Diphenhydramine is an antihistamine that reduces the effects of histamine, which causes symptoms like sneezing, itching, and runny nose. It is used to treat hives, rashes, cold or allergy symptoms, insomnia, and even Parkinson’s disease.

Dosage and Usage

Common dosages include:

• Allergy: 10–25 mg (max 300 mg daily) or 10–50 mg (max 400 mg daily).
• Insomnia: 50 mg, typically 30 minutes before bedtime.
• Cough: 25–50 mg (max 150 mg daily).
• Motion sickness: 10–50 mg (max 100 mg daily).
• Parkinson’s disease: 25–50 mg (max 300–400 mg daily).

Doses for children and the elderly may vary based on the condition.

Drug Interactions

Concurrent use with oligostat is not recommended. It interacts severely with drugs like calcium, magnesium, potassium, and sodium oxybates, eluxadoline, isocarboxazid, metoclopramide (nasal), pitolisant, and others. Always consult a doctor for alternatives.

Side Effects

Common side effects include:

• Drowsiness
• Cognitive decline in elderly patients
• Anticholinergic effects (dry mouth, constipation, blurred vision, increased heart rate)
• Decreased saliva and nasal mucus
• Severe effects like seizures, low blood pressure, or hemolytic anemia (rare)

Contraindications

• Hypersensitivity to the drug
• Respiratory conditions like acute asthma
• Use in newborns, breastfeeding mothers, or children under 6 years without a prescription

Warnings

• Can cause CNS depression, affecting activities like driving.
• May amplify sedative effects of alcohol.
• Use cautiously in patients with glaucoma, prostate hypertrophy, or thyroid dysfunction.
• High risk for elderly patients due to fall hazards and anticholinergic effects.

Pregnancy and Lactation

Limited human studies suggest it may be safe during pregnancy, but animal studies indicate minor risks. It is not recommended during breastfeeding.

Sources: Medscape, Drugs.com