Tamoxifen exerts its therapeutic effects by selectively modulating estrogen receptors (ERs) in different tissues. Its mechanism of action includes:

1. Estrogen Receptor Antagonism in Breast Tissue:
   • Tamoxifen competitively binds to estrogen receptors in breast tissue, blocking the binding of endogenous estrogen.
   • This inhibits estrogen-mediated gene transcription, preventing the growth and proliferation of estrogen-dependent breast cancer cells.
2. Estrogen Receptor Agonism in Other Tissues:
   • In bone tissue, tamoxifen acts as an estrogen agonist, helping to maintain bone density and reduce the risk of osteoporosis.
   • In the endometrium (uterine lining), tamoxifen has partial estrogenic effects, which can lead to endometrial thickening and, in rare cases, increase the risk of endometrial cancer.
3. Reduction of Circulating Estrogen Levels:
   • Tamoxifen indirectly reduces estrogen levels by inhibiting the release of prolactin and gonadotropins, which are involved in estrogen production.

Tamoxifen is primarily used for the following conditions:

1. Breast Cancer Treatment:
   • Adjuvant Therapy: Used after surgery, radiation, or chemotherapy to reduce the risk of cancer recurrence in premenopausal and postmenopausal women with ER+ breast cancer.
   • Metastatic Breast Cancer: Used to treat advanced or metastatic breast cancer in both men and women.
2. Breast Cancer Prevention:
   • Used to reduce the risk of developing breast cancer in high-risk individuals, such as those with a strong family history or genetic predisposition (e.g., BRCA mutations).
3. Ductal Carcinoma in Situ (DCIS):
   • Used to reduce the risk of invasive breast cancer in women with DCIS, a non-invasive form of breast cancer.
4. Off-Label Uses:
   • Infertility Treatment: Used to induce ovulation in women with anovulatory disorders.
   • Gynecomastia: Used to treat or prevent breast enlargement in men, often due to hormone therapy or steroid use.

The dosage of tamoxifen varies depending on the condition being treated. Always follow the prescribing physician's instructions.

Common Dosages:

• Breast Cancer Treatment: 20 mg orally once daily, with or without food. Treatment duration is typically 5–10 years.
• Breast Cancer Prevention: 20 mg orally once daily for up to 5 years.
• DCIS: 20 mg orally once daily for 5 years.
• Off-Label Uses: Dosages may vary; consult a healthcare provider for specific recommendations.

Administration Tips:

• Take the medication at the same time each day to maintain consistent levels in the body.
• Do not crush or chew the tablet; swallow it whole with water.
• For long-term use, regular monitoring (e.g., bone density scans, endometrial evaluations) is recommended.

Tamoxifen is generally well-tolerated, but some patients may experience side effects. These can include:

Common Side Effects:

• Hot flashes
• Nausea
• Fatigue
• Vaginal discharge or dryness
• Menstrual irregularities
• Mood changes

Rare but Serious Side Effects:

• Endometrial Cancer: Increased risk due to estrogenic effects on the uterus.
• Thromboembolic Events: Increased risk of blood clots, stroke, or pulmonary embolism.
• Ocular Toxicity: Cataracts or retinopathy.
• Liver Dysfunction: Elevated liver enzymes or rare cases of hepatitis.

If any severe side effects occur, discontinue use and seek medical attention immediately.

Before using tamoxifen, inform your healthcare provider if you have:

• A history of blood clots or thromboembolic disorders
• Liver disease
• Endometrial abnormalities or a history of endometrial cancer
• High cholesterol levels
• Allergies to tamoxifen or any of its components

Special Populations:

• Pregnancy: Tamoxifen is contraindicated during pregnancy as it may harm the fetus. It is classified as Pregnancy Category D.
• Breastfeeding: Tamoxifen is not recommended for use during breastfeeding.
• Elderly Patients: Use with caution due to the potential for increased side effects, particularly bone loss and thromboembolic events.

tamoxifen may interact with other medications, altering their effectiveness or increasing the risk of side effects. Notable interactions include:

1. CYP2D6 Inhibitors:
   • Drugs like paroxetine, fluoxetine, and quinidine may reduce the conversion of tamoxifen to its active metabolite, endoxifen, decreasing its efficacy.
2. Anticoagulants:
   • Tamoxifen may increase the risk of bleeding when used with anticoagulants like warfarin.
3. Estrogen-Containing Therapies:
   • Estrogen-containing medications (e.g., hormone replacement therapy) may counteract the effects of tamoxifen.
4. Aromatase Inhibitors:
   • Concurrent use with aromatase inhibitors (e.g., letrozole, anastrozole) is generally avoided due to opposing mechanisms of action.
5. CYP3A4 Inducers:
   • Drugs like rifampicin and phenytoin may reduce tamoxifen levels by increasing its metabolism.

Always inform your healthcare provider about all medications, supplements, and herbal products you are taking.

Generic Names in Pharmacy

• Tamoxifen (the active ingredient in Nolvadex)
• Tamoxifen citrate (the salt form commonly used in medications)