Tamoxifen: Targeted Estrogen Receptor Modulation for Breast Cancer
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Synonyms
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Tamoxifen is a selective estrogen receptor modulator (SERM) widely utilized in the management of hormone receptor-positive breast cancer. It functions as an estrogen antagonist in breast tissue, competitively inhibiting estrogen binding to its receptors, thereby impeding the proliferation of malignant cells. Its well-established efficacy profile and extensive clinical history make it a cornerstone of endocrine therapy for both early and advanced disease stages. This agent is also indicated for risk reduction in high-risk populations, underscoring its dual therapeutic and preventive roles.
Features
- Chemical name: (Z)-2-[4-(1,2-Diphenylbut-1-enyl)phenoxy]-N,N-dimethylethanamine
- Molecular formula: C₂₆H₂₉NO
- Available as 10 mg and 20 mg oral tablets
- Brand and generic formulations available
- Typically administered as a once or twice daily regimen
- Stable at room temperature; does not require refrigeration
Benefits
- Significantly reduces the risk of recurrence in hormone receptor-positive early breast cancer
- Lowers incidence of contralateral breast cancer in treated patients
- Provides effective palliative treatment for metastatic breast cancer
- Offers risk reduction option for women at high risk of developing breast cancer
- Generally well-tolerated with a manageable side effect profile
- Oral administration facilitates long-term outpatient management
Common use
Tamoxifen is primarily indicated for the treatment of hormone receptor-positive breast cancer in both premenopausal and postmenopausal women. It is used as adjuvant therapy following primary treatment (surgery, radiation, chemotherapy) for early-stage disease to prevent recurrence. In metastatic settings, it provides systemic control of estrogen-sensitive tumors. Additionally, it is prescribed for ductal carcinoma in situ (DCIS) following surgery and radiation to reduce the risk of invasive cancer development. Off-label uses include management of gynecomastia and infertility in certain clinical scenarios.
Dosage and direction
The standard adult dosage for breast cancer treatment is 20 mg once daily, though doses may range from 20-40 mg daily based on disease stage and clinical response. For DCIS, the recommended dose is 20 mg daily for 5 years. Risk reduction in high-risk patients follows the same 20 mg daily regimen. Tablets should be taken with water, with or without food, at approximately the same time each day. Treatment duration typically spans 5-10 years based on individual risk assessment and clinical guidelines. Hepatic impairment requires careful dose consideration and monitoring.
Precautions
Regular monitoring of liver function tests is recommended due to potential hepatotoxicity. Annual gynecological examinations are essential for early detection of endometrial changes. Ophthalmologic evaluations should be conducted for patients reporting visual disturbances. Blood lipid profiles require monitoring as tamoxifen may alter lipid metabolism. Patients should be advised about the increased risk of thromboembolic events, particularly during periods of immobility. Calcium and vitamin D supplementation should be considered to mitigate bone density effects in premenopausal women.
Contraindications
Tamoxifen is contraindicated in patients with known hypersensitivity to the drug or any component of the formulation. Concurrent anticoagulant therapy with warfarin requires extreme caution due to potentiated anticoagulant effects. It should not be used during pregnancy (Pregnancy Category D) due to potential fetal harm. Patients with history of deep vein thrombosis or pulmonary embolism should generally avoid therapy. Those with pre-existing endometrial pathology may require alternative treatments. Concomitant use with strong CYP2D6 inhibitors may reduce efficacy and is generally discouraged.
Possible side effect
Common adverse effects include hot flashes (up to 80% of patients), vaginal discharge or dryness, and menstrual irregularities. Gastrointestinal disturbances such as nausea and vomiting occur in approximately 25% of patients. Less frequently, users may experience fatigue, dizziness, or headache. Serious side effects requiring immediate medical attention include unusual vaginal bleeding, leg pain or swelling suggestive of thrombosis, visual changes, and signs of hepatotoxicity (jaundice, dark urine). Endometrial cancer risk increases approximately 2-3 fold with long-term use.
Drug interaction
Tamoxifen interacts significantly with CYP2D6 inhibitors (e.g., paroxetine, fluoxetine) which may reduce conversion to active metabolites. Warfarin coadministration potentiates anticoagulant effect requiring frequent INR monitoring. Aromatase inhibitors may antagonize therapeutic effects when used concurrently. Drugs inducing CYP3A4 (e.g., rifampin, carbamazepine) may decrease tamoxifen concentrations. Estrogen-containing products counteract therapeutic effects and are contraindicated. Tamoxifen may reduce letrozole plasma concentrations by approximately 37% through induction of CYP2A6.
Missed dose
If a dose is missed, it should be taken as soon as remembered unless it is nearly time for the next scheduled dose. In that case, the missed dose should be skipped and the regular dosing schedule resumed. Patients should not double the dose to make up for a missed administration. Consistent daily adherence is important for maintaining stable drug levels and optimal therapeutic effect. If multiple doses are missed, medical consultation is recommended to assess need for dose adjustment or additional monitoring.
Overdose
Acute overdose may manifest as tremor, hyperreflexia, unsteady gait, dizziness, or seizures. QT prolongation and cardiac arrhythmias have been reported. There is no specific antidote for tamoxifen overdose. Management consists of supportive care including cardiac monitoring, antiarrhythmic therapy if indicated, and seizure control measures. Gastric lavage may be considered if presentation is early after ingestion. Activated charcoal can be administered to reduce absorption. Dialysis is unlikely to be effective due to high protein binding and extensive tissue distribution.
Storage
Store at controlled room temperature (20-25°C or 68-77°F) in the original container. Protect from light and moisture. Keep tightly closed and out of reach of children. Do not store in bathroom areas where humidity fluctuations may occur. Discard any medication that appears discolored or shows signs of degradation. Proper disposal through medication take-back programs is recommended to prevent environmental contamination and accidental ingestion.
Disclaimer
This information is provided for educational purposes only and does not constitute medical advice. Treatment decisions should be made exclusively by qualified healthcare professionals based on individual patient characteristics. The prescribing physician should be consulted for specific dosage recommendations and therapeutic monitoring. Actual clinical effects may vary among patients. Full prescribing information including boxed warnings should be reviewed before initiation of therapy.
Reviews
“Twenty-year follow-up data from the NSABP B-14 trial continues to demonstrate durable benefit with tamoxifen therapy, showing persistent reduction in recurrence risk years after treatment completion.” - Journal of Clinical Oncology
“Meta-analysis of 55 randomized trials confirms 39% reduction in annual recurrence risk and 31% mortality reduction with 5 years of adjuvant tamoxifen in ER-positive disease.” - The Lancet
“The IBIS-I prevention trial demonstrated 32% risk reduction in high-risk women with 5 years of tamoxifen, though benefit persists for at least 20 years following treatment.” - Journal of the National Cancer Institute