Drug interactions
Phenobarbital + Promethazine
Severity: Major
Phenobarbital and Promethazine: 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
Phenobarbital is a long-acting barbiturate that causes sedation and respiratory depression. Combined with promethazine, CNS depression is increased, and phenobarbital enzyme induction may also modestly lower promethazine exposure.
What this interaction means in plain words
Phenobarbital and Promethazine 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.
Why it happens
Additive CNS depression via GABA-A potentiation and H1 blockade, plus CYP induction by phenobarbital.
What to do
Monitor sedation and respiratory status and use the lowest effective doses. Avoid additional CNS depressants.
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 Promethazine
- Ibutilide + Promethazine Contraindicated
- Pimozide + Promethazine Contraindicated
- Promethazine + Thioridazine Contraindicated
- Alfentanil + Promethazine Major
- Amiodarone + Promethazine Major
- Amisulpride + Promethazine Major
Frequently asked questions
Can I take Phenobarbital with Promethazine?
MediHelp rates the interaction as major. Phenobarbital and Promethazine 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. Phenobarbital is a long-acting barbiturate that causes sedation and respiratory depression. Combined with promethazine, CNS depression is increased, and phenobarbital enzyme induction may also modestly lower promethazine exposure.
How serious is the interaction between Phenobarbital and Promethazine?
MediHelp rates it major, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor sedation and respiratory status and use the lowest effective doses. Avoid additional CNS depressants.
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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