remains relatively small in absolute terms, especially for shorter durations of use.

Is estrogen-only HRT safer than combined HRT?

Estrogen alone does not significantly increase breast cancer risk, making it safer than combined HRT in that regard. However, estrogen alone raises endometrial cancer risk substantially if you have a uterus, which is why progestin is added. If you have had a hysterectomy, estrogen-only HRT carries a different risk profile than combined HRT.

Can I take HRT if I have a family history of breast cancer?

Family history does not automatically disqualify you from HRT, but it does mean the conversation with your doctor needs to weigh your increased baseline risk against the symptom relief HRT might provide. Some people with family history do take HRT; others choose alternatives. Your doctor can help you assess whether the trade-off makes sense for you.

How much does HRT increase my cancer risk?

The increase varies by cancer type and duration of use. Combined HRT raises breast cancer risk by roughly 20 to 30 percent after five years — meaning if your baseline risk was 30 cases per 10,000 people per year, it might rise to 38 to 39. Ovarian cancer risk increases modestly with long-term use. The absolute numbers are small, but they are real.

What should I do if I am already taking HRT?

Do not stop HRT without talking to your doctor. If you are concerned about cancer risk, schedule a conversation about your individual situation, how long you plan to take it, and whether your dose and type are still appropriate for your needs. Your doctor can help you weigh the benefits and risks specific to you.

This guide is for general information only and is not medical, financial, legal, or other professional advice. For decisions specific to your situation, consult a qualified professional. See our Editorial Policy.