Drug interactions
Promethazine + Quinidine
Severity: Major
Promethazine and Quinidine: major interaction.
Last updated Sources: UK SmPC
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
Quinidine is a well-known cause of QT prolongation and torsades de pointes, and promethazine can add to this effect. The combination raises the risk of life-threatening ventricular arrhythmias.
What this interaction means in plain words
Promethazine and Quinidine 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.
Promethazine belongs to Respiratory; Quinidine belongs to Anti Parasitic.
Why it happens
Additive QT prolongation through cardiac potassium channel blockade; quinidine may also inhibit CYP2D6, which metabolizes promethazine.
What to do
Prefer an alternative antihistamine or antiemetic with no QT effect. If both are required, monitor the ECG and keep potassium and magnesium within normal range.
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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References
Other interactions of Promethazine
- Ibutilide + Promethazine Contraindicated
- Pimozide + Promethazine Contraindicated
- Promethazine + Thioridazine Contraindicated
- Alfentanil + Promethazine Major
- Amiodarone + Promethazine Major
- Amisulpride + Promethazine Major
Other interactions of Quinidine
- Adagrasib + Quinidine Contraindicated
- Arsenic trioxide + Quinidine Contraindicated
- Artemether / lumefantrine + Quinidine Contraindicated
- Chloroquine + Quinidine Contraindicated
- Dofetilide + Quinidine Contraindicated
- Dronedarone + Quinidine Contraindicated
Frequently asked questions
Can I take Promethazine with Quinidine?
MediHelp rates the interaction as major. Promethazine and Quinidine 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 is a well-known cause of QT prolongation and torsades de pointes, and promethazine can add to this effect. The combination raises the risk of life-threatening ventricular arrhythmias.
How serious is the interaction between Promethazine and Quinidine?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Prefer an alternative antihistamine or antiemetic with no QT effect. If both are required, monitor the ECG and keep potassium and magnesium within normal range.
What other medicines interact with Promethazine?
MediHelp also lists interactions of Promethazine with, for example, Ibutilide, Pimozide, Thioridazine. The full list is on the Promethazine page.
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