Diphenhydramine is a first-generation antihistamine with anticholinergic and sedative properties indicated for allergic reactions, motion sickness, insomnia, and various other conditions. It is one of the most widely used over-the-counter medications but requires careful consideration due to its significant anticholinergic effects, particularly in elderly patients.
Clinical Overview
Primary Clinical Applications
Diphenhydramine is indicated for allergic reactions, seasonal allergies, motion sickness, insomnia, cough suppression, and as an adjunct in treating extrapyramidal symptoms. It is available in multiple formulations including oral, topical, and injectable forms for various clinical situations.
Mechanism and Clinical Benefits
Diphenhydramine blocks histamine H1 receptors and has significant anticholinergic activity. It provides antihistaminic, sedative, antiemetic, antitussive, and local anesthetic effects. The sedating properties make it useful for sleep disorders but also contribute to its side effect profile.
Safety Considerations
While generally safe when used appropriately, diphenhydramine can cause significant anticholinergic toxicity, especially in elderly patients or with overdose. It interacts with numerous medications and can impair cognitive function and increase fall risk in susceptible populations.
Key Clinical Points
Listed on the Beers Criteria for avoidance in older adults
High anticholinergic burden makes diphenhydramine inappropriate in geriatric patients, where it contributes to confusion, urinary retention, and falls. Its over-the-counter status means patients frequently self-medicate without disclosing it.
Paradoxical excitation in children
Rather than sedating, diphenhydramine can produce agitation and hyperactivity in paediatric patients. Contraindicated outright in newborns and premature infants.
Tolerance to the hypnotic effect develops quickly
Sedation diminishes within days of regular use while the anticholinergic burden persists, which makes it a poor choice for anything beyond short-term insomnia.
Additive burden with other anticholinergics
Frequently co-administered with tricyclics, low-potency antipsychotics, and antiparkinsonian agents. The cumulative anticholinergic load, rather than any single drug, is what precipitates toxicity.
Also used for acute extrapyramidal reactions
Serves as an adjunct in acute dystonic reactions where benztropine is unavailable, and has antiemetic and antitussive activity alongside its antihistamine effect.
Prescribing Information
Dosing & Administration
Allergic Reactions – Adults:
- Oral: 25-50 mg every 4-6 hours
- Maximum: 300 mg daily
- IV/IM: 10-50 mg, maximum 400 mg daily
Insomnia – Adults:
- Oral: 25-50 mg at bedtime
Motion Sickness – Adults:
- Oral: 25-50 mg every 4-6 hours
- Prevention: 30 minutes before travel
Pediatric (≥2 years):
- Oral: 1.25 mg/kg every 4-6 hours
- Maximum: 37.5 mg every 4-6 hours
Elderly:
- Use lower doses due to increased sensitivity
- Avoid if possible due to anticholinergic effects
Indications
- Allergic reactions and seasonal allergies
- Motion sickness
- Insomnia (short-term)
- Cough suppression
- Extrapyramidal symptoms (adjunct)
Contraindications
- Hypersensitivity to diphenhydramine
- Newborns and premature infants
- Nursing mothers (if high doses used)
- Acute asthma attack
- Narrow-angle glaucoma
- Bladder neck obstruction
Warnings & Precautions
- Anticholinergic toxicity: Especially in elderly patients
- CNS depression: Impaired alertness and coordination
- Respiratory depression: Risk in compromised patients
- Paradoxical excitation: Particularly in children
- Falls risk: Due to sedation and orthostatic hypotension
- Cognitive impairment: Memory and attention effects
Drug Interactions
- CNS depressants: Enhanced sedation
- Anticholinergic drugs: Additive anticholinergic effects
- MAOIs: Prolonged anticholinergic effects
- Alcohol: Increased CNS depression
Adverse Reactions
Common:
- Drowsiness, dizziness, dry mouth, blurred vision, constipation
Anticholinergic Effects:
- Urinary retention, confusion, memory impairment
Serious:
- Anticholinergic toxicity, respiratory depression, seizures
Special Populations
- Elderly: Avoid due to high anticholinergic burden (Beers Criteria)
- Pregnancy: Category B – generally considered safe
- Pediatric: Risk of paradoxical excitation
- Hepatic impairment: Use with caution, consider dose reduction
Medical Disclaimer: This information is for educational purposes only and is not intended as medical advice. Always consult with qualified healthcare professionals before making any treatment decisions. Individual patient circumstances may vary significantly.
