Pharmacology. Diphenhydramine is an antihistamine with anticholinergic, antitussive, antiemetic, and local anesthetic properties. The antihistaminic property affords relief from itching and minor irritation caused by plant-induced dermatitis and insect bites, and when used as pretreatment, it provides partial protection against anaphylaxis caused by animal serum–derived antivenoms or antitoxins. Drug-induced extrapyramidal symptoms respond to the anticholinergic effect of diphenhydramine. The effects of diphenhydramine are maximal at 1 hour after intravenous injection and last up to 7 hours. The drug is eliminated by hepatic metabolism, with a serum half-life of 3–7 hours.

Indications

1. 
   

   Relief of symptoms caused by excessive histamine effect (eg, ingestion of scombroid-contaminated fish or niacin and rapid intravenous administration of acetylcysteine). Diphenhydramine may be combined with cimetidine or another histamine2 (H2) receptor blocker (See Cimetidine and Other H2 Blockers).

   
   
2. 
   

   Pretreatment before administration of animal serum–derived antivenoms or antitoxins, especially in patients with a history of hypersensitivity or with a positive skin test. Diphenhydramine can be combined with cimetidine or another H2 receptor blocker.

   
   
3. 
   

   Neuroleptic drug–induced extrapyramidal symptoms and priapism (one case report).

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