Meclizine antiemetic effects

Motion sickness. Effective prophylaxis can be achieved with the parasympatholytic scopolamine (p. 106) and H antihistamines (p. 114) of the diphenyl-methane type (e.g., diphenhydramine, meclizine). Antiemetic activity is not a property shared by all parasympatho-lytics or antihistamines. The efficacy of the drugs mentioned depends on the actual situation of the in vidual (gastric filling, ethanol consumption), environ-... 

Diarylmethylene groups (benzhydryl substituents, as in diphenhydramine) are attached to one of the nitrogen atoms, and an alkyl or aralkyl substituent is attached to the other nitrogen. Early compounds, such as cyclizine, chlorcyclizine, meclizine, buclizine, and hydroxyzine, have been widely used as antihistamines and as agents for treatment of motion sickness, because they have useful central antiemetic effects. 

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). 

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