Drug interactions
Carbachol + Diphenhydramine
Severity: Minor
Carbachol and Diphenhydramine: minor 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
Carbachol is a cholinergic agonist, and diphenhydramine has anticholinergic activity through muscarinic receptor blockade. Diphenhydramine may weaken the effect of carbachol, for example on pupil constriction or intraocular pressure when carbachol is used in the eye. With intraocular use the systemic exposure is small, so the clinical effect is usually limited.
What this interaction means in plain words
Carbachol and Diphenhydramine have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised.
Carbachol belongs to Miscellaneous; Diphenhydramine belongs to Respiratory.
Why it happens
Pharmacodynamic antagonism at muscarinic receptors.
What to do
No routine change is needed. If the response to carbachol is weaker than expected, consider the antihistamine as a contributing factor.
What to ask your pharmacist
- Is there anything I should notice while taking both?
- Do I need to tell my doctor at the next visit?
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Other interactions of Diphenhydramine
Other interactions of Carbachol
- Atropine + Carbachol Moderate
- Bethanechol + Carbachol Moderate
- Carbachol + Darifenacin Moderate
- Carbachol + Dicyclomine Moderate
- Carbachol + Donepezil Moderate
- Carbachol + Flavoxate Moderate
Frequently asked questions
Can I take Carbachol with Diphenhydramine?
MediHelp rates the interaction as minor. Carbachol and Diphenhydramine have a small, usually unimportant interaction. Most people can take both; the pharmacist may still mention it so that any unexpected effect is recognised. Carbachol is a cholinergic agonist, and diphenhydramine has anticholinergic activity through muscarinic receptor blockade. Diphenhydramine may weaken the effect of carbachol, for example on pupil constriction or intraocular pressure when carbachol is used in the eye. With intraocular use the systemic exposure is small, so the clinical effect is usually limited.
How serious is the interaction between Carbachol and Diphenhydramine?
MediHelp rates it minor, on a four-level scale (minor, moderate, major, contraindicated) based on published labels and references. What to do, in brief: No routine change is needed. If the response to carbachol is weaker than expected, consider the antihistamine as a contributing factor.
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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