Tamoxifen is a first-generation selective estrogen receptor modulator (SERM) and a cornerstone of endocrine therapy for hormone receptor-positive (HR+) breast cancer. It acts as a competitive antagonist of estrogen at its receptor in breast tissue, while exhibiting partial agonist activity in other tissues like bone and endometrium. In the Indian context, it is a widely used, cost-effective option for both early and advanced breast cancer, as well as for chemoprevention in high-risk individuals.
Adult: Breast Cancer (Adjuvant & Metastatic): 20 mg once daily. Some regimens use 10 mg twice daily. For chemoprevention or DCIS: 20 mg once daily for 5 years.
Note: Oral tablet. Can be taken with or without food. Should be taken at the same time each day. If a dose is missed, it should be taken as soon as remembered unless it is almost time for the next dose. Do not double the dose.
Tamoxifen is a non-steroidal agent with mixed agonist-antagonist activity. Its primary therapeutic action in breast cancer is competitive antagonism of estrogen binding to the estrogen receptor (ER) in breast tissue. This blockade inhibits estrogen-mediated gene transcription, leading to cell cycle arrest (G0/G1 phase) and apoptosis in ER-positive cancer cells.
Pregnancy: Pregnancy Category D. Tamoxifen is teratogenic and can cause fetal harm (craniofacial defects, ambiguous genitalia). Contraindicated. Women of childbearing potential must use effective non-hormonal contraception during and for at least 2 months after therapy.
Driving: May cause dizziness, visual disturbances, or fatigue. Patients should be cautioned about operating machinery or driving until they know how the drug affects them.
| Warfarin | Tamoxifen potentiates anticoagulant effect, increasing INR and risk of bleeding. | Major |
| Strong CYP2D6 Inhibitors (Paroxetine, Fluoxetine, Bupropion, Quinidine) | Reduce conversion to active endoxifen, potentially decreasing tamoxifen efficacy. | Major |
| CYP3A4 Inducers (Rifampicin, Phenytoin, Carbamazepine, St. John's Wort) | Increase tamoxifen metabolism, reducing plasma levels and efficacy. | Moderate |
| CYP3A4 Inhibitors (Ketoconazole, Itraconazole, Clarithromycin) | May increase tamoxifen levels, potentially increasing toxicity. | Moderate |
| SSRIs/SNRIs (especially CYP2D6 inhibitors) | See above. Use alternatives like venlafaxine or citalopram which are weak CYP2D6 inhibitors. | Moderate |
| Aromatase Inhibitors (Letrozole, Anastrozole, Exemestane) | Concomitant use is not recommended as it may antagonize effects; used sequentially, not concurrently. | Major |
| Cytotoxic Chemotherapy | Additive risk of thromboembolism and possibly thrombocytopenia. | Moderate |
Same composition (Tamoxifen (10mg)), different brands: