Nursing care
Diphenhydramine: what to check before you give it
Written and reviewed by Dana Whitfield, RN, MSN · 4 min read · Updated September 2026
Short answer
Diphenhydramine is a first-generation antihistamine used for allergic reactions, itching, and short-term insomnia. Its anticholinergic effects, including confusion, urinary retention, and increased fall risk, make it a poor choice for sleep in older adults, and it appears on the Beers Criteria for that reason. Assess mental status and voiding before and after administration.
What it does and why it is prescribed
Diphenhydramine is a first-generation H1 antihistamine that crosses the blood-brain barrier readily, which is what gives it both its antihistamine effect and its sedating properties. It also has significant anticholinergic activity, blocking muscarinic receptors throughout the body, not just in the airway.
It's prescribed for allergic reactions, urticaria, and as an adjunct in anaphylaxis alongside epinephrine, for antiemetic and antitussive purposes in some formulations, and for short-term insomnia because of its sedating effect. It's also used for extrapyramidal symptoms caused by antipsychotics, where its anticholinergic action helps counteract dystonia.
Nursing considerations before giving it
Assess age and cognitive baseline before administration. In older adults, diphenhydramine's anticholinergic effects, confusion, urinary retention, and increased fall risk, make it a poor choice for sleep, and it's listed on the Beers Criteria as a medication to avoid in this population for exactly that reason. If it's ordered for an older adult as a sleep aid, that's worth questioning with the prescriber rather than simply administering it.
Check for narrow-angle glaucoma, benign prostatic hyperplasia, and asthma or COPD, all of which are relative contraindications given the drug's anticholinergic and drying effects. Review the medication list for other anticholinergic or CNS-depressant drugs, since diphenhydramine's effects are additive with both.
What to monitor
Monitor mental status closely in older adults, since confusion or delirium can develop even at standard doses and is easy to mistake for baseline cognitive decline if there's no clear before-and-after comparison. Monitor voiding pattern and bladder distension, particularly in men with prostatic enlargement, since urinary retention is a common anticholinergic effect.
Track fall risk continuously for the duration the drug is active, given the combination of sedation, dizziness, and blurred vision. Respiratory status is worth watching too, especially in combination with other CNS depressants or in patients with compromised airway reflexes.
Side effects versus adverse effects
Common side effects are drowsiness, dry mouth, dry eyes, and mild constipation, all direct consequences of the anticholinergic and antihistamine action. These are expected and usually tolerable for short-term use.
Adverse effects to escalate include acute confusion or delirium, especially in older adults, urinary retention severe enough to require catheterisation, and paradoxical excitation, which is more common in children and can present as agitation rather than sedation. Anticholinergic toxicity, with flushing, tachycardia, hyperthermia, and altered mental status, is the most serious and requires immediate notification.
What to hold for and when to call
Hold the dose for significant sedation that impairs airway protection, for new-onset confusion in an older adult, or for signs of urinary retention such as inability to void or a palpably distended bladder. Hold if the patient reports blurred vision severe enough to affect safety or if heart rate is elevated beyond expected parameters.
Call the provider for any sign of anticholinergic toxicity, for delirium that doesn't resolve once the drug clears, or for respiratory depression in combination with other sedating medications. In an older adult, flag the order itself if it's being used for sleep, since safer alternatives usually exist.
Patient teaching
Tell the patient to expect drowsiness and to avoid driving or operating machinery until they know how it affects them. Advise against alcohol and other sedating medications during treatment, since the combined effect increases fall and confusion risk substantially.
For older adults or their families, explain plainly that this isn't a good long-term option for sleep because of the confusion and fall risk it carries, and that a conversation with the prescriber about alternatives is worthwhile. Advise increasing fluid and fibre intake to offset constipation, and to report any difficulty urinating promptly rather than waiting for it to resolve on its own.
The next step on this is the same as on everything else here: answer questions and read the rationales. Our pharmacology practice questions are the closest set to what this page covers.
One question from the pharmacology set
A client with heart failure is started on furosemide 40 mg PO daily. Which findings should the nurse report to the provider before administering the next dose? Select all that apply.
Rationale
Furosemide is a loop diuretic, so the two things you are watching are potassium and kidney function. A potassium of 2.9 mEq/L is below the 3.5–5.0 reference range and puts the client at risk for dysrhythmia — hold and report. Muscle cramps with palpitations are the clinical face of that same hypokalemia, so they are reported together, not separately. A creatinine that doubles signals the diuresis has outrun renal perfusion. A blood pressure of 132/78 and a 1 kg loss are the expected response to the drug working, not reasons to hold it.
Answer: A, D, E
Common questions
Why is diphenhydramine discouraged for sleep in older adults?
Its anticholinergic effects cause confusion, urinary retention, and increased fall risk in this population, and these risks often outweigh the benefit of sedation. It's included on the Beers Criteria list of potentially inappropriate medications for older adults for these reasons.
Can diphenhydramine cause delirium?
Yes, particularly in older adults, where even standard doses can produce acute confusion that mimics or worsens baseline cognitive impairment. This is one of the main reasons it's avoided as a routine sleep aid in this age group.
What are signs of anticholinergic toxicity from diphenhydramine?
Flushed, dry skin, dry mouth, blurred vision, tachycardia, urinary retention, hyperthermia, and altered mental status are the classic signs, sometimes remembered as red as a beet, dry as a bone, blind as a bat, mad as a hatter. Severe cases can progress to seizures or arrhythmia and need prompt medical attention.
Is diphenhydramine safe with other sedating medications?
Not without caution. Combining it with opioids, benzodiazepines, or alcohol increases CNS and respiratory depression risk, and combining it with other anticholinergic drugs compounds confusion, dry mouth, and urinary retention. Review the full medication list before administering.
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