Tamoxifen is a prescription medicine that is used to treat breast cancer in men and women. It is in a class known as a selective estrogen receptor modulator (SERM) and is used to treat hormone receptor-positive breast cancer. It may also be used to help prevent breast cancer occurrence in women at high risk for breast cancer (for example, with a family history).

Tamoxifen works by blocking the action of estrogen, a female hormone. Some types of breast cancer require estrogen to grow.

Tamoxifen may cause wanted and unwanted effects. Although not all of these side effects may occur, if they do occur, some of them may require medical attention.

Due to the extensive list of side effects that may occur with tamoxifen, discuss your risk for any side effects with your healthcare provider before you start treatment.

US Boxed Warning

If you are using tamoxifen to lower your risk of getting breast cancer (for example, women with Ductal Carcinoma in Situ (DCIS) and women at high risk for breast cancer), discuss your risk for any side effects with your healthcare provider before you start treatment.

For women with Ductal Carcinoma in Situ (DCIS) and women at high risk for breast cancer, serious, life-threatening, and fatal events from use of tamoxifen may include:

• Uterine cancer (endometrial cancer and uterine sarcoma)
• Stroke
• Pulmonary embolism (blood clot in the lung)

Do not take tamoxifen to lower your chance of getting breast cancer if:

• you ever had a blood clot that needed medical treatment.
• you are taking medicines to thin your blood, like warfarin, (also called Coumadin).
• your ability to move around is limited for most of your waking hours.
• you are at risk for blood clots. Your doctor can tell you if you are at high risk for blood clots.
• you do not have a higher than normal chance of getting breast cancer. Your doctor can tell you if you are a high-risk woman.

The benefits of tamoxifen outweigh its risks in women already diagnosed with breast cancer.

What are the more common side effects of tamoxifen?

This medication is usually well-tolerated and most patients do not need to stop treatment due to side effects.

The most common tamoxifen side effects may include:

• Hot flashes
• Irregular periods or missing periods
• Vaginal discharge
• Vaginal bleeding
• Mood changes
• Nausea, Vomiting
• Fluid retention (edema or swelling)

If you have rust or brown-colored vaginal discharge or vaginal bleeding, contact your doctor right away.

This drug may cause you to miss periods (menstrual bleeding), but it is not used as a form of birth control.

Other common side effects include:

• Weight gain
• Weight loss
• Fatigue or tiredness
• Skin changes or rashes
• High blood pressure
• Ischemic heart disease
• Pain, back pain
• Depression
• Weakness
• Joint pain
• Sore throat (pharyngitis)
• Blood vessels dilate
• Chest pain
• Deep vein thrombosis (DVT)
• Venous thromboembolism
• Hair thinning or loss
• Excess sweating
• Changes in blood lipids and cholesterol
• Stomach pain
• Loss of appetite
• Constipation
• Diarrhea
• Heartburn
• Urinary tract infection
• Sepsis  (body’s extreme response to an infection)
• Low platelets (thrombocytopenia)
• Increased liver enzymes
• Allergies or hypersensitivity reactions
• Tingling sensation or numbness in extremities

What are some mild side effects of tamoxifen?

Call your doctor or get medical help if any of these side effects concern you or do not go away:

• Hot flashes or flushing
• Nausea, vomiting
• Constipation
• Weight gain
• Weight loss
• Dizziness
• Fatigue
• Weakness
• Bone or muscle pain
• Trouble sleeping or restlessness
• Anxiety

What are some serious side effects of tamoxifen? 

Like many drugs used for cancer, tamoxifen has many reported side effects, but those that are serious or deadly usually occur rarely. If you experience any serious side effects, contact your doctor right away.

• Increased risk of blood clots, such as deep vein thrombosis (DVT) and pulmonary embolism (blood clot in the lung)
• Stroke
• Abnormal vaginal bleeding or discharge, pain or pressure in your pelvis (below your belly button)
• Cataracts or increased chance of needing cataract surgery
• Liver problems, including jaundice, liver cancer (rare)
• A serious or life-threatening allergic reaction
• Depression
• ​​Low blood cell counts
• High blood pressure
• Infection, fever
• High calcium levels

Call your doctor right away if you have signs of a blood clot like chest pain or pressure, coughing up blood, shortness of breath; swelling, warmth, numbness, change of color, or pain in a leg or arm; or trouble speaking or swallowing.

Signs of a stroke can include: sudden weakness, tingling, or numbness in your face, arm or leg, especially on one side of your body, sudden confusion, trouble speaking or understanding, sudden trouble seeing in one or both eyes, sudden trouble walking, dizziness, loss of balance or coordination, sudden severe headache with no known cause.

If you are taking tamoxifen, tell your doctor if you have any abnormal vaginal bleeding or discharge, spotting (or rust-colored discharge), menstrual irregularities, or pelvic pain / pressure, as these are possible symptoms of uterine cancer (endometrial carcinoma, uterine sarcoma), which may be deadly.

Signs of liver problems may include dark urine, light-colored stools, loss of appetite, yellowing of your skin or whites of your eyes. Your doctor may check your liver function with blood tests.

Blurring of your vision that occurs over a period of time may mean you are developing cataracts. Inform your doctor of any changes in vision.

Low blood cell counts can cause bleeding problems, infections, or anemia. Call your healthcare provider right away if you have fever, chills, sore throat, unexplained bruising or bleeding, or if you feel very tired or weak.

Signs of high calcium may include weakness, confusion, feeling tired, headache, loss of appetite, increased thirst or urination, nausea, vomiting, weight loss, constipation, or bone pain.

Tell your doctor right away if you have any new breast lumps. Get your mammogram or other breast imaging done and do a breast self exam as directed by your healthcare provider.

Do not take tamoxifen if you are allergic to it or any inactive ingredients found in the medicine. Get emergency medical help right away if you have signs of an allergic reaction such as: rash, hives, itching; trouble breathing, swallowing, or talking; tightness in the chest or throat; wheezing; swelling of your face, lips, mouth tongue, or throat or anywhere on your body; red, swollen, blistered, or peeling skin with or without fever.

Other side effects or warnings

Less common side effects may include:

• Insomnia
• Dizziness
• Headache
• Weight gain
• Abdominal (stomach) pain
• Diarrhea
• Indigestion
• Urinary tract infections
• Thrombocytopenia (low blood platelet counts)
• Back pain
• Alopecia (hair thinning or hair loss)
• Cataracts
• Bone pain, tumor flare
• High blood calcium levels
• Loss of interest in sex or inability to get an erection (males)

Do not take tamoxifen if you take a coumarin-type anticoagulant (blood thinner) therapy (such as warfarin) or if you have had a deep-vein thrombosis (DVT) or a lung embolism (types of blood clots).

Tamoxifen may have a serious drug interaction with other medicines you take, which may lead to side effects. Tell your healthcare provider about all of your medicines, including prescription, over-the-counter, vitamins and herbal products. Do not start, stop, or change the dose of any drug you take without checking with your healthcare provider first.

Pregnancy / Breastfeeding

Tamoxifen can harm your unborn baby. Tell your healthcare provider if you are pregnant, plan to become pregnant, or breastfeeding before starting treatment with tamoxifen. A pregnancy test will be done before you start this drug to be sure you are not pregnant.

Do not become pregnant while taking tamoxifen or within 2 months of discontinuing tamoxifen. It takes about 2 months to clear tamoxifen from your body. If you get pregnant while taking tamoxifen citrate tablets, stop taking them right away and contact your doctor.

If you are of childbearing age, use a birth control method that does not use hormones (like a condom, diaphragms with spermicide, or plain IUDs) if sexually active to prevent pregnancy. If you are not sure what kind of birth control to use talk to your doctor.

It is not known if tamoxifen is excreted in human milk. Because of the potential for serious side effects in nursing infants, women taking tamoxifen should not breastfeed.

FAQs about tamoxifen’s side effects

Does tamoxifen cause eye problems?

Yes, tamoxifen has been reported as causing cataracts and may increase the need for cataract surgery. Tell your doctor right away if you have eye symptoms like blurred vision, trouble seeing at night, seeing halos around lights, or sensitivity to light.

Does tamoxifen cause hair loss?

Yes, hair loss and hair thinning (alopecia) can occur with tamoxifen. These side effects are reported infrequently. In clinical studies 5.2% of women taking tamoxifen reported alopecia compared to 4.4% of women taking a placebo (an inactive pill). However, some women report much higher levels of hair thinning. Hair loss due to tamoxifen use will usually be temporary and grow back when treatment is stopped.

This is not a complete list of side effects and others may occur. Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.