Tamoxifen is used for treating breast cancer that has spread to other sites in the body. It is also used along with other medicines to treat other types of breast cancer. It is used in women who are at high risk for breast cancer and in women with DCIS (after surgery and radiation) to decrease the risk of developing breast cancer. Tamoxifen is an antiestrogen. It works by blocking the effect of estrogen on certain tumors. This may prevent the growth of tumors that are activated by estrogen.
Use Tamoxifen as directed by your doctor.
- Tamoxifen may be taken by mouth with or without food.
- Take Tamoxifen with water or another nonalcoholic liquid.
- Swallow this medication whole. Do not break, crush, or chew before swallowing.
- Continue to take Tamoxifen even if you feel well. Do not miss any dose. Taking Tamoxifen at the same time each day will help you to remember to take it.
- If you miss a dose of Tamoxifen, take it as soon as possible. If it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not take 2 doses at once.
Ask your health care provider any questions you may have about how to use Tamoxifen.
Store Tamoxifen between 68 and 77 degrees F (20 and 25 degrees C) in a tightly closed, light-resistant container. Store away from moisture, heat, and light. Do not store in the bathroom. Do not take tablets after the expiration date printed on the label. Keep Tamoxifen out of the reach of children and away from pets.
Do NOT use Tamoxifen if:
- you are allergic to any ingredient in Tamoxifen
- you are using Tamoxifen to reduce your risk of breast cancer and you have a history of blood clots in the lung or leg
- you are using Tamoxifen to reduce your risk of breast cancer and you also take certain anticoagulants (eg, warfarin)
- you are taking anastrozole.
Contact your doctor or health care provider right away if any of these apply to you.
Some medical conditions may interact with Tamoxifen. Tell your doctor or pharmacist if you have any medical conditions, especially if any of the following apply to you:
- if you are pregnant, planning to become pregnant, or are breast-feeding
- if you are taking any prescription or nonprescription medicine, herbal preparation, or dietary supplement
- if you have allergies to medicines, foods, or other substances
- if you have a history of blood clots in the legs or lungs
- if you have high levels of calcium in your blood, a weakened immune system, or low levels of white blood cells or platelets in your blood
- if you have high cholesterol or lipid levels
- if you have cataracts or other vision problems
- if you are using cytotoxic cancer medicines
- if you are confined to a bed or chair.
Some medicines may interact with Tamoxifen. Tell your health care provider if you are taking any other medicines, especially any of the following:
- Anticoagulants (eg, warfarin) because the risk of bleeding may be increased
- Cytotoxic cancer medicines (eg, cisplatin) because the risk of developing blood clots may be increased
- Rifampin because it may decrease Tamoxifen’s effectiveness
- Fluorouracil or mitomycin C because they may increase the risk of Tamoxifen’s side effects
- Aromatase inhibitors (eg, anastrozole, letrozole) because their effectiveness may be decreased by Tamoxifen.
This may not be a complete list of all interactions that may occur. Ask your health care provider if Tamoxifen may interact with other medicines that you take. Check with your health care provider before you start, stop, or change the dose of any medicine.
Important safety information:
- Tamoxifen may cause dizziness. These effects may be worse if you take it with alcohol or certain medicines. Use Tamoxifen with caution. Do not drive or perform other possible unsafe tasks until you know how you react to it.
- Tamoxifen may reduce the number of clot-forming cells (platelets) in your blood. Avoid activities that may cause bruising or injury. Tell your doctor if you have unusual bruising or bleeding. Tell your doctor if you have dark, tarry, or bloody stools.
- Tamoxifen may lower the ability of your body to fight infection. Avoid contact with people who have colds or infections. Tell your doctor if you notice signs of infection like fever, sore throat, rash, or chills.
- New tumors have occurred in body sites other than the uterus in patients taking Tamoxifen. It is unknown if Tamoxifen may be the cause. Discuss any questions or concerns with your doctor.
- Women who take Tamoxifen to reduce the risk of breast cancer should have a breast exam, mammogram, and gynecological exam before starting and during treatment with Tamoxifen.
- Women who may become pregnant should begin taking Tamoxifen during a menstrual period. Women who have irregular menstrual periods should have a pregnancy test immediately before starting Tamoxifen. Talk to your doctor if you have any questions about how to begin taking Tamoxifen.
- Women who may become pregnant must use effective nonhormonal birth control (eg, condoms, diaphragm) during sexual intercourse while they are taking Tamoxifen and for 2 months after they stop taking it. Do not use hormonal birth control (eg, birth control pills); it may not work as well while you are taking Tamoxifen. Discuss any questions about effective nonhormonal birth control with your doctor.
- Tamoxifen will not prevent you from becoming pregnant.
- Lab tests, including breast exams, mammograms, gynecologic exams, complete blood counts, and liver function tests, may be performed while you use Tamoxifen. These tests may be used to monitor your condition or check for side effects. Be sure to keep all doctor and lab appointments.
- Tamoxifen should be used with extreme caution in children; safety and effectiveness in children have not been confirmed.
- Pregnancy and breast-feeding: Tamoxifen may cause harm to the fetus. Do not become pregnant while you are using it and for 2 months after you stop using it. If you think you may be pregnant, contact your doctor. You will need to discuss the benefits and risks of using Tamoxifen while you are pregnant. It is not known if Tamoxifen is found in breast milk. Do not breastfeed while taking Tamoxifen.
All medicines may cause side effects, but many people have no, or minor, side effects.
Check with your doctor if any of these most common side effects persist or become bothersome:
Bone pain; constipation; coughing; hot flashes; muscle pain; nausea; tiredness; vaginal discharge; weight loss.
Seek medical attention right away if any of these severe side effects occur:
Severe allergic reactions (rash; hives; itching; difficulty breathing; tightness in the chest; swelling of the mouth, face, lips, or tongue; unexplained hoarseness); abnormal menstrual periods; abnormal vaginal bleeding or bloody discharge; chest pain; coughing up blood; dark urine; decreased sexual desire or ability; depression; fever, chills, or persistent sore throat; groin or pelvic pain or pressure; loss of appetite; loss of balance or coordination; missed menstrual period; new or increased breast tumor or pain; new or unusual lumps; one-sided weakness; pain or swelling in one or both legs; red, swollen, blistered, or peeling skin; severe or persistent tiredness or weakness; shortness of breath; skin changes; stomach pain; sudden severe headache; swelling of the arms or the legs; unusual bleeding or bruising; vision or speech problems; yellowing of the eyes or skin.