DESCRIPTION:-
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. It has some undesired antimuscarinic and sedative effects.
This compound belongs to the class of organic compounds known as diphenylmethanes. These are compounds containing a diphenylmethane moiety, which consists of a methane wherein two hydrogen atoms are replaced by two phenyl groups.
PHARMACOLOGY:-
Diphenhydramine is an antihistamine of the ethanolamine class. Ethanolamine antihistamines have significant antimuscarinic activity and produce marked sedation in most patients. In addition to the usual allergic symptoms, the drug also treats irritant cough and nausea, vomiting, and vertigo associated with motion sickness. It also is used commonly to treat drug-induced extrapyramidal symptoms as well as to treat mild cases of Parkinson's disease. Rather than preventing the release of histamine, as do cromolyn and nedocromil, diphenhydramine competes with free histamine for binding at HA-receptor sites. 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. 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.
PHARMACOKINETICS:-
Absorption:- Quickly absorbed with maximum activity occurring in approximately one hour.
Protein binding:- 98 to 99%
Route of elimination:-Little, if any, is excreted unchanged in the urine; most appears as the degradation products of metabolic transformation in the liver, which are almost completely excreted within 24 hours.
Half life:-1-4 hours
Toxicity:-LD50=500 mg/kg (orally in rats). Considerable overdosage can lead to myocardial infarction (heart attack), serious ventricular dysrhythmias, coma and death. Affected organism:- Humans and other mammals.
DRUG INTERACTION:-
3,4-Methylenedioxyamphetamine+diphenhydramine=3,4-Methylenedioxyamphetamine may decrease the sedative activities of Diphenhydramine.
3,4-Methylenedioxymethamphetamine + diphenhydramine = 3,4-Methylenedioxymethamphetamine may decrease the sedative activities of Diphenhydramine.
4-Methoxyamphetamine + diphenhydramine = The metabolism of 4-Methoxyamphetamine can be decreased when combined with Diphenhydramine.
7-Nitroindazole + diphenhydramine = The risk or severity of adverse effects can be increased when Diphenhydramine is combined with 7-Nitroindazole.
Abiraterone + diphenhydramine =The serum concentration of Diphenhydramine can be increased when it is combined with Abiraterone.
Acepromazine+ diphenhydramine= The risk or severity of adverse effects can be increased when Diphenhydramine is combined with Acepromazine.
Aceprometazine+ diphenhydramine= The risk or severity of adverse effects can be increased when Diphenhydramine is combined with Aceprometazine.
Acetaminophen+ diphenhydramine= The metabolism of Acetaminophen can be decreased when combined with Diphenhydramine.
Acetylcholine+ diphenhydramine= The metabolism of Acetylcholine can be decreased when combined with Diphenhydramine.
SIDE EFFECTS:-
Nervous system side effects have been reported frequently. These have included depression with drowsiness and sedation in nearly all patients treated. Motor skills may be impaired. Dystonic reactions have been reported after single doses of diphenhydramine.
The CNS depressant effect of diphenhydramine parallels its plasma concentrations. The plasma concentration threshold for sedation is 30 to 42 ng/mL, and to cause mental impairment is 58 to 74 ng/mL. Patients should be warned against driving while taking diphenhydramine.
Dystonic reactions have been accompanied by dizziness, mental confusion, rigidity, lip and tongue protrusion, trismus, torticollis, and swallowing difficulties and generally resolve spontaneously. Toxic encephalopathy has been reported in a child with chicken pox treated generously with topical diphenhydramine.
Delirium has been reported in elderly patients with mild dementia following a small oral dose of diphenhydramine.
Hypersensitivity side effects have included rash, pruritus and eczema. Photosensitivity reactions have also been reported.
Most commonly, hypersensitivity has manifested itself in patients receiving systemic drug after being sensitized to it by topical application. Sensitization with systemic administration has also been reported.
Gastrointestinal side effects have been usually mild and included nausea and dry mouth.
Cardiovascular
Cardiovascular side effects have included hypotension, tachycardia, and palpitations.
Ocular
Ocular side effects have included blurred vision, diplopia, and dry eyes due to anticholinergic effects.
Genitourinary side effects have included urinary retention and dysuria as a result of the anticholinergic effects of diphenhydramine.
Hematologic side effects such as hemolytic anemia, thrombocytopenia, and agranulocytosis have been rarely caused by antihistamine.
THERAPEUTIC USES:-
Diphenhydramine is an antihistamine used to relieve symptoms of allergy, hay fever, and the common cold. These symptoms include rash, itching, watery eyes, itchy eyes/nose/throat, cough, runny nose, and sneezing. It is also used to prevent and treat nausea, vomiting and dizziness caused by motion sickness.
Diphenhydramine also has local anesthetic properties, and has been used as such in people allergic to common local anesthetics such as lidocaine.
REFERENCE:-Tripathi K D"Essential of medical pharmacology",7th edition ,page no.164,165,168,177,663.
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