Overview
Tamoxifen is a selective estrogen receptor modulator (SERM) used primarily as adjuvant therapy for hormone receptor-positive breast cancer in pre- and postmenopausal women. It reduces the risk of recurrence and mortality by blocking estrogen's effects on breast tissue. It is also used for breast cancer risk reduction in high-risk individuals.
Mechanism of Action
Tamoxifen competitively inhibits estrogen binding to estrogen receptors in breast tissue, functioning as a selective estrogen receptor modulator (SERM). It acts as an estrogen antagonist in breast tissue while having partial agonist effects in other tissues like bone and endometrium.
Indications
- Adjuvant treatment of hormone receptor-positive breast cancer in women
- Reduction of breast cancer incidence in high-risk women
- Treatment of metastatic breast cancer in women and men
Common Doses
- 10 mg
- 20 mg
Dosage
For breast cancer, the recommended daily dose is 20-40 mg, with doses above 20 mg divided into morning and evening administration. Adjuvant therapy is typically continued for 5 years based on clinical trial evidence showing optimal benefit at this duration.
Contraindications
- Known hypersensitivity to tamoxifen or any tablet ingredients
- Concomitant use with anastrozole
- Pregnancy (Category D)
Side Effects
- Hot flashes, night sweats, vaginal discharge
- Endometrial changes including hyperplasia, polyps, and cancer
- Thromboembolic events
- Cataracts and visual disturbances
- Elevated liver enzymes
Interactions
- Coumarin-type anticoagulants: significant increase in anticoagulant effect requiring careful monitoring
- CYP3A4 inducers (rifampin, aminoglutethimide, phenobarbital): reduce tamoxifen plasma concentrations
- Anastrozole: concomitant use not recommended
- Letrozole: tamoxifen reduces letrozole plasma concentrations by 37%
Counseling Points
- Take at the same time(s) daily, with or without food
- Report any unusual vaginal bleeding, pelvic pain/pressure, or vision changes immediately
- Use effective non-hormonal contraception during treatment
- Regular gynecological exams are essential due to endometrial cancer risk
- Report signs of blood clots (leg pain/swelling, chest pain, shortness of breath)