Mechanism of action of sedating antihistamines

Doxepin has potent effects on H-adrenergic receptor antagonism (amitriptyline may be the worst in this regard).

Diphenhydramine also blocks the action of acetylcholine (anticholinergic effect) and is used as a sedative because it causes drowsiness.Yes (Injection) Diphenhydramine should be used with caution (if at all) in persons with narrow-angle glaucoma, prostatic hypertrophy (enlarged prostate gland), hyperthyroidism, cardiovascular disease, high blood pressure (hypertension), and asthma.: Diphenhydramine adds to (exaggerates) the sedating effects of alcohol and other drugs than can cause sedation such as the benzodiazepine class of anti-anxiety (for example, diazepam [Valium], lorazepam [Ativan], clonazepam [Klonopin], alprazolam [Xanax]), the narcotic class of pain medications and its derivatives (for example, oxycodone and acetaminophen [Percocet], and hydrocodone and acetaminophen [Vicodin], guaifenesin with hydromorphone [Dilaudid], codeine, propoxyphene [Darvon]), the tricyclic class of antidepressants (for example, amitriptyline [Elavil, Endep], imipramine [Tofranil], desipramine [Norpramin]), and certain antihypertensive medications (for example, clonidine [Catapres], propranolol [Inderal]).In modern medicine they have been largely replaced by the benzodiazepines, primarily because they can induce tolerance, physical dependence and serious withdrawal symptoms.Nevertheless, certain barbiturates are still employed as anticonvulsants (phenobarbital) and to induce anesthesia (thiopental).