Home Drug Guide Diphenhydramine Hydrochloride

Drug Guide

Generic Name

Diphenhydramine Hydrochloride

Brand Names Benadryl, Benylin, Benadryl Preservative Free, Diphen, Hydramine, Vicks Formula 44 Antitussive, Silphen, Diphenhydramine Hydrochloride Preservative Free, Belix, Dibenil, Beldin

Classification

Therapeutic: Antihistamine, Antitussive

Pharmacological: First-generation H1 antihistamine, Muscarinic receptor antagonist

FDA Approved Indications

Relief of allergy symptoms (rash, itching, watery eyes, runny nose, hay fever)
Prevention and treatment of motion sickness
Insomnia
Cough suppression (antitussive)

Mechanism of Action

Diphenhydramine blocks H1 histamine receptors, reducing allergy symptoms; it also has anticholinergic and sedative properties, which contribute to its effect in motion sickness and sleep induction.

Dosage and Administration

Adult: 25-50 mg every 4-6 hours as needed, not exceeding 300 mg/day

Pediatric: 2-6 years: 6.25-12.5 mg every 4-6 hours (max 75 mg/day); 6-12 years: 25-50 mg every 4-6 hours (max 150 mg/day)

Geriatric: Use with caution, starting at lower doses due to increased sensitivity and risk of anticholinergic effects

Renal Impairment: Adjust dose based on severity, consult dosing guidelines

Hepatic Impairment: Use with caution; dose adjustments may be necessary

Pharmacokinetics

Absorption: Well absorbed from gastrointestinal tract

Distribution: Widely distributed; crosses blood-brain barrier

Metabolism: Hepatic, primarily by CYP2D6 and other liver enzymes

Excretion: Renal; unchanged drug and metabolites

Half Life: 2.4 to 9.3 hours, longer in the elderly

Contraindications

Newborns and premature infants
Liver failure
Acute asthma exacerbation

Precautions

Use with caution in elderly; may cause sedation, confusion, or anticholinergic effects
May impair cognitive or motor performance
Monitor for paradoxical excitation in children

Adverse Reactions - Common

Drowsiness (Very common)
Dizziness (Common)
Dry mouth (Common)
Constipation (Common)

Adverse Reactions - Serious

Respiratory depression (Rare)
Allergic reactions (rash, angioedema, anaphylaxis) (Rare but serious)
Hallucinations, seizures (in overdose) (Rare)

Drug-Drug Interactions

CNS depressants (additive sedation)
Monoamine oxidase inhibitors (risk of anticholinergic effects)
QT prolonging drugs

Drug-Food Interactions

N/A

Drug-Herb Interactions

N/A

Nursing Implications

Assessment: Monitor for effectiveness (relief of allergy symptoms, sleep quality, cough suppression) and adverse effects (sedation, anticholinergic effects)

Diagnoses:

Risk for injury due to sedation or dizziness
Impaired sleep pattern

Implementation: Administer with food if GI upset occurs, monitor for CNS depression, educate about avoiding alcohol and other CNS depressants

Evaluation: Assess symptom relief and adverse effects regularly

Patient/Family Teaching

Take exactly as prescribed or directed on the label
Avoid alcohol and sedatives during therapy
Caution when driving or operating machinery
Notify healthcare provider if experiencing excessive drowsiness, confusion, or allergic reactions

Special Considerations

Black Box Warnings:

Potential for severe CNS depression in children, especially under 2 years old

Genetic Factors: CYP2D6 metabolizer status may affect drug levels

Lab Test Interference: May interfere with certain skin allergy and other tests

Overdose Management

Signs/Symptoms: Extreme sedation, agitation, hallucinations, seizures, tachycardia, dry mouth, urinary retention, hypo- or hypertension

Treatment: Supportive care, activated charcoal if within 1 hour of ingestion, physostigmine in severe anticholinergic toxicity, airway management, hemodynamic support as needed

Storage and Handling

Storage: Store at room temperature between 20-25°C (68-77°F), away from moisture and light

Stability: Stable under recommended storage conditions for the duration of the labeled shelf life