Drug interactions
Diphenhydramine + Trihexyphenidyl
Severity: Moderate
Diphenhydramine and Trihexyphenidyl: 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 have significant central antimuscarinic activity, so their combination markedly increases anticholinergic load. Confusion, hallucinations, sedation, urinary retention, constipation and tachycardia may result, particularly in older patients.
What this interaction means in plain words
Diphenhydramine and Trihexyphenidyl 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.
Diphenhydramine belongs to Respiratory; Trihexyphenidyl belongs to Central Nervous System.
Why it happens
Additive muscarinic receptor antagonism.
What to do
Avoid regular co-use in older adults and use a less anticholinergic alternative for allergy or sleep. Monitor mental status and bladder and bowel function if combined.
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 Trihexyphenidyl
Other interactions of Diphenhydramine
Frequently asked questions
Can I take Diphenhydramine with Trihexyphenidyl?
MediHelp rates the interaction as moderate. Diphenhydramine and Trihexyphenidyl 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 have significant central antimuscarinic activity, so their combination markedly increases anticholinergic load. Confusion, hallucinations, sedation, urinary retention, constipation and tachycardia may result, particularly in older patients.
How serious is the interaction between Diphenhydramine and Trihexyphenidyl?
MediHelp rates it moderate, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: Avoid regular co-use in older adults and use a less anticholinergic alternative for allergy or sleep. Monitor mental status and bladder and bowel function if combined.
What other medicines interact with Trihexyphenidyl?
MediHelp also lists interactions of Trihexyphenidyl with, for example, Orphenadrin, Oxybutynin, Scopolamine. The full list is on the Trihexyphenidyl page.
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