Dimenhydrinate antiemetic effects

When ondansetron is administered with rifampin, blood levels of ondansetron may be reduced, decreasing the antiemetic effect. Dimenhydrinate may mask the signs and symptoms of ototoxicity when administered with ototoxic drugp, such as the aminoglycosides (see Chap. 10), causing irreversible hearing damage. When lithium is administered with prochlorperazine, the risk of extrapyramidal reactions increases (see Chap. 32). 

Pharmacokinetics Dimenhydrinate has a depressant action on hyperstimulated labyrinthine function. The precise mode of action is not known. The antiemetic effects are believed to be caused by the diphenhydramine, an antihistamine also used as an antiemetic agent. 

Antagonists. Most of the so-called Hi-antihistamines also block other receptors, including M-cholinoceptors and D-receptors. Hi-antihistamines are used for the symptomatic relief of allergies (e.g., bamipine, chlorpheniramine, clemastine, dimethindene, mebhydroline pheniramine) as antiemetics (meclizine, dimenhydrinate, p. 330), as over-the-counter hypnotics (e.g., diphenhydramine, p. 222). Promethazine represents the transition to the neuroleptic phenothiazines (p. 236). Unwanted effects of most Hi-antihistamines are lassitude (impaired driving skills) and atropine-like reactions (e.g., dry mouth, constipation). At the usual therapeutic doses, astemizole, cetrizine, fexofenadine, and loratidine are practically devoid of sedative and anticholinergic effects. Hj-antihistamines (cimetidine, ranitidine, famotidine, nizatidine) inhibit gastric acid secretion, and thus are useful in the treatment of peptic ulcers. 

Some Hj antagonists, notably dimenhydrinate and meclizine, often are of benefit in vestibular disturbances such as Meniere s disease and in other types of true vertigo. Only promethazine has usefulness in treating the nausea and vomiting subsequent to chemotherapy or radiation thercq y for malignancies however, other effective antiemetic drugs are available (see Chapter 37). 

---