Tamoxifen Citrate

Introduction

Written by Rosanna Sutherby, PharmD
Medically reviewed by Alex Brewer, PharmD, MBA
Updated on August 14, 2023

Tamoxifen (ta MOKS I fen) is a generic medication prescribed to treat and help lower the risk of certain types of breast cancer. Tamoxifen belongs to a group of drugs called antiestrogens.

The Food and Drug Administration (FDA) has approved tamoxifen to:

• treat breast cancer that has spread to other parts of the body in males* and females*
• help reduce the risk of breast cancer from returning after surgery, chemotherapy, or radiation therapy in females
• help prevent cancer in the milk ducts from spreading to other parts of the breast after surgery or radiation treatment in females
• help reduce the risk of breast cancer in females with a high risk of developing breast cancer

* Sex and gender exist on spectrums. In this article, we use the terms “male” and “female” to refer to a person’s sex assigned at birth.

Boxed warning: Risk of uterine cancer, stroke, or certain blood clots in some people

Tamoxifen has a boxed warning about the risk of uterine cancer, stroke, or certain blood clots in some people. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA).

For details, see the “Interactions and warnings for tamoxifen” section.

Side effects of tamoxifen

Tamoxifen may cause mild or serious side effects. More common mild side effects of tamoxifen and its serious side effects are listed below. This article doesn’t include all possible side effects of the drug. Side effects can vary based on your age, overall health, and any other medications you take.

To learn more about tamoxifen’s side effects, ask your doctor or pharmacist. You can also refer to the prescribing information for tamoxifen listed in the “Resources” section.

Mild side effects

More common mild side effects reported with tamoxifen are listed below.

This article doesn’t include all possible mild side effects of the drug. To learn more about tamoxifen’s side effects, refer to the prescribing information for tamoxifen listed in the “Resources” section.

With many drugs, mild side effects may go away within a few days or a couple of weeks. If the side effects are bothersome, tell your doctor or pharmacist. They may be able to suggest ways to manage them.

Tamoxifen oral tablet’s mild side effects include:

• hot flashes
• irregular periods
• nausea
• fluid retention (swelling)
• mild allergic reaction*

* For details about this side effect, see the “Interactions and warnings for tamoxifen” section.

Serious side effects

Serious side effects of tamoxifen are listed below.

This article doesn’t include all possible serious side effects of the drug. To learn more about tamoxifen’s side effects, refer to the prescribing information for tamoxifen listed in the “Resources” section.

With many drugs, serious side effects are possible but not common. If you have serious side effects from this drug, call your doctor right away. If you’re having severe symptoms or a medical emergency, call 911 or a local emergency number.

In general, tamoxifen’s serious side effects include:

• high blood calcium level
• changes in the lining of your uterus, including thickening of the lining
• eye problems, such as cataracts
• liver problems, including hepatitis (liver inflammation) and liver cancer
• boxed warnings: risk of uterine cancer, stroke, or certain blood clots in some people*
• severe allergic reaction*

* For details about this side effect, see the “Interactions and warnings for tamoxifen” section.

Common questions about tamoxifen

Below you’ll find answers to a few commonly asked questions about tamoxifen.

What is tamoxifen’s mechanism of action? How does the drug work?

Tamoxifen works by blocking the effects of estrogen in the breast. Estrogen is a hormone that helps regulate the menstrual cycle and the reproductive system. It also plays a role in breast development.

With some types of cancer, including certain breast cancers, estrogen causes tumors to grow. Tamoxifen blocks estrogen from attaching to breast cancer tumors. This prevents the tumors from growing.

It may take 4–8 weeks to get the full effect of tamoxifen.

If you’d like to know more about this, talk with your doctor or pharmacist.

Can tamoxifen cause certain side effects after 5 years?

Possibly. Tamoxifen’s side effects are the same whether you take it for 5 years or longer. However, your risk of certain side effects may increase if you take it for more than 5 years.

In a large study, people who took tamoxifen for 10 years had a higher risk of uterine cancer than those who stopped treatment after 5 years.

However, this study also found that taking tamoxifen for 10 years can reduce the risk of death due to breast cancer by half compared with taking the drug for 5 years. The reduced risk of death from breast cancer was greater than the increased risk of uterine cancer in people who took tamoxifen for 10 years.

If you have questions about taking tamoxifen for longer than 5 years, talk with your doctor.

Is tamoxifen a chemotherapy?

No, tamoxifen is not chemotherapy. It is endocrine therapy, also known as hormone therapy.

Endocrine therapy can be used with other treatments, including chemotherapy, to help slow or stop breast cancer tumors from growing.

If you’d like to know more about this, talk with your doctor or pharmacist.

Uses of tamoxifen

The Food and Drug Administration (FDA) approves prescription drugs such as tamoxifen for certain conditions. Approved uses for tamoxifen are described below.

Use for breast cancer

Doctors may prescribe tamoxifen tablets for treating breast cancer that has spread to other parts of the body. It’s used for this purpose in adult males* and females.*

Doctors may also prescribe tamoxifen to adult females for the following uses:

• help reduce the risk of breast cancer from returning after treatment with surgery, chemotherapy, or radiation
• help prevent cancer in the milk ducts from spreading to other parts of the breast after surgery or radiation therapy
• help reduce the risk of breast cancer in females with a high risk of developing breast cancer in the next 5 years

Tamoxifen is usually prescribed for 5 years.

With breast cancer, certain gene mutations cause breast cells to grow uncontrollably. Cells growing uncontrollably can group together to form a tumor. They can also spread to other parts of the body and take over healthy cells.

Typically, the first symptom of breast cancer many people notice is a lump in the breast. Other symptoms can include:

• inverted nipple
• breast pain
• discolored, pitted skin on the breast
• swelling or a lump under the arm

Doctors may not prescribe tamoxifen tablets for females with cancer in their milk ducts who also:

• take the blood thinner warfarin
• have had certain blood clots in the past

* Sex and gender exist on spectrums. In this article, we use the terms “male” and “female” to refer to a person’s sex assigned at birth.

Interactions and warnings for tamoxifen

Below, you’ll find information about tamoxifen’s possible interactions and warnings.

Interactions of tamoxifen

Certain things may affect how a drug works. These include getting vaccines, consuming alcohol or certain foods, or taking the drug with other medications. This effect is called a drug interaction.

Before you take tamoxifen, ask your doctor to check for possible interactions. Be sure to tell them about any of the following you take or use:

• prescription or over-the-counter drugs
• vitamins, herbs, or supplements

To learn about drug-condition interactions, see the “Warnings for tamoxifen” section below.

Warnings for tamoxifen

Some people should not take tamoxifen, and others should take it cautiously.

Boxed warning: Risk of uterine cancer, stroke, or certain blood clots in some people

Tamoxifen has a boxed warning about the risk of uterine cancer, stroke, or certain blood clots in females* taking this drug to help reduce the risk of breast cancer from spreading or coming back. Boxed warnings are the most serious warnings from the Food and Drug Administration (FDA).

Uterine cancer is cancer that starts in the inner lining of your uterus. Taking tamoxifen may increase the risk of uterine cancer. This side effect was rarely reported in studies of tamoxifen.

Symptoms may include abnormal vaginal bleeding or bloody vaginal discharge.

Let your doctor know if you think you have symptoms of uterine cancer. They can review your treatment and determine possible causes.

Tamoxifen may also increase your risk of stroke or certain blood clots. This side effect was reported rarely in tamoxifen studies.

Some symptoms of stroke include:

• difficulty speaking or understanding others
• numbness, especially on one side of the face or body
• vision problems

Stroke is a medical emergency requiring immediate treatment. Call 911 or go to the closest emergency room if you think you’re having symptoms of a stroke.

Some blood clot symptoms in your lungs or veins to watch for include:

• trouble breathing
• pain in your chest
• pain or tenderness in your leg
• skin on your leg that is warm to the touch

If you experience trouble breathing, sudden shortness of breath, or chest pressure, this may indicate a blood clot in your lungs. Immediately call 911 or go to the closest emergency room.

For other symptoms of a blood clot described above, call your doctor right away. Do not delay seeking medical attention if you feel your blood clot symptoms are severe or feel life threatening.

To learn more, talk with your doctor or pharmacist.

* Sex and gender exist on spectrums. In this article, we use the terms “male” and “female” to refer to a person’s sex assigned at birth.

Other warnings

Tamoxifen could cause harm to people with certain health conditions. This effect is called a drug-condition interaction. Other factors can also affect whether tamoxifen is a safe option for you.

Ask your doctor about specific warnings for tamoxifen, and be sure to tell your doctor about your:

• current health, including any allergies to medications
• past health conditions or surgeries

Allergic reaction

Tamoxifen can cause an allergic reaction in some people. Severe allergic reactions are rare but possible.

If you’ve had an allergic reaction to tamoxifen or any of its ingredients, your doctor will likely not prescribe tamoxifen. They can tell you about medications that are safer options for you.

A mild allergic reaction may cause the following symptoms:

• skin rash
• itchiness
• flushing (temporary warmth, redness, or deepening of skin color)

A severe allergic reaction may cause the following symptoms:

• swelling under your skin, usually in your hands, feet, eyelids, or lips
• swelling of your mouth, throat, or tongue, which can cause breathing problems

If you have an allergic reaction to tamoxifen, call your doctor right away. If you have severe symptoms, call 911 or a local emergency number.

Overdose of tamoxifen

If you take tamoxifen by mouth, you should not take more tamoxifen than your doctor prescribes. For some drugs, doing so may lead to harmful effects or overdose.

If you think you’ve taken too much tamoxifen, call your doctor or pharmacist right away. Or you could call 800-222-1222 to speak with someone at America’s Poison Centers. You can also use its online resource. If you have concerning symptoms, call 911 or a local emergency number immediately. You can also go to the closest emergency room.

Pregnancy or breastfeeding and tamoxifen

Information about tamoxifen and pregnancy and breastfeeding is described below.

Tamoxifen and pregnancy

Tamoxifen should not be taken during pregnancy. If you’re planning a pregnancy or can become pregnant, talk with your doctor before taking this medication. Your doctor may suggest birth control options to use during treatment with tamoxifen.

Tamoxifen and breastfeeding

Tamoxifen should not be taken while breastfeeding. If you’re breastfeeding or planning to do so, talk with your doctor before taking this medication. Your doctor may recommend other ways to feed your child during treatment with tamoxifen.

What to ask your doctor

Let your doctor know if you have questions about tamoxifen or would like more details about it.

Here’s a list of some possible questions you may want to ask your doctor:

• Can my breast cancer come back after I finish tamoxifen treatment?
• Can I take tamoxifen if I take a different blood thinner than warfarin?
• Can I take tamoxifen for less than 5 years?
• Can you prescribe other medications to treat my condition?

Resources

• Davies C, et al. (2012). Long-term effects of continuing adjuvant tamoxifen to 10 years versus stopping at 5 years after diagnosis of oestrogen receptor-positive breast cancer: ATLAS, a randomised trial. https://www.thelancet.com/journals/lanonc/article/PIIS0140-6736(12)61963-1/fulltext
• Food and Drug Administration. (2023). Orange Book: Approved drug products with therapeutic equivalence evaluations. https://www.accessdata.fda.gov/scripts/cder/ob/index.cfm
• Tamoxifen citrate tablet, film coated. (2022). https://nctr-crs.fda.gov/fdalabel/services/spl/set-ids/cadef490-5dc6-7230-e053-2a95a90a5551/spl-doc

Disclaimer: Optum Perks has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.