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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")

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)

3. Drug Class

  • 1st-Generation Ethanolamine Antihistamine

  • Potent H₁-receptor antagonist

  • Strong anticholinergic & sedative properties

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

5. Side Effects

Common (>20%)Serious (Seek Immediate Care)
Profound sedationAnaphylaxis
Dry mouth/noseSeizures
DizzinessCardiac arrhythmias
Blurred visionAcute glaucoma
Urinary retentionSevere hypotension
ConstipationToxic psychosis (elderly)

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)

7. How to Take

IndicationAdult DoseMax DailyTiming
Allergies25-50 mg q4-6hrs300 mgWith food
Insomnia50 mg at bedtime50 mg30 min before sleep
Motion Sickness25-50 mg pre-travel300 mg1-2 hrs pre-exposure

  • Key Notes:

    • Never drive/operate machinery after dosing

    • Liquid: Use calibrated syringe (household spoons inaccurate)

    • Duration: ≤7 days for allergies/insomnia

8. Pediatric Dosing

AgeDose (Liquid)Frequency
6-11 years12.5-25 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)*

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

10. Storage

  • Temperature: 15-30°C (59-86°F)

  • Liquid: Discard after 6 months

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

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.

Diphenhydramine vs. Newer Antihistamines

ParameterDiphenhydramineCetirizine
Half-life4-8 hours8-10 hours
SedationSevere (50-80%)Mild (10-15%)
Onset15-30 min30-60 min
Pediatric SafetyHigh risk>6 months safe
OTC Cost$0.05/dose$0.20/dose

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.

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