Drug interactions
Guanfacine + Promethazine
Severity: Moderate
Guanfacine and Promethazine: moderate 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
Both drugs can cause sedation and lower blood pressure, so using them together may produce excessive drowsiness, dizziness, hypotension and impaired alertness. Older or volume-depleted patients are particularly vulnerable.
What this interaction means in plain words
Guanfacine and Promethazine can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken.
Guanfacine belongs to Cardiovascular; Promethazine belongs to Respiratory.
Why it happens
Additive pharmacodynamic effects: central alpha-2 agonism by guanfacine plus H1 antagonism and alpha-blockade by promethazine increase sedation and hypotension.
What to do
Monitor blood pressure and level of sedation when starting or adjusting either drug, and counsel patients to rise slowly and avoid driving until response is known. Reduce the dose of one agent if symptomatic hypotension or oversedation occurs.
What to ask your pharmacist
- Should I take them at different times of day?
- Does the dose of either medicine need to change?
- Is there anything I should avoid, such as alcohol or certain foods, while on both?
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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
Other interactions of Guanfacine
- Clonidine + Guanfacine Major
- Guanfacine + Lofexidine Major
- Aldesleukin + Guanfacine Moderate
- Amiodarone + Guanfacine Moderate
- Amisulpride + Guanfacine Moderate
- Amitriptyline + Guanfacine Moderate
Frequently asked questions
Can I take Guanfacine with Promethazine?
MediHelp rates the interaction as moderate. Guanfacine and Promethazine can affect each other. The combination is often manageable, but it may need a change of dose, a change of timing, or extra checks while both are taken. Both drugs can cause sedation and lower blood pressure, so using them together may produce excessive drowsiness, dizziness, hypotension and impaired alertness. Older or volume-depleted patients are particularly vulnerable.
How serious is the interaction between Guanfacine and Promethazine?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Monitor blood pressure and level of sedation when starting or adjusting either drug, and counsel patients to rise slowly and avoid driving until response is known. Reduce the dose of one agent if symptomatic hypotension or oversedation occurs.
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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