Amitriptyline and Diphenhydramine: Interaction Details
AI-assisted, pharmacist-reviewed · AI content regenerated Jul 11, 2026 · Source data updated Jul 2, 2026 · Sources: FDA labeling, DDInter 2.0, cited literature
Amitriptyline
Diphenhydramine
How we grade severity & evidence
Severity levels
- Contraindicated: These should generally not be used together.
- Major: Potentially serious — often needs a change or close monitoring.
- Moderate: Can be significant — usually manageable with monitoring.
- Minor: Usually limited clinical impact.
Evidence grades
- Established: Well documented — supported by controlled studies or strong clinical data.
- Probable: Good supporting evidence, though not definitively proven.
- Suspected: Some evidence suggests this interaction, but it is not well established.
- Possible: Limited or conflicting evidence; the interaction may occur.
- Theoretical: Predicted from the drugs' pharmacology; not yet confirmed in people.
Ratings come from the documented interaction literature and are reviewed by a pharmacist. They describe the documented risk of the combination, not what will necessarily happen to you — your dose, timing, and health picture all matter.
Both of these medicines have drying effects on the body. Amitriptyline (Elavil) is an antidepressant, and diphenhydramine (Benadryl) is an antihistamine found in many allergy, cold, and nighttime sleep products. When you take them together, those drying effects can add up.
That can mean more dry mouth, constipation, blurry vision, trouble urinating, and extra drowsiness. Older adults tend to feel these effects more strongly. The good news is this is very manageable. Please don't stop either medicine on your own, but do let your pharmacist or doctor know you take both so they can watch for these symptoms and adjust things if needed.
Mechanism: additive anticholinergic burden. Amitriptyline (a strongly anticholinergic TCA) plus diphenhydramine (a first-generation H1 antihistamine with marked antimuscarinic activity) compound peripheral and central anticholinergic effects. Neither is a prodrug; this is a pharmacodynamic (additive), not pharmacokinetic, interaction.
- Effect/direction: increased anticholinergic effects, dry mouth, urinary retention, risk of adynamic ileus and worsening narrow-angle glaucoma.
- Onset: delayed. Severity: moderate. Evidence: theoretical.
- Population: more prominent in the elderly.
- Management: monitor for dry mouth, sedation, and urinary difficulty; consider a lower diphenhydramine dose or a non-sedating, less anticholinergic antihistamine.
What happens
Increased anticholinergic effects (dry mouth, urinary retention)
Interaction Deep Dive
The simultaneous use of antihistamines together with antidepressants that carry potent anticholinergic activity (for instance, amitriptyline, trimipramine, amoxapine, doxepin, imipramine, nortriptyline, and maprotiline) can raise the likelihood of adynamic ileus, retention of urine, or chronic glaucoma. Older patients may experience this interaction to a greater degree1.
Why it happens (mechanism)
Additive anticholinergic effects
How to manage this interaction
This combination is usually manageable, and you should keep taking your medicines as prescribed unless your care team tells you otherwise. Here is what typically helps:
- Tell your pharmacist or prescriber you take amitriptyline, and ask before using any over-the-counter antihistamine, sleep aid, or cold and allergy product, since many contain diphenhydramine.
- Watch for dry mouth, heavy drowsiness, constipation, and trouble urinating, and report them.
- A lower diphenhydramine dose may be considered by your team, or they may suggest a less drying antihistamine.
- Extra caution applies if you are older or have glaucoma or prostate issues.
Management is individual — confirm any change with your pharmacist or prescriber.
Common questions
Can I take Amitriptyline and Diphenhydramine together?
Taking amitriptyline with diphenhydramine can stack up drying side effects like dry mouth, constipation, drowsiness, and trouble urinating, especially in older adults. Keep taking both as prescribed, but check with your pharmacist before using Benadryl-type products so they can lower the dose or suggest an alternative. Always confirm with your pharmacist or prescriber before making any change.
How serious is the Amitriptyline and Diphenhydramine interaction?
It is rated moderate. Can be significant — usually manageable with monitoring.
How quickly could this interaction happen?
The documented onset is "delayed". Effects tend to build up gradually over days to weeks.
How is the Amitriptyline and Diphenhydramine interaction managed?
This combination is usually manageable, and you should keep taking your medicines as prescribed unless your care team tells you otherwise. Here is what typically helps: Tell your pharmacist or prescriber you take amitriptyline, and ask before using any over-the-counter antihistamine, sleep aid, or cold and allergy product, since many contain diphenhydramine. Watch for dry mouth, heavy drowsiness,… Management is individual — always follow your own care team's guidance.
How strong is the evidence for this interaction?
The evidence is graded "theoretical". Predicted from the drugs' pharmacology; not yet confirmed in people.
From our Q&A
Real reader questions about these medications, each personally answered by our pharmacist:
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- Benadryl (Diphenhydramine) With Ibuprofen Interaction Details
- Benadryl (Diphenhydramine) Interaction With Prozac (Fluoxetine)
- Can You Overdose On Benadryl (Diphenhydramine)? What Are The Symptoms?
Questions for your pharmacist
- Does my dose of Amitriptyline or Diphenhydramine need adjusting while I take them together?
- What symptoms should prompt me to call you or my prescriber right away?
- Does the timing of my doses matter for this combination?
- Is there anything you'd monitor while I'm on both?
References (1)
- Blazer DG, Federspiel CF, Ray WA, et al: The risk of anticholinergic toxicity in the elderly: a study of prescribing practices in two populations. J Gerontol 1983; 38:31-35. PubMed
Keep reading about Amitriptyline
Keep reading about Diphenhydramine
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Prescription drugs aren't the whole picture — herbal and dietary supplements can interact with them too. From the evidence-graded Natural Medicines database:
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