👤 By the CinnaRN Clinical Content Team🕐 Updated 2026-08-23🏷️ Pharmacological Therapies🔖 Free to read, print, and share
Also known as: Benadryl · first-generation antihistamine · allergy/sleep pill
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Use this quick-reference guide to spot, treat, and prevent Diphenhydramine on the NCLEX. Keep it handy during review and on exam day!
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📒 The 1-minute cheat sheet
📌 Uses
Allergy, sleep aid, EPS, motion sick
Blocks H1 → anticholinergic effects
🩺 Signs
Drowsiness, dry mouth, constipation
Urinary retention, blurred vision
📌 Avoid
Elderly → falls, confusion (Beers)
Glaucoma, BPH, no alcohol/CNS depress
🎓 Teach
No driving until effect known
Sugarless gum for dry mouth
📚 Diphenhydramine — full study notes
The cheat sheet above is your quick recall card. These notes go deeper — what it is, what to do first, what must be reported, and what to teach.
Diphenhydramine is a first-generation H1 antihistamine that relieves allergy symptoms, itching, and motion sickness and is also used for mild sleep aid and acute allergic/anaphylactic reactions. Because it crosses the blood-brain barrier, it causes marked sedation and strong anticholinergic effects. Memory aid: classic anticholinergic mnemonic 'can't see, can't pee, can't spit, can't poop.'
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Key points
Understand these first
It causes pronounced drowsiness and CNS depression, so it is sedating compared with second-generation antihistamines.
Anticholinergic effects include dry mouth, urinary retention, constipation, blurred vision, and tachycardia.
It is on the Beers list as potentially inappropriate in older adults due to confusion and fall risk.
It is used as an adjunct in allergic reactions and to counter extrapyramidal/dystonic reactions.
Effects are additive with alcohol and other CNS depressants.
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Nursing priorities
What to do, in order
Implement fall precautions, especially in older adults, due to sedation and dizziness.
Assess for urinary retention and constipation from anticholinergic effects.
Caution the patient against driving or operating machinery after dosing.
Avoid combining with alcohol or other sedatives.
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Red flags — report now
Escalate immediately
Report confusion, severe agitation, or hallucinations, particularly in older adults.
Never give to patients with narrow-angle glaucoma or significant urinary retention without provider clearance.
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Patient teaching
What patients must know
This medicine makes most people sleepy; do not drive until you know how it affects you.
Avoid alcohol while taking it.
Increase fluids and fiber to prevent constipation and report trouble urinating.
❓ Diphenhydramine: NCLEX FAQs
What are the priority nursing interventions for Diphenhydramine?
Implement fall precautions, especially in older adults, due to sedation and dizziness. Assess for urinary retention and constipation from anticholinergic effects. Caution the patient against driving or operating machinery after dosing. Avoid combining with alcohol or other sedatives.
What are the warning signs of Diphenhydramine a nurse must report?
Report confusion, severe agitation, or hallucinations, particularly in older adults. Never give to patients with narrow-angle glaucoma or significant urinary retention without provider clearance.
What do I need to know about Diphenhydramine for the NCLEX?
It causes pronounced drowsiness and CNS depression, so it is sedating compared with second-generation antihistamines. Anticholinergic effects include dry mouth, urinary retention, constipation, blurred vision, and tachycardia. It is on the Beers list as potentially inappropriate in older adults due to confusion and fall risk. It is used as an adjunct in allergic reactions and to counter extrapyramidal/dystonic reactions.
What patient teaching is important for Diphenhydramine?
This medicine makes most people sleepy; do not drive until you know how it affects you. Avoid alcohol while taking it. Increase fluids and fiber to prevent constipation and report trouble urinating.
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Quick Tip
It causes pronounced drowsiness and CNS depression, so it is sedating compared with second-generation antihistamines.