Drug interactions
Quinidine + Tamoxifen
Severity: Major
Quinidine and Tamoxifen: major interaction.
Last updated
For healthcare professionals — reference only. This page is a brief summary, not medical advice and not a substitute for official labeling or clinical judgement. Always verify before prescribing or dispensing.
Summary
Tamoxifen is converted to its active metabolite endoxifen mainly by CYP2D6. Quinidine strongly inhibits CYP2D6, so endoxifen concentrations fall, which may reduce the anticancer benefit of tamoxifen.
What this interaction means in plain words
Quinidine and Tamoxifen can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for.
Quinidine belongs to Anti Parasitic; Tamoxifen belongs to Endocrine.
Why it happens
Quinidine strongly inhibits CYP2D6, reducing the conversion of tamoxifen to endoxifen.
What to do
Avoid quinidine in patients taking tamoxifen for breast cancer if possible, and choose an alternative antiarrhythmic. If an inhibitor is unavoidable, consider another endocrine therapy such as an aromatase inhibitor where appropriate.
What to ask your pharmacist
- Does the benefit of taking both outweigh the risk in my case?
- Do I need any blood test, ECG or other check while on both?
- What early warning signs should I watch for, and what should I do if they appear?
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Other interactions of Quinidine
- Adagrasib + Quinidine Contraindicated
- Arsenic trioxide + Quinidine Contraindicated
- Artemether / lumefantrine + Quinidine Contraindicated
- Chloroquine + Quinidine Contraindicated
- Dofetilide + Quinidine Contraindicated
- Dronedarone + Quinidine Contraindicated
Other interactions of Tamoxifen
- Amiodarone + Tamoxifen Major
- Anastrozole + Tamoxifen Major
- Bupropion + Tamoxifen Major
- Carbamazepine + Tamoxifen Major
- Cinacalcet + Tamoxifen Major
- Dacomitinib + Tamoxifen Major
Frequently asked questions
Can I take Quinidine with Tamoxifen?
MediHelp rates the interaction as major. Quinidine and Tamoxifen can interact in a way that may be serious. Using them together is sometimes necessary, but it needs a deliberate decision by the prescriber, a plan for monitoring, and clear advice on the warning signs to watch for. Tamoxifen is converted to its active metabolite endoxifen mainly by CYP2D6. Quinidine strongly inhibits CYP2D6, so endoxifen concentrations fall, which may reduce the anticancer benefit of tamoxifen.
How serious is the interaction between Quinidine and Tamoxifen?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid quinidine in patients taking tamoxifen for breast cancer if possible, and choose an alternative antiarrhythmic. If an inhibitor is unavoidable, consider another endocrine therapy such as an aromatase inhibitor where appropriate.
What other medicines interact with Quinidine?
MediHelp also lists interactions of Quinidine with, for example, Adagrasib, Arsenic trioxide, Artemether / lumefantrine. The full list is on the Quinidine page.
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