Warnings

Tamoxifen

Medically Reviewed by Dr. Harshi Dhingra, M.D.

Last Reviewed: Jun 12, 2025

Warnings

Tamoxifen Citrate Risks, Warnings, and Complications

Tamoxifen citrate can cause or worsen health problems in some people. Talk to your doctor or pharmacist about other breast cancer treatments if you are at an increased risk of adverse reactions from this drug. Some of the risks of taking tamoxifen citrate include:

HYPERCALCEMIA: In breast cancer patients with bone metastases (spread of the cancer from breast tissue to bone), tamoxifen citrate can cause hypercalcemia (high blood calcium levels). Your doctor will check serum calcium levels periodically. You may need to stop tamoxifen therapy if blood calcium levels are severely elevated.
EFFECTS ON THE UTERUS: Studies have shown an increased incidence of uterine malignancies, including fatal malignancies, in women receiving tamoxifen treatment. Most of these malignancies are endometrial cancers (adenocarcinomas of the endometrium or lining of the uterus); however, there are rare reports of uterine sarcomas, including malignant mixed mullerian tumors (MMMT). Uterine sarcomas are associated with a poorer prognosis and shorter life expectancy. The risk of uterine sarcoma is higher in women who are on tamoxifen for more than 2 years.
Seek immediate medical care if you develop unusual vaginal bleeding, menstrual irregularities, changes in vaginal discharge, or pelvic pressure or pain during or after treatment with tamoxifen. If you are taking or have taken tamoxifen in the past, you should have an annual gynecological examination.
In addition to the increased risk of malignancies, women taking tamoxifen are also at an increased risk of endometrial changes such as hyperplasia and polyps. There are reports of endometriosis and uterine fibroids in women on tamoxifen as well. Other effects noted in premenopausal women on hormonal therapy with tamoxifen include ovarian cysts, menstrual irregularity, and amenorrhea.
The underlying mechanism for these effects on the uterus, both malignant and non-malignant, could be the partial estrogenic effect of tamoxifen.
THROMBOEMBOLIC EFFECTS: Tamoxifen can increase the risk of blood clots (thromboembolic events), including deep vein thrombosis (blood clot in the leg) and pulmonary embolism (blood clot in the lungs), and to a lesser extent, strokes (blood clot in the brain).
Co-administration of endocrine therapy (tamoxifen) with chemotherapy further increases the risk. Your healthcare providers will carefully weigh the benefits versus risks of treatment before starting tamoxifen therapy.
EFFECTS ON THE LIVER: Clinical trials have found that adjuvant tamoxifen therapy for 2-5 years may increase the risk of liver cancer. In addition, tamoxifen can have non-malignant effects on the liver, including changes in the levels of liver enzymes and, rarely, fatty liver, hepatitis, cholestasis, and hepatic necrosis. Some of these liver abnormalities have been fatal in reported cases, but the exact relationship to tamoxifen is unclear.
EFFECTS ON THE EYE: There are reports of ocular changes, including changes in the cornea, decreased color vision, retinal vein thrombosis, retinopathy, and an increased incidence of cataracts in patients receiving tamoxifen.
PREGNANCY CATEGORY D: Tamoxifen can cause harm to the unborn baby if it is taken by a pregnant woman. Your doctor will advise using non-hormonal birth control measures such as barrier methods if you are sexually active. You will need to avoid pregnancy while on tamoxifen and for 2 months after stopping treatment. If you are a sexually active woman with childbearing potential, you will be asked to start tamoxifen therapy during menstruation (to rule out pregnancy). If you have irregular menses, your doctor will do a pregnancy test before starting tamoxifen.

Note: Tamoxifen may cause menstrual irregularities, but it does not cause infertility. However, there may be some effects on reproductive function due to its anti-estrogenic properties.

Precautions Before Starting Tamoxifen Citrate

Tell your doctor if you have ever had an allergic reaction to tamoxifen citrate, any of the active or inactive ingredients in tamoxifen tablets, or any other drugs. Your pharmacy can give you a list of ingredients.

Certain medications can affect how tamoxifen citrate works. Give your doctor and pharmacist a complete list of your medications, including prescription drugs, over-the-counter medicines, vitamins, dietary supplements, and herbal products. This can help avoid possible interactions between tamoxifen citrate and your other medicines.

Give your doctor a complete medical history. Tamoxifen citrate may not be safe for people with certain medical conditions such as pulmonary embolism, deep vein thrombosis, heart attack, stroke, high blood pressure, diabetes, high cholesterol, or smoking.

Tell your doctor if you are pregnant, could be pregnant, plan to get pregnant, or are breast-feeding.

Precautions During Use of Tamoxifen Citrate

Keep all your medical and lab appointments while on tamoxifen citrate.

Use a non-hormonal method of contraception to prevent pregnancy during treatment with tamoxifen citrate and for 2 months afterward.

Get an annual gynecological examination to look for early signs of uterine cancer. Get screening tests for breast cancer (self-breast exam, breast exam by a doctor, mammogram or X-ray of the breasts). While tamoxifen can help to prevent breast cancer, it does not lower your risk to zero. Women with breast cancer are at risk of developing it in the opposite breast.

Tell all your healthcare providers you are on tamoxifen. Call your doctor right away if you become pregnant while on tamoxifen. Do not breastfeed while on this drug.

Seek immediate medical care for any abnormal vaginal bleeding, irregular menstrual periods, changes in vaginal discharge (especially if the vaginal discharge is brown, rusty, or bloody appearing), pelvic pain or pressure, chest pain, shortness of breath, coughing up blood, leg swelling or tenderness, sudden weakness or numbness on one side of the body, sudden confusion, sudden severe headache, difficulty speaking or understanding speech, difficulty walking, poor balance or coordination, vision problems such as blurred vision in one or both eyes, or dizziness.

Tamoxifen Citrate Drug Interactions

Certain medications and tamoxifen interact and increase the risk of serious adverse effects. Your doctor may change the doses of your medications, switch you to different medicines, and/or monitor you carefully for side effects if there are known interactions between tamoxifen citrate and your other medicines. Some of the drugs that can interact with tamoxifen citrate include:

Anti-cancer drugs such as anastrozole (Arimidex) and letrozole (Femara)
Aminoglutethimide (Cytadren)
Bromocriptine (Parlodel)
Chemotherapy drugs such as cyclophosphamide (Cytoxan, Neosar)
Medroxyprogesterone (Depo-Provera, Provera, in Prempro)
Antiepileptic drugs such as phenobarbital
Antitubercular drugs such as rifampin (Rifadin, Rimactane)

The above list may not include all the potential interactions of tamoxifen citrate. Give your doctor or pharmacist a complete list of your medications, including prescription drugs, over-the-counter medicines, nutritional supplements, and herbal remedies. Also, tell your healthcare professional if you smoke, drink alcohol, or use illegal drugs because some of these substances can cause serious health complications when used with prescription medications.

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