Source: Health Products Regulatory Authority (IE) Revision Year: 2021 Publisher: Chefaro Ireland DAC, The Sharp Building, Hogan Place, Dublin 2, Ireland
Pharmacotherapeutic group: Antihistamines for systemic use, aminoalkyl ethers
ATC code: R06AA02
Diphenhydramine is an ethanolamine-derivative antihistamine. It has anticholinergic and marked sedative effects. It acts by inhibiting the effects on H1-receptors.
Diphenhydramine is effective in reducing sleep onset (i.e., time to fall asleep) and increasing the depth and quality of sleep.
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.
It is rapidly distributed throughout the body including CNS. 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 extensively metabolised, mainly in the liver. The drug is metabolised principally to Diphenylmetoxyacetic acid and is also dealkylated. The metabolites are conjugated with glycine and glutamine and excreted in urine. Multiple cytochrome P450 enzymes contribute to the metabolism of diphenhydramine, with the highest activity shown by CYP2D6. Diphenhydramine has been shown to competitively inhibit CYP2D6 activity.
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.
Diphenhydramine hydrochloride has a well-established safety profile. There are no pre-clinical data of relevance to the prescriber which are additional to that already included in other sections of the SmPC.
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