Drug interactions
Diphenhydramine + Olanzapine
Severity: Moderate
Diphenhydramine and Olanzapine: 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 are sedating and anticholinergic, so combined use can cause marked drowsiness, confusion, constipation, urinary retention and tachycardia. Older patients are particularly vulnerable to delirium and falls. Injectable olanzapine given with parenteral diphenhydramine may increase oversedation.
What this interaction means in plain words
Diphenhydramine and Olanzapine 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; Olanzapine belongs to Central Nervous System.
Why it happens
Additive H1 antihistaminic and antimuscarinic effects leading to CNS depression and anticholinergic toxicity.
What to do
Monitor for oversedation and anticholinergic effects and use the lowest effective doses. Avoid in frail elderly patients and consider a non-sedating antihistamine.
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 Olanzapine
Other interactions of Diphenhydramine
Frequently asked questions
Can I take Diphenhydramine with Olanzapine?
MediHelp rates the interaction as moderate. Diphenhydramine and Olanzapine 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 are sedating and anticholinergic, so combined use can cause marked drowsiness, confusion, constipation, urinary retention and tachycardia. Older patients are particularly vulnerable to delirium and falls. Injectable olanzapine given with parenteral diphenhydramine may increase oversedation.
How serious is the interaction between Diphenhydramine and Olanzapine?
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 oversedation and anticholinergic effects and use the lowest effective doses. Avoid in frail elderly patients and consider a non-sedating antihistamine.
What other medicines interact with Olanzapine?
MediHelp also lists interactions of Olanzapine with, for example, Amisulpride, Amoxapine, Apomorphine. The full list is on the Olanzapine page.
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