It also is used commonly to treat drug-induced extrapyramidal symptoms as well as to treat mild cases of Parkinson's disease. This anticholinergic action appears to be due to a central antimuscarinic effect, which also may be responsible for its antiemetic effects, although the exact mechanism is unknown. Rather than preventing the release of histamine, as do cromolyn and nedocromil, diphenhydramine competes with free histamine for binding at HA-receptor sites. In addition to the usual allergic symptoms, the drug also treats irritant cough and nausea, vomiting, and vertigo associated with motion sickness. Diphenhydramine is an antihistamine of the ethanolamine class. Ethanolamine antihistamines have significant antimuscarinic activity and produce marked sedation in most patients. Diphenhydramine competitively antagonizes the effects of histamine on HA-receptors in the GI tract, uterus, large blood vessels, and bronchial muscle. Ethanolamine derivatives have greater anticholinergic activity than do other antihistamines, which probably accounts for the antidyskinetic action of diphenhydramine.
Alison B. Lukacsko, Joseph J. Piala, "Effect of a combination of a terbutaline, diphenhydramine and ranitidine composition on gastrointestinal injury produced by nonsteroidal anti-inflammatory compositions." U.S. Patent US5260333, issued December, 1983.
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1-4 hours. Little, if any, is excreted unchanged in the urine; most appears as the degradation products of metabolic transformation in the liver, which are almost compley excreted within 24 hours.
Quickly absorbed with maximum activity occurring in approximay one hour. 98 to 99% Hepatic and renal.
Diphenhydramine competes with free histamine for binding at HA-receptor sites. This antagonizes the effects of histamine on HA-receptors, leading to a reduction of the negative symptoms brought on by histamine HA-receptor binding.
LD 50 =500 mg/kg (orally in rats). Considerable overdosage can lead to myocardial infarction (heart attack), serious ventricular dysrhythmias, coma and death.
For the treatment of symptoms associated with Vertigo/Meniere's disease, nausea and vomiting, motion sickness and insect bite.
It has some undesired antimuscarinic and sedative effects. A histamine H1 antagonist used as an antiemetic, antitussive, for dermatoses and pruritus, for hypersensitivity reactions, as a hypnotic, an antiparkinson, and as an ingredient in common cold preparations.Diphenhydramine