Mechanism of action
Diphenhydramine competitively antagonizes histamine at H1 receptors in the peripheral and central nervous systems, preventing histamine-mediated vasodilation, increased vascular permeability, and pruritus. Unlike second-generation antihistamines, diphenhydramine crosses the blood-brain barrier readily due to its lipophilic structure, producing marked CNS depression and sedation.
The drug also exhibits substantial antimuscarinic (anticholinergic) activity by blocking muscarinic acetylcholine receptors, leading to dry mouth, urinary retention, constipation, and blurred vision. Additional mechanisms include weak sodium channel blockade (membrane stabilization) and inhibition of serotonin reuptake. Its anticholinergic properties make it useful for treating drug-induced extrapyramidal symptoms and dystonic reactions, particularly those caused by antipsychotics and metoclopramide.
- First-generation antihistamine that crosses the blood-brain barrier, causing sedation and anticholinergic effects.
- Used for allergies, insomnia, motion sickness, and acute dystonic reactions from antipsychotics or metoclopramide.
- Included in Beers Criteria—avoid in elderly due to delirium, falls, and urinary retention risk.
- Anticholinergic toxidrome in overdose: hyperthermia, delirium, mydriasis, dry mucosa, urinary retention, tachycardia.
- Paradoxical CNS excitation (agitation, insomnia) can occur in children instead of sedation.
- Contraindicated in acute asthma—thickens secretions and worsens airway obstruction.
- Additive effects with other anticholinergics and CNS depressants increase toxicity risk.
- Diphenhydramine 50 mg IV/IM rapidly reverses drug-induced dystonic reactions.
Indications
Diphenhydramine is FDA-approved and commonly used for:
- Allergic reactions: acute urticaria, angioedema, allergic rhinitis, pruritus from various causes
- Anaphylaxis adjunct: alongside epinephrine to reduce histamine-mediated symptoms
- Insomnia: short-term treatment of sleep-onset insomnia (over-the-counter)
- Motion sickness and nausea: prevention and treatment of nausea, vomiting, and vertigo
- Drug-induced extrapyramidal symptoms: acute dystonic reactions from antipsychotics or metoclopramide
- Parkinsonism: mild adjunctive therapy for tremor and rigidity
- Cough suppression: antitussive in combination cold preparations
It is also used off-label for hyperemesis gravidarum in pregnancy when other antiemetics fail, though newer agents are preferred.
Pharmacokinetics
Diphenhydramine is well absorbed orally with peak plasma concentrations reached in 1 to 4 hours. It undergoes extensive hepatic metabolism via CYP2D6 and other enzymes to inactive metabolites, which are excreted renally. The elimination half-life ranges from 4 to 8 hours in adults, but may be prolonged in elderly patients and those with hepatic or renal impairment.
The drug is highly lipophilic and distributes widely into tissues, including the CNS. It is approximately 80% protein-bound. Onset of action occurs within 15 to 30 minutes after oral administration, with maximal effects at 1 to 3 hours. Duration of action is typically 4 to 6 hours. Dosing follows the product label and varies by indication and patient age.
Adverse effects
Diphenhydramine's adverse effect profile is dominated by its anticholinergic and sedative properties:
Common adverse effects
- Sedation and drowsiness: most frequent, dose-dependent
- Anticholinergic effects: dry mouth, blurred vision, urinary retention, constipation, tachycardia
- Cognitive impairment: confusion, dizziness, poor coordination
- Gastrointestinal: nausea, epigastric distress
Serious adverse effects
- Anticholinergic toxicity: delirium, hallucinations, hyperthermia, seizures, arrhythmias (especially in overdose)
- Paradoxical CNS stimulation: excitation, insomnia, tremor, especially in children
- QT prolongation: risk of torsades de pointes at high doses or in susceptible patients
- Respiratory depression: particularly when combined with other CNS depressants
Special populations
- Elderly: increased risk of falls, delirium, cognitive decline, urinary retention; listed on the Beers Criteria as potentially inappropriate
- Children: paradoxical excitation and seizures possible
- Pregnancy: category B; generally considered safe but not first-line
Contraindications and warnings
Absolute contraindications:
- Hypersensitivity to diphenhydramine or other ethanolamines
- Neonates and premature infants (risk of CNS depression)
- Acute asthma attack (anticholinergic effects thicken secretions)
- Concurrent MAO inhibitor therapy (prolongs anticholinergic effects)
Relative contraindications and warnings:
- Narrow-angle glaucoma: anticholinergic effects increase intraocular pressure
- Prostatic hypertrophy or bladder neck obstruction: risk of urinary retention
- Stenosing peptic ulcer or pyloroduodenal obstruction: anticholinergics worsen obstruction
- Hyperthyroidism: increased sensitivity to tachycardia
- Cardiovascular disease: risk of tachycardia, arrhythmias, hypotension
- Seizure disorders: may lower seizure threshold
- Hepatic impairment: reduced metabolism increases drug levels
Beers Criteria: diphenhydramine is potentially inappropriate in adults 65 years and older due to high anticholinergic burden and CNS effects. Avoid chronic use as a sleep aid in the elderly.
Drug interactions
Diphenhydramine has multiple clinically significant drug interactions:
CNS depressants
Additive sedation and respiratory depression occur with alcohol, benzodiazepines, opioids, barbiturates, and other sedative-hypnotics. Patients should be warned to avoid alcohol and use caution with concurrent CNS depressants.
Anticholinergic agents
Additive anticholinergic effects with tricyclic antidepressants, antipsychotics (especially first-generation), antiparkinson drugs, and antispasmodics increase the risk of anticholinergic toxicity, including delirium, urinary retention, and hyperthermia.
MAO inhibitors
MAOIs prolong and intensify the anticholinergic effects of antihistamines. Concurrent use is contraindicated.
QT-prolonging drugs
Diphenhydramine may enhance QT prolongation when combined with antiarrhythmics, macrolides, fluoroquinolones, or antipsychotics, increasing torsades de pointes risk.
CYP2D6 substrates and inhibitors
Drugs that inhibit CYP2D6 (e.g., fluoxetine, paroxetine, quinidine) may increase diphenhydramine levels and toxicity. Diphenhydramine may also inhibit metabolism of other CYP2D6 substrates at high doses.
Clinical pearls
- Beers Criteria drug: avoid diphenhydramine in elderly patients due to anticholinergic burden, falls risk, and cognitive impairment. Second-generation antihistamines like cetirizine or loratadine are preferred for allergy management.
- Acute dystonic reaction treatment: diphenhydramine 50 mg IV/IM is first-line for metoclopramide- or antipsychotic-induced dystonia, with rapid symptom reversal in minutes.
- Tolerance develops quickly: with chronic use for insomnia, efficacy diminishes within days. It is not appropriate for long-term sleep management.
- Anticholinergic toxidrome: in overdose, patients present with classic findings—hot as a hare, blind as a bat, dry as a bone, red as a beet, mad as a hatter. Treatment is supportive; physostigmine is reserved for severe cases with life-threatening delirium or arrhythmias.
- Paradoxical excitation in children: young children may experience CNS stimulation (agitation, insomnia, tremor) instead of sedation.
- Not for asthma: anticholinergic effects thicken respiratory secretions and worsen airway obstruction; avoid in acute asthma or COPD exacerbations.
- Topical formulations: topical diphenhydramine for pruritus is discouraged due to risk of contact dermatitis and systemic absorption; oral or alternative topical agents are preferred.
- Drug screen: diphenhydramine can cause false-positive urine immunoassays for methadone and tricyclic antidepressants.
Anticholinergic toxidrome—Hot as a hare, Blind as a bat, Dry as a bone, Red as a beet, Mad as a hatter—classic for diphenhydramine overdose.
Practice questions
1. A 72-year-old woman with hypertension and benign prostatic hypertrophy presents to the ED with confusion, dry mucous membranes, dilated pupils, and urinary retention. Her daughter reports she has been taking an over-the-counter sleep aid for the past week. Vital signs show temperature 38.2°C, heart rate 118 bpm, and blood pressure 158/92 mmHg. Which of the following is the most likely cause?
2. A 28-year-old woman receiving metoclopramide for nausea develops sudden onset of neck twisting, upward eye deviation, and jaw muscle spasm. She is alert and distressed. Which of the following is the most appropriate immediate treatment?
3. A 5-year-old boy is brought to the clinic with rhinorrhea, sneezing, and itchy eyes due to seasonal allergies. His mother asks for a medication recommendation. Past medical history is unremarkable. Which of the following antihistamines is most appropriate?
Frequently asked questions
How does diphenhydramine work?
Diphenhydramine works by competitively blocking histamine H1 receptors in peripheral tissues and the CNS, preventing allergic symptoms like itching and hives. It also has strong anticholinergic effects, blocking muscarinic receptors and causing sedation, dry mouth, and urinary retention. Its ability to cross the blood-brain barrier accounts for its sedative and CNS side effects.
What are the side effects of diphenhydramine?
The most common side effects are sedation, drowsiness, dry mouth, blurred vision, constipation, and urinary retention due to anticholinergic activity. In elderly patients, diphenhydramine can cause confusion, delirium, and falls. Children may experience paradoxical excitation. Overdose leads to anticholinergic toxicity with hyperthermia, hallucinations, seizures, and arrhythmias.
Is diphenhydramine safe for elderly patients?
No, diphenhydramine is listed on the Beers Criteria as potentially inappropriate for adults 65 and older. Its strong anticholinergic effects increase the risk of cognitive impairment, delirium, falls, urinary retention, and constipation in elderly patients. Second-generation antihistamines like cetirizine or loratadine are safer alternatives for allergy management in this population.
Diphenhydramine vs loratadine—what is the difference?
Diphenhydramine is a first-generation antihistamine that crosses the blood-brain barrier, causing sedation and anticholinergic effects like dry mouth and urinary retention. Loratadine is a second-generation antihistamine that does not cross the blood-brain barrier significantly, resulting in minimal sedation and no anticholinergic side effects. Loratadine is preferred for chronic allergy management, especially in elderly patients.
Can diphenhydramine be used for acute dystonic reactions?
Yes, diphenhydramine 50 mg IV or IM is first-line treatment for acute dystonic reactions caused by antipsychotics or metoclopramide. Its anticholinergic properties rapidly reverse extrapyramidal symptoms like torticollis, oculogyric crisis, and muscle spasms, usually within minutes. Benztropine is an alternative. Diphenhydramine is effective because it restores the balance between dopamine and acetylcholine in the basal ganglia.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: D04AA32WHO ATC/DDD index: R06AA02
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.