Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2017 Publisher: Norma Chemicals Ltd., 51-53 Stert Street, Abingdon, Oxfordshire OX14 3JF United Kingdom
Pharmacotherapeutic group: Antihistamines for systemic use – diphenhydramine
ATC Code: R06AA02
Diphenhydramine is an ethanolamine-derivative anti-histamine with anti-cholinergic (anti-spasmodic), anti-tussive and sedative activity. It acts by inhibiting the effects on H1-receptors.
Diphenhydramine is effective in reducing sleep onset (ie, time to fall asleep) and increasing the depth and quality of sleep.
Diphenhydramine is a histamine H1 receptor antagonist.
The main site of metabolism is the liver.
Diphenhydramine hydrochloride is rapidly absorbed following oral administration. Apparently it undergoes first-pass metabolism in the liver and only about 40-60% of an oral dose reaches systematic circulation as unchanged diphenhydramine.
Diphenhydramine is rapidly distributed throughout the whole body. Peak plasma concentrations are attained within 1-4 hours. The sedative effect also appears to be maximal within 1-3 hours after administration of a single dose. It is positively correlated with the plasma drug concentration.
Diphenhydramine is approx 80-85% bound to plasma proteins. Diphenhydramine is rapidly and almost completely metabolised. It is metabolised principally to diphenylmetoxyacetic acid and is also dealkylated. The metabolites are conjugated with glycine and glutamine and excreted in urine. Only about 1% of a single dose is excreted unchanged in urine.
The elimination half-life ranges from 2.4-9.3 hours in healthy adults. The terminal elimination half-life is prolonged in liver cirrhosis.
Not applicable.
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