What is Tamoxifen?

Serious and life-threatening uterine malignancies, stroke, and pulmonary embolism have been associated with tamoxifen use in the risk reduction setting (women with Ductal Carcinoma in Situ (DCIS) and women at high risk for breast cancer). Some of these adverse events were fatal. Health care providers should discuss the potential benefits versus the potential risks of these serious events with women at high risk of breast cancer and women with DCIS considering tamoxifen to reduce their risk of developing breast cancer. The benefits of tamoxifen outweigh its risks in women already diagnosed with breast cancer .

Tamoxifen is a medicine that blocks the effects of the estrogen hormone in the body. It is used to treat breast cancer in women or men. It may also be used to treat other kinds of cancer, as determined by your doctor.

Tamoxifen also may be used to reduce the risk of developing breast cancer in women who have a high risk of developing breast cancer. Women at high risk for developing breast cancer are at least 35 years of age and have a combination of risk factors that make their chance of developing breast cancer 1.67% or more over the next 5 years. Your doctor will help to determine your risk of developing breast cancer.

The following are risk factors that may increase your chance of developing breast cancer:

• If you have close family members (mother, sister, or daughter) with breast cancer.
• If you have ever had a breast biopsy or if high-risk changes in your breast(s) have been found from a breast biopsy.
• If you have never been pregnant or if your first pregnancy occurred at a late age.
• If your first menstrual period occurred at an early age.

The exact way that tamoxifen works against cancer is not known, but it may be related to the way it blocks the effects of estrogen on the body.

Before you begin treatment with tamoxifen, you and your doctor should talk about the good this medicine will do as well as the risks of using it.

Tamoxifen is available only with your doctor's prescription.

Use this medicine only as directed by your doctor. Do not use more or less of it, and do not use it more often than your doctor ordered. The exact amount of medicine you need has been carefully worked out. Taking too much may increase the chance of side effects, while taking too little may not improve your condition.

Tamoxifen sometimes causes mild nausea and vomiting. However, it may have to be taken for several weeks or months to be effective. Even if you begin to feel ill, do not stop using this medicine without first checking with your doctor. Ask your health care professional for ways to lessen these effects.

Swallow the tablets whole with a drink of water. You can take the tablets with or without food.

If you vomit shortly after taking a dose of tamoxifen, check with your doctor. You will be told whether to take the dose again or to wait until the next scheduled dose.

The dose of this medicine will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of this medicine. If your dose is different, do not change it unless your doctor tells you to do so.

The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.

• For oral dosage form (tablets):
  • For breast cancer in women or men:
    • Adults—20 to 40 milligrams (mg) daily.
    • Children—Use and dose must be determined by your doctor.
  • For reducing the risk of developing breast cancer in high-risk women:
    • Adults—20 milligrams (mg) a day for five years.
    • Children—Use and dose must be determined by your doctor.
  • For reducing the risk of developing invasive breast cancer in women with ductal carcinoma in situ:
    • Adults—20 milligrams (mg) a day for five years.
    • Children—Use and dose must be determined by your doctor.

If you miss a dose of this medicine, skip the missed dose and go back to your regular dosing schedule. Do not double doses.

Call your doctor or pharmacist for instructions.

Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.

Keep out of the reach of children.

Do not keep outdated medicine or medicine no longer needed.

Ask your healthcare professional how you should dispose of any medicine you do not use.

In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For this medicine, the following should be considered:

Tell your doctor if you have ever had any unusual or allergic reaction to this medicine or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.

Many medicines have not been studied specifically in older people. Therefore, it may not be known whether they work exactly the same way they do in younger adults. Although there is no specific information comparing use of tamoxifen in the elderly with use in other age groups, this medicine is not expected to cause different side effects or problems in older people than it does in younger adults.

There are no adequate studies in women for determining infant risk when using this medication during breastfeeding. Weigh the potential benefits against the potential risks before taking this medication while breastfeeding.

Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking this medicine, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.

Using this medicine with any of the following medicines is not recommended. Your doctor may decide not to treat you with this medication or change some of the other medicines you take.

• Amifampridine
• Amisulpride
• Bepridil
• Cisapride
• Dronedarone
• Fluconazole
• Ketoconazole
• Mesoridazine
• Nelfinavir
• Pimozide
• Piperaquine
• Posaconazole
• Saquinavir
• Sparfloxacin
• Terfenadine
• Thioridazine
• Warfarin
• Ziprasidone

Using this medicine with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

• Abiraterone Acetate
• Acenocoumarol
• Amiodarone
• Anagrelide
• Aripiprazole
• Aripiprazole Lauroxil
• Arsenic Trioxide
• Buprenorphine
• Buserelin
• Capecitabine
• Ceritinib
• Chloroquine
• Chlorpromazine
• Clarithromycin
• Clobazam
• Clozapine
• Cobicistat
• Conivaptan
• Crizotinib
• Cyclophosphamide
• Dabrafenib
• Darunavir
• Dasabuvir
• Dasatinib
• Degarelix
• Delamanid
• Desipramine
• Deslorelin
• Deutetrabenazine
• Dicumarol
• Domperidone
• Donepezil
• Doxifluridine
• Efavirenz
• Encorafenib
• Escitalopram
• Fingolimod
• Fluorouracil
• Fluoxetine
• Fluphenazine
• Fluvoxamine
• Foscarnet
• Fosnetupitant
• Fosphenytoin
• Genistein
• Gonadorelin
• Goserelin
• Histrelin
• Hydroxychloroquine
• Hydroxyzine
• Idelalisib
• Inotuzumab Ozogamicin
• Ipriflavone
• Ivabradine
• Ivosidenib
• Leuprolide
• Levofloxacin
• Lofexidine
• Macimorelin
• Methadone
• Methotrexate
• Metronidazole
• Mitomycin
• Moxifloxacin
• Nafarelin
• Netupitant
• Nilotinib
• Ondansetron
• Osimertinib
• Panobinostat
• Paroxetine
• Pasireotide
• Pazopanib
• Phenprocoumon
• Phenytoin
• Pimavanserin
• Pitolisant
• Quetiapine
• Red Clover
• Ribociclib
• Ritonavir
• Sertraline
• Sevoflurane
• Sotalol
• St John's Wort
• Sulpiride
• Tacrolimus
• Tegafur
• Triptorelin
• Vandetanib
• Vemurafenib
• Vinflunine
• Zuclopenthixol

Using this medicine with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.

• Aldesleukin
• Aminoglutethimide
• Anastrozole
• Bexarotene
• Letrozole
• Rifampin

Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. Discuss with your healthcare professional the use of your medicine with food, alcohol, or tobacco.

The presence of other medical problems may affect the use of this medicine. Make sure you tell your doctor if you have any other medical problems, especially:

For all patients

• Blood problems or
• Cataracts or other eye problems—Tamoxifen may also cause these problems.

• High cholesterol levels in the blood—Tamoxifen can increase cholesterol levels.

When used for reducing the risk for developing breast cancer in high-risk women or in women with Ductal Carcinoma in Situ (DCIS)

• Blood clots (or history of) or
• Pulmonary embolism (or history of) or
• Stroke or
• Uterine (womb) cancer—May increase risk of serious side effects from tamoxifen.

It is very important that your doctor check your progress at regular visits to make sure that this medicine is working properly and to check for unwanted effects.

A woman should contact her doctor right away if she develops:

• Changes in vaginal discharge or
• Changes in vision or
• Coughing up blood or
• Leg swelling or tenderness or
• Menstrual irregularities or
• New breast lumps or
• Pelvic pain or pressure or
• Sudden chest pain or
• Unexplained shortness of breath or
• Vaginal bleeding

If you seek medical attention for any reason, be sure to tell your doctor that you take tamoxifen or have taken tamoxifen.

For women: Tamoxifen may make you more fertile. It is best to use some type of birth control while you are taking it. However, do not use oral contraceptives (“the Pill”) since they may change the effects of tamoxifen. Tell your doctor right away if you think you have become pregnant while taking this medicine.