Drug interactions
Diphenhydramine + Homatropine
Severity: Moderate
Diphenhydramine and Homatropine: 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 antimuscarinic activity, so combined use increases dry mouth, constipation, urinary retention, blurred vision and confusion. Older adults are particularly susceptible.
What this interaction means in plain words
Diphenhydramine and Homatropine 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; Homatropine belongs to Genitourinary.
Why it happens
Additive antimuscarinic receptor blockade.
What to do
Monitor for anticholinergic effects and use the lowest effective doses. Avoid in older adults, and in patients with glaucoma or urinary retention.
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 Diphenhydramine
Other interactions of Homatropine
- Amantadine + Homatropine Moderate
- Amitriptyline + Homatropine Moderate
- Atropine + Homatropine Moderate
- Benztropine + Homatropine Moderate
- Bethanechol + Homatropine Moderate
- Chlorpromazine + Homatropine Moderate
Frequently asked questions
Can I take Diphenhydramine with Homatropine?
MediHelp rates the interaction as moderate. Diphenhydramine and Homatropine 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 antimuscarinic activity, so combined use increases dry mouth, constipation, urinary retention, blurred vision and confusion. Older adults are particularly susceptible.
How serious is the interaction between Diphenhydramine and Homatropine?
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 for anticholinergic effects and use the lowest effective doses. Avoid in older adults, and in patients with glaucoma or urinary retention.
What other medicines interact with Diphenhydramine?
MediHelp also lists interactions of Diphenhydramine with, for example, Alfentanil, Belladonna and opium, Benzhydrocodone / acetaminophen. The full list is on the Diphenhydramine page.
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