Diphenhydramine

Diphenhydramine is an antihistamine and is used in particular as a drug against nausea and as a sedative. Originally developed for the treatment of allergic reactions, it is currently used mainly for vomiting, sleep disorders and restlessness.

Pharmacodynamics

The antihistamine displaces histamine from H1 receptors and thereby prevents their activation, thus acting as an inverse agonist. Nausea and emesis can be induced via histamine, which explains the antiemetic effect. In addition, diphenhydramine binds to muscarinic receptors, resulting in various anticholinergic side effects.

Pharmacokinetics

After oral ingestion, bioavailability is about 50%, and maximum concentration in the blood is reached after one hour. Metabolization occurs via the enzyme CYP2D6, the half-life is 4 to 6 hours and the drug is excreted mainly via the kidneys.

Drug interactions

In combination with other centrally depressant drugs, such as opioids or benzodiazepines, increased sedation occurs. Inhibitors of the enzyme CYP2D6 result in increased blood concentrations and increased side effects. Other anticholinergic agents, such as clozapine or chlorprothixene, increase anticholinergic side effects, including urinary retention, constipation, dry mouth, or visual disturbances.

Side effects

Due to the sedative effect, fatigue, dizziness, muscle weakness and concentration disorders often occur even on the following day. In addition to the aforementioned anticholinergic effects, gastrointestinal complaints and prolongation of the QT interval with the occurrence of cardiac arrhythmias may occur. Since the skin is more sensitive during application, direct sunlight should be avoided.

Toxicological data

Application during pregnancy must be avoided as this may lead to malformations such as cleft palate in the child. Furthermore, due to sedation under this medication, active participation in road traffic and operation of dangerous machines must not take place.