Tamoxifen: Effective Estrogen Receptor Modulation in Breast Cancer
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Synonyms
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Tamoxifen citrate is a selective estrogen receptor modulator (SERM) widely utilized in the management of hormone receptor-positive breast cancer. It functions by competitively inhibiting estrogen binding at receptor sites, thereby impeding the proliferative signaling that drives tumor growth in estrogen-sensitive malignancies. Its established efficacy spans both early and advanced disease settings, as well as risk reduction in high-risk populations, supported by decades of clinical evidence and extensive real-world use. This agent remains a cornerstone of endocrine therapy due to its favorable benefit-risk profile and oral administration.
Features
- Active ingredient: Tamoxifen citrate
- Standard dosage forms: 10 mg and 20 mg oral tablets
- Mechanism: Selective estrogen receptor modulator (SERM) with mixed agonist/antagonist effects
- Half-life: Approximately 5–7 days with active metabolites
- Bioavailability: Well-absorbed orally with peak plasma concentrations within 4–7 hours
- Metabolism: Hepatic, primarily via CYP2D6 and CYP3A4
Benefits
- Significantly reduces the risk of recurrence in early-stage, hormone receptor-positive breast cancer
- Lowers incidence of contralateral breast cancer in treated patients
- Provides effective palliative treatment for advanced or metastatic hormone-sensitive breast cancer
- Demonstrated efficacy in reducing breast cancer incidence in high-risk individuals
- Oral administration supports long-term adherence and outpatient management
- Well-established long-term safety profile with decades of clinical use
Common use
Tamoxifen is primarily indicated for the treatment of hormone receptor-positive breast cancer in both pre- and postmenopausal women. It is used as adjuvant therapy following primary treatment (surgery, radiation, or chemotherapy) in early-stage disease to prevent recurrence. In metastatic settings, it provides systemic control of estrogen-sensitive tumors. Additionally, it is approved for risk reduction in women with high lifetime risk of developing breast cancer, including those with lobular carcinoma in situ (LCIS) or atypical hyperplasia. Off-label uses include management of gynecomastia and infertility treatment in certain contexts.
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 indication and clinical response. For risk reduction in high-risk patients, 20 mg daily for 5 years is typically recommended. Tablets should be taken with or without food, preferably at the same time each day. Treatment duration for adjuvant therapy generally spans 5–10 years based on individual risk assessment. Hepatic impairment requires careful dose consideration and monitoring. No routine dosage adjustment is necessary for renal impairment.
Precautions
Patients should undergo comprehensive ophthalmologic examination at baseline and periodically during treatment due to risk of retinopathy and corneal changes. Regular gynecologic monitoring is essential for women with intact uteri due to increased risk of endometrial changes, including hyperplasia and malignancy. Liver function tests should be performed periodically. Bone mineral density monitoring is advised in premenopausal women due to potential estrogen antagonist effects on bone. Patients should be advised about potential increased risk of thromboembolic events, particularly during prolonged immobilization.
Contraindications
Tamoxifen is contraindicated in patients with known hypersensitivity to tamoxifen citrate or any component of the formulation. Concurrent use with anastrozole or other aromatase inhibitors is contraindicated. It should not be used during pregnancy (Pregnancy Category D) or in women who may become pregnant without adequate contraception. Additional contraindications include personal history of deep vein thrombosis or pulmonary embolism, and in some cases, concomitant warfarin therapy requiring careful risk-benefit assessment.
Possible side effects
Common side effects (≥10%) include hot flashes, vaginal discharge or dryness, and menstrual irregularities. Frequent adverse reactions (1–10%) comprise nausea, fatigue, skin rash, and headache. Serious but less common side effects (<1%) include endometrial cancer, thromboembolic events (deep vein thrombosis, pulmonary embolism), stroke, cataracts, and hepatotoxicity. Rare cases of hypercalcemia have been reported in patients with bone metastases upon initiation of therapy. Most side effects are dose-dependent and often manageable with symptomatic treatment or dose adjustment.
Drug interaction
Tamoxifen exhibits significant interactions with drugs affecting CYP2D6 metabolism. Strong CYP2D6 inhibitors (e.g., paroxetine, fluoxetine, quinidine) may reduce conversion to active metabolites, potentially diminishing efficacy. Warfarin coadministration requires frequent INR monitoring due to increased anticoagulant effect. Aromatase inhibitors may antagonize tamoxifen’s therapeutic effect. Drugs that induce CYP3A4 (e.g., rifampin, phenytoin) may reduce tamoxifen concentrations. Tamoxifen may enhance the thrombogenic potential of estrogen-containing therapies.
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. Doubling the dose to make up for a missed dose is not recommended. Patients should be advised to maintain a consistent dosing routine and use reminder systems if needed to support adherence, particularly given the long half-life of the medication.
Overdose
Acute overdose may manifest as tremor, hyperreflexia, unsteady gait, dizziness, or nausea. There is no specific antidote for tamoxifen overdose. Management should include supportive care with monitoring of vital signs and symptomatic treatment. Gastric lavage may be considered if presentation is early after ingestion. Given the drug’s long half-life, extended observation and monitoring may be necessary. Dialysis is not expected 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 use after the expiration date printed on the packaging. Do not transfer tablets to other containers unless they provide equivalent protection from moisture and light.
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. Actual clinical use may differ from described information based on evolving evidence, regional guidelines, and individual patient factors. Patients should consult their prescribing physician for personalized medical advice and report any adverse effects promptly.
Reviews
Tamoxifen maintains a favorable evaluation in clinical guidelines based on extensive evidence from multiple large-scale trials including NSABP B-14 and IBIS-I. Meta-analyses consistently demonstrate approximately 30–50% reduction in recurrence risk and 30% reduction in mortality in hormone receptor-positive early breast cancer. Long-term follow-up data confirm sustained benefit beyond active treatment period. The drug is generally well-tolerated with manageable side effect profile, though individual experiences may vary. Ongoing research continues to refine its role in treatment algorithms and identify biomarkers for optimized patient selection.