Diphenhydramine

 

1. Generic Name & Brand Names

• Generic Name: Diphenhydramine HCl

• Common Brands:

  • Benadryl® (Global)

  • Sominex® (sleep aid)

  • Unisom SleepGels® (US)

  • Nytol® (UK)

  • Generic/store brands ("Allergy Relief," "Sleep Aid")

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2. Dosage Forms

• Oral:

  • Tablets/Capsules (25 mg, 50 mg)

  • Liquid (12.5 mg/5mL)

  • Chewables (12.5 mg)

• Topical:

  • Cream/gel (1-2% for itch/rashes)

• Injectable: IV/IM (hospital use)

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3. Drug Class

• 1st-Generation Ethanolamine Antihistamine

• Potent H₁-receptor antagonist

• Strong anticholinergic & sedative properties

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4. Uses

• Allergic Reactions:

  • Acute urticaria, angioedema, allergic rhinitis

• Insomnia: Short-term sleep aid

• Motion Sickness: Prevention/treatment

• Antitussive: Cough suppression (in syrups)

• Extrapyramidal Symptoms: Drug-induced dystonia

• Topical: Insect bites, poison ivy, mild sunburn

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5. Side Effects

Common (>20%)	Serious (Seek Immediate Care)
Profound sedation	Anaphylaxis
Dry mouth/nose	Seizures
Dizziness	Cardiac arrhythmias
Blurred vision	Acute glaucoma
Urinary retention	Severe hypotension
Constipation	Toxic psychosis (elderly)

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6. Critical Warnings

• 🚫 Absolute Contraindications:

  • Newborns/premature infants

  • Breastfeeding

  • Narrow-angle glaucoma

  • Severe COPD/asthma exacerbation

  • Concurrent MAOIs

• High-Risk Populations:

  • Elderly (>65 yrs): ↑ Delirium, falls, dementia risk

  • Children <6 yrs: Fatal respiratory depression risk

  • Pregnancy: Category B (avoid in 3rd trimester)

• Drug Interactions:

  • ⚠️ CNS Depressants: Alcohol, opioids, benzodiazepines (↑ coma risk)

  • ⚠️ Anticholinergics: TCAs, scopolamine (↑ toxicity)

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7. How to Take

Indication	Adult Dose	Max Daily	Timing
Allergies	25-50 mg q4-6hrs	300 mg	With food
Insomnia	50 mg at bedtime	50 mg	30 min before sleep
Motion Sickness	25-50 mg pre-travel	300 mg	1-2 hrs pre-exposure

• Key Notes:

  • Never drive/operate machinery after dosing

  • Liquid: Use calibrated syringe (household spoons inaccurate)

  • Duration: ≤7 days for allergies/insomnia

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8. Pediatric Dosing

Age	Dose (Liquid)	Frequency
6-11 years	12.5-25 mg (5-10 mL)	q4-6hrs
2-5 years*	6.25 mg (2.5 mL)	q6-8hrs

**Avoid under age 2 unless directed by a physician (respiratory arrest risk)*

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9. Overdose

• Symptoms:

  Hallucinations, tachycardia, dilated pupils, seizures, coma

• Toxic Dose: >1 g (adults); >7.5 mg/kg (children)

• Emergency Management:

  • Call Poison Control (1-800-222-1222)

  • IV benzodiazepines for seizures

  • Naloxone ineffective

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10. Storage

• Temperature: 15-30°C (59-86°F)

• Liquid: Discard after 6 months

• Lock Away: High abuse/suicide risk - keep in childproof container

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Critical Safety Alerts

⚠️ "Benadryl Challenge" Danger:

Social media trends promoting high doses cause seizures/deaths - educate teens.

⚠️ Elderly Vulnerability:

↑ 50% dementia risk with chronic use; avoid for insomnia in >65 yrs (use melatonin/trazodone instead).

⚠️ Asthma Paradox:

Dries secretions → avoid in acute asthma (impairs mucus clearance).

⚠️ Topical Caution:

Systemic absorption occurs - don't combine with oral diphenhydramine.

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Diphenhydramine vs. Newer Antihistamines

Parameter	Diphenhydramine	Cetirizine
Half-life	4-8 hours	8-10 hours
Sedation	Severe (50-80%)	Mild (10-15%)
Onset	15-30 min	30-60 min
Pediatric Safety	High risk	>6 months safe
OTC Cost	$0.05/dose	$0.20/dose

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Clinical Guidance

✅ Appropriate Uses:

• Acute allergic reactions (with epinephrine for anaphylaxis)

• Nighttime pruritus (e.g., eczema flares)

• Drug-induced dystonic reactions
  ❌ Avoid For:

• Daytime allergies (use 2nd-gen agents)

• Chronic insomnia

• Children <6 yrs without medical supervision
  💡 Pro Tip:

• In anaphylaxis: IM epinephrine first, diphenhydramine is adjunctive only.

OTC Status: Widely available, but high-risk populations require strict medical guidance.

📊 Efficacy Note:

• 50 mg diphenhydramine ≈ 10 mg diazepam for sedation (J Clin Pharmacol), but with greater anticholinergic burden.