Nursing care
Antihistamines: what to check before you give it
Written and reviewed by Dana Whitfield, RN, MSN · 5 min read · Updated September 2026
Short answer
Antihistamines block H1 receptors to relieve allergy symptoms and induce sedation, but the same dose behaves differently across the lifespan. In children they tend to sedate; in older adults, particularly first-generation agents like diphenhydramine, they are more likely to cause confusion, agitation, or frank delirium due to anticholinergic burden and reduced drug clearance.
Mechanism, simply
Antihistamines compete with histamine for H1 receptors, blocking the vasodilation, capillary permeability, and itch signalling that histamine drives during an allergic response. First-generation agents such as diphenhydramine and chlorpheniramine cross the blood-brain barrier freely, which is why they sedate. Second-generation agents such as cetirizine, loratadine, and fexofenadine are more selective for peripheral H1 receptors and largely spare the central nervous system.
Diphenhydramine also carries significant anticholinergic activity, blocking muscarinic receptors alongside H1. This dual action explains its use for motion sickness and as a sleep aid, but it is also the source of most of the adverse effects you will be asked to manage.
Indications you will see on the ward
Diphenhydramine appears on most units for acute allergic reactions, urticaria, and as premedication before blood transfusions or agents like vancomycin that trigger histamine release. It is also given for extrapyramidal symptoms from antipsychotics, since its anticholinergic effect counters the dystonia. Second-generation agents are more common for routine seasonal allergy management, given once daily and without the sedation that would interfere with a patient's ability to function.
You will also see antihistamines combined with other agents, such as diphenhydramine paired with acetaminophen in over-the-counter sleep products, or as one component of a rapid-acting anaphylaxis protocol alongside epinephrine and corticosteroids. Antihistamines are never a substitute for epinephrine in anaphylaxis; they treat the itch and hives, not the airway compromise.
Assessment before administration
Ask the patient's age before you draw up diphenhydramine. In a young adult, expect drowsiness and plan around it. In a patient over 65, the same dose raises real risk of delirium, urinary retention, and falls, and many facilities restrict or avoid first-generation antihistamines in this group entirely under Beers Criteria guidance.
Check for narrow-angle glaucoma and urinary retention or bladder outlet obstruction, both relative contraindications because of the anticholinergic effect. Ask about current sedatives, opioids, or alcohol use, since combined CNS depression compounds quickly. Review respiratory status in patients with asthma or COPD, as the drying effect on secretions can thicken mucus and worsen a productive cough. Baseline mental status and orientation matter more here than with most drug classes, because a shift in cognition after the dose is your earliest warning sign.
Toxicity and the antidote
Overdose produces a classic anticholinergic toxidrome: flushed dry skin, dry mouth, dilated pupils, tachycardia, urinary retention, fever, and in severe cases agitation, hallucinations, or seizures. The mnemonic taught for this picture is red as a beet, dry as a bone, blind as a bat, mad as a hatter, hot as a hare.
There is no specific reversal agent in routine use. Management is supportive: airway protection, cardiac monitoring for QRS widening, benzodiazepines for agitation or seizures, and cooling measures for hyperthermia. Physostigmine can reverse severe anticholinergic delirium in select cases but carries its own risk of bradycardia and seizure, so it is reserved for toxicology-guided use, not a first-line nursing action. Activated charcoal may be given if the patient presents early and the airway is protected.
Interactions that matter
Other CNS depressants, opioids, benzodiazepines, and alcohol all potentiate the sedation of first-generation antihistamines, raising the risk of respiratory depression and falls. Combining diphenhydramine with other anticholinergic drugs, including some antidepressants, antipsychotics, and overactive bladder medications, stacks the anticholinergic burden and increases delirium risk sharply in older adults.
MAOIs prolong and intensify antihistamine effects and should not be combined without specialist input. Second-generation agents carry fewer interactions overall, though some, like the older formulation of terfenadine, were withdrawn specifically for cardiac interaction risk, a reminder that not every antihistamine in this class behaves identically.
What the patient must be told
Tell the patient not to drive or operate machinery until they know how a first-generation antihistamine affects them, since the sedation is dose-dependent and varies by individual. Warn that alcohol will intensify the drowsiness.
For an older patient or family member, be explicit that confusion, agitation, or a fall risk can appear instead of sleepiness, and that this is a reason to call the prescriber rather than simply push through. Advise against combining an over-the-counter sleep aid containing diphenhydramine with a separately prescribed sedative, since patients frequently do not realise they are doubling up. For second-generation agents, reassure the patient that daytime dosing is generally safe, but recommend a trial dose on a day without driving plans the first time.
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 does diphenhydramine cause confusion in the elderly instead of drowsiness?
Older adults have reduced hepatic and renal clearance, so the drug accumulates, and age-related changes in brain cholinergic function make them more sensitive to anticholinergic blockade. The result is often paradoxical agitation or frank delirium rather than sedation.
Can antihistamines be given for anaphylaxis instead of epinephrine?
No. Antihistamines relieve itching and hives but do nothing for airway swelling, bronchospasm, or hypotension. Epinephrine is the first-line drug in anaphylaxis; an antihistamine is adjunct therapy given afterward.
Is it safe to give diphenhydramine with an opioid for pain and itching?
It can be done but requires closer monitoring, since both drugs depress the CNS and respiratory drive. Watch sedation level and respiratory rate more frequently than with either drug alone.
What is the mnemonic for anticholinergic toxicity from antihistamine overdose?
Red as a beet, dry as a bone, blind as a bat, mad as a hatter, hot as a hare — covering flushed skin, dry mucous membranes, dilated pupils, delirium, and fever.
Why do some facilities avoid diphenhydramine in patients over 65?
It appears on the Beers Criteria list of potentially inappropriate medications for older adults because of high delirium, fall, and urinary retention risk relative to its benefit. Second-generation antihistamines are preferred when an allergy medication is needed in this age group.
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