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

🛡️ 5 Critical Medication Safety Tips for Nurses

1

Triple-Check High-Risk Medications

Always have another nurse verify insulin, heparin, warfarin, and chemotherapy drugs. These "high-alert" medications cause the most serious errors. Check concentration, dose calculation, and pump settings twice.

2

Know Look-Alike, Sound-Alike Drugs

Common mix-ups: hydromorphone/morphine, Celebrex/Celexa, Zyprexa/Zyrtec. Always use BOTH generic and brand names, read labels twice, and use barcode scanning when available. One wrong letter can be fatal.

3

Assess Before AND After Giving Meds

Check vitals before cardiac meds, pain levels before analgesics, and blood glucose before insulin. Always reassess within 30 minutes to evaluate effectiveness and watch for adverse reactions.

4

Watch for Drug Interactions

Common dangerous combinations: warfarin + aspirin (bleeding), ACE inhibitors + potassium (hyperkalemia), digoxin + diuretics (toxicity). Always check drug interactions before administering new medications.

5

Educate Your Patients

Teach patients medication names, purposes, major side effects, and what to report. Informed patients catch errors and improve compliance. Always encourage questions - an educated patient is a safer patient.

⚡ Remember: When in doubt, don't give it out! It's always safer to double-check than regret later.

⚠️ Medical Disclaimer

This drug guide is for educational purposes only and is NOT intended for clinical use. Always consult current prescribing information, healthcare providers, and institutional protocols before administering any medication. Do not use this information for patient care decisions.