Diphenhydramine

Oral
Allergic conditions
Adult: 25-50 mg 3 or 4 times daily. Max: 300 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Child: 2-6 years 6.25 mg 4-6 hourly; 6-12 years 12.5-25 mg 4-6 hourly; >12 years Same as adult dose. For prevention of motion sickness, administer 30 minutes before exposure to motion.

Oral
Motion sickness
Adult: 25-50 mg 3 or 4 times daily. Max: 300 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Child: 2-6 years 6.25 mg 4-6 hourly; 6-12 years 12.5-25 mg 4-6 hourly; >12 years Same as adult dose. For prevention of motion sickness, administer 30 minutes before exposure to motion.

Oral
Short-term management of insomnia
Adult: 50 mg given 30 minutes before bedtime as needed.

Parenteral
Motion sickness
Adult: 10-50 mg up to 100 mg if needed via IV inj at a rate of 25 mg/min or deep IM inj. Max: 400 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Child: 5 mg/kg via IV inj at a rate of 25 mg/min or deep IM inj in 4 divided doses. Max: 300 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Incompatibility: IV/IM: Incompatible with amphotericin B, cefmetazole, cefalotin, hydrocortisone Na succinate, soluble barbiturates, contrast media, and strong acids or alkali solutions.

Parenteral
Allergic conditions
Adult: 10-50 mg up to 100 mg if needed via IV inj at a rate of 25 mg/min or deep IM inj. Max: 400 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Child: 5 mg/kg via IV inj at a rate of 25 mg/min or deep IM inj in 4 divided doses. Max: 300 mg daily. For prevention of motion sickness, administer 30 minutes before exposure to motion.
Incompatibility: IV/IM: Incompatible with amphotericin B, cefmetazole, cefalotin, hydrocortisone Na succinate, soluble barbiturates, contrast media, and strong acids or alkali solutions.

Parenteral
Parkinson's disease
Adult: 10-50 mg up to 100 mg if needed, via IV inj at a rate not exceeding 25 mg/min or via deep IM inj, when oral therapy is impossible or contraindicated. Max: 400 mg daily.
Incompatibility: IV/IM: Incompatible with amphotericin B, cefmetazole, cefalotin, hydrocortisone Na succinate, soluble barbiturates, contrast media, and strong acids or alkali solutions.

Topical/Cutaneous
Pruritic skin disorders
Adult: As 2% cream: Apply to affected area(s) up to max bid for not more than 3 days.
Child: ≥2 years Same as adult dose.

May be taken with or without food.

Stenosing peptic ulcer or pyloroduodenal obstruction. Premature infants and neonates. Lactation. Concomitant use with other antihistamine-containing medicines (including topical antihistamines).

Patient with narrow-angle glaucoma, prostatic hypertrophy or genitourinary obstruction, urinary retention, asthma, bronchitis, COPD, CV (e.g. hypertension, ischaemic heart disease), thyroid disorder, myasthenia gravis, seizure disorders (e.g. epilepsy). Moderate to severe renal and hepatic impairment. Elderly. Pregnancy. Patient Counselling This drug may cause drowsiness or sedation, dizziness, blurred vision, cognitive and psychomotor impairment, if affected, do not drive or operate machinery. Monitoring Parameters Monitor mental alertness.

Significant: Drug tolerance (continuous use). Rarely, eczematous reactions (topical). Blood and lymphatic system disorders: Haemolytic anaemia, thrombocytopenia, agranulocytosis. Cardiac disorders: Palpitations, tachycardia, extrasystoles. Gastrointestinal disorders: Dry mouth, epigastric distress, nausea, vomiting, diarrhoea, constipation. General disorders and administration site conditions: Fatigue. Metabolism and nutrition disorders: Anorexia. Nervous system disorders: Sedation, drowsiness, restlessness, dizziness, excitability (children), headache. Psychiatric disorders: Disturbance in attention, confusion, nervousness. Renal and urinary disorders: Urinary frequency, urinary retention, difficulty in micturition. Respiratory, thoracic and mediastinal disorders: Thickening of bronchial secretions. Skin and subcutaneous tissue disorders: Sensitisation reaction, photosensitivity. Vascular disorders: Hypotension.

Symptoms: Drowsiness, sedation, mydriasis, fever, flushing, agitation, tremor, dystonic reactions, hallucination, ECG changes. Severe cases: Rhabdomyolysis, convulsions, delirium, toxic psychosis, arrhythmias, coma, cardiovascular collapse. May result in hallucinations, convulsions and death in paediatrics patients. Management: Supportive and symptomatic treatment. Administer parenteral diazepam for treatment of convulsions and marked CNS stimulation. May give vasopressors to treat hypotension.

May increase sedative effects of other CNS depressants (e.g. tranquilizers, hypnotics, anxiolytics). Enhanced and prolonged anticholinergic effects with MAOIs. May potentiate effects of anticholinergic agents (e.g. atropine, TCA). May diminish therapeutic effect of betahistine.
Potentially Fatal: Enhanced CNS depressant effects with other antihistamines (e.g. hydroxyzine).

May potentiate sedative effects of alcohol.

May give false-positive result in urine detection of methadone and phencyclidine, serum TCA screening. May suppress wheal and flare reactions to skin test antigens.

Diphenhydramine, a monoethanolamine derivative, is an antihistamine with sedative and anti-cholinergic activity. It competes with histamine for H₁ receptor sites on effector cells in the blood vessels, gastrointestinal tract, and respiratory tract.
Duration: Histamine-induced wheal suppression: ≤10 hours; histamine-induced flare suppression: ≤12 hours.
Absorption: Well absorbed from the gastrointestinal tract. Bioavailability: 42-62%. Time to peak plasma concentration: Approx: 1-4 hours.
Distribution: Widely distributed throughout the body including the CNS. Crosses the placenta, enters breastmilk. Volume of distribution: 17 L/kg. Plasma protein binding: 98.5%.
Metabolism: Extensively metabolised in the liver by CYP2D6 isoenzyme via N-demethylation and minor demethylation by CYP1A2, 2C9, 2C19; smaller degrees in pulmonary and renal systems; undergoes significant first-pass effect.
Excretion: Mainly via urine (as metabolites and unchanged drug). Elimination half-life: 2.4-9.3 hours.

Oral: Store below 25°C. Protect from light and moisture. Parenteral: Store below 25°C. Protect from light and moisture. Topical/Cutaneous: Store below 25°C. Protect from light and moisture.

Antihistamines & Antiallergics / Antiparkinsonian Drugs / Antivertigo Drugs / Hypnotics & Sedatives

D04AA32 - diphenhydramine ; Belongs to the class of topical antihistamines used in the treatment of pruritus.
R06AA02 - diphenhydramine ; Belongs to the class of aminoalkyl ethers used as systemic antihistamines.