Some breast cancer survivors have a higher-than-average risk of later developing another type of cancer, such as uterine cancer. Certain breast cancer treatments may raise the risk of second cancers. Tamoxifen is often prescribed to prevent breast cancer recurrence (when breast cancer comes back) in people who have treated hormone-positive breast cancer. Along with other risk factors, tamoxifen may affect your chances of developing uterine cancer.

Even if a breast cancer treatment may raise your risk of developing a second cancer, its benefits might outweigh this risk. It’s also important to understand that risk factors for cancer are complex. Continue reading to learn more about how tamoxifen treatment may affect your risk of uterine cancer and the signs and symptoms of uterine cancer.

What Is Tamoxifen?

Tamoxifen, sold under the brand name Soltamox, is a hormonal therapy used to prevent cancer from returning after treatment. Tamoxifen is also categorized as an adjuvant therapy, which means it’s used after treatments like surgery or chemotherapy to decrease the likelihood of cancer returning. Many people take tamoxifen for five to 10 years to treat or prevent certain types of breast cancer. It can decrease the risk of breast cancer recurring or new breast cancer developing by 30 percent to 50 percent.

Tamoxifen is in a class of medications called selective estrogen receptor modulators (SERMs). Tamoxifen is also known as an antiestrogen drug that works by blocking the effect of estrogen on hormone receptor-positive breast cancer. In this type of cancer, estrogen encourages breast cancer cells to grow and divide. By blocking the production of estrogen, tamoxifen can slow or stop cancer cell growth.

Your doctor may recommend tamoxifen if:

• You have a high risk of breast cancer but haven’t been diagnosed yet.
• You’ve had breast-conserving surgery for hormone receptor-positive ductal carcinoma in situ.
• You have hormone receptor-positive invasive breast cancer that’s been treated with surgery.
• You have hormone receptor-positive breast cancer that’s spread to other parts of your body.

Tamoxifen can be taken whether or not you have gone through menopause.

What Is Uterine Cancer?

The uterus is the largest component of the female reproductive system. This pear-shaped, hollow organ is located between the bladder and rectum in the pelvis (the area between the hip bones in the lower abdomen).

Uterine cancer — also called endometrial cancer or endometrial carcinoma — can develop when cells of the inner lining of the uterus begin to grow out of control. Uterine cancer is the most common gynecologic cancer, or cancer of the female reproductive system. In 2023, it’s estimated that more than 66,000 people in the United States will be diagnosed with uterine cancer, and more than 13,000 will die from the disease.

A tumor in the uterus can be benign (noncancerous) or cancerous. Examples of benign tumors include:

• Fibroids — Benign tumors
• Endometrial polyps — Abnormal growths in the endometrium, the layer that lines the inside of the uterus
• Endometrial hyperplasia — A condition with increased cells in the endometrium
• Endometriosis — When endometrial tissue is found outside of the uterus

There are two main types of cancerous tumors in the uterus:

• Adenocarcinoma — Cancer that starts in the endometrium, the most common type
• Sarcoma — Cancer that starts in the myometrium (the thickest, muscular layer of the uterus)

What Causes Uterine Cancer?

The exact cause of uterine cancer is unknown, but certain factors can increase your risk of endometrial cancer.

Tamoxifen Use and Uterine Cancer

The use of tamoxifen is one factor associated with an increased risk of uterine cancer. If you’re taking (or have taken) tamoxifen, you have about a two to five times higher risk of developing uterine cancer. Although the odds may seem high with these numbers, the risk is still relatively low — about 1 in 500, according to the American Cancer Society.

Your risk of uterine cancer may increase the longer you take tamoxifen.

Breast Cancer and Uterine Cancer

Tamoxifen use isn’t the only risk factor for uterine cancer. In fact, a history of breast cancer is also a risk factor for uterine cancer.

Additionally, breast cancer and uterine cancer share many of the same risk factors, including:

• Age — Both breast and uterine cancer are more common over age 50.
• Overweight and obesity — Obesity is linked to breast cancer and about 70 percent of uterine cancer cases, per Cancer.Net.
• Race and ethnicity — White people are more likely to develop both breast and uterine cancer.
• Genetics — People with Lynch syndrome are more likely to develop breast and uterine cancer.
• Prolonged estrogen exposure — Early menstruation and late menopause cause longer lifetime exposure to the hormones estrogen and progesterone, increasing the risk of breast and uterine cancer.

Other Risk Factors for Uterine Cancer

Your risk of developing uterine cancer can be influenced by many different factors, some of which you have control over and some you don’t. You may have more or fewer of these risk factors, including:

• Type 2 diabetes
• Radiation therapy in the pelvic area
• A diet high in animal fat
• Use of hair-straightening chemicals called relaxers
• Personal history of breast, colon, or ovarian cancer

Talk to your doctor about your risk factors for uterine cancer.

Can You Reduce Your Risk of Uterine Cancer?

Talk to your breast cancer health care team about which hormone therapy is best for you based on your risk factors. Treatment is a shared decision you should make together with your doctor to feel confident about your treatment plan. You can also talk to your doctor about potential risks and benefits of treatments to reduce the risk of tamoxifen-associated uterine cancer, such as:

• Medroxyprogesterone (Provera)
• Intrauterine device with levonorgestrel
• Lower tamoxifen dose

Other Hormonal Treatments for Breast Cancer and Risk for Uterine Cancer

If you’re worried about the effects of tamoxifen therapy on your risk for uterine cancer, you may wonder whether other breast cancer treatment options raise the risk as well.

Raloxifene (Evista) — another SERM used in breast cancer treatment — seems to have no effect on your risk of developing uterine cancer.

If you’ve already gone through menopause, you may be able to take an aromatase inhibitor — another type of hormone therapy that can decrease estrogen levels. Aromatase inhibitors have different side effects than tamoxifen and don’t cause uterine cancer. Taking aromatase inhibitors may result in fewer cases of uterine cancer compared to tamoxifen users. Examples of aromatase inhibitors include:

• Letrozole (Femara)
• Anastrozole (Arimidex)
• Exemestane (Aromasin)

Know These Symptoms of Uterine Cancer

Regular testing for uterine cancer isn’t recommended unless you have symptoms or a high risk.

If you’re taking tamoxifen, you should be mindful of the signs and symptoms of uterine cancer. According to the American College of Obstetricians and Gynecologists (ACOG), postmenopausal women taking tamoxifen should be closely monitored for uterine cancer. Postmenopausal women should look for any vaginal bleeding during or after menopause. Pain in the pelvic area (the lower part of the abdomen between the hip bones) can also be a symptom of uterine cancer.

ACOG states that premenopausal women taking tamoxifen do not have an increased risk of uterine cancer. However, premenopausal women should still watch for the following symptoms:

• Spotting between periods
• Abnormally heavy periods
• Watery or blood-streaked vaginal discharge

If you experience symptoms of uterine cancer, you should talk to your doctor right away. They may order additional tests to help diagnose uterine cancer, such as:

• Pelvic exam — Your doctor checks your vagina, uterus, ovaries, and rectum to look for abnormalities during a pelvic exam.
• Endometrial biopsy — In an endometrial biopsy, a sample of endometrial tissue is removed and tested.
• Imaging tests — These tests produce an image of the inside of your body.

If you have concerns about tamoxifen and your risk of developing uterine cancer, talk to your doctor. They can help you weigh the risks and benefits of taking tamoxifen as part of your breast cancer treatment.

Talk With Others Who Understand

MyBCTeam is the social network for people with breast cancer and their loved ones. On MyBCTeam, more than 65,000 members come together to ask questions, give advice, and share their tips with others who understand life with breast cancer.

Are you taking tamoxifen to prevent a recurrence of breast cancer? Did your doctor discuss the signs and symptoms of uterine cancer with you? Share your thoughts in the comments below, or start a conversation by posting on your Activities page.