Antiemetics antihistamines

Mechanism of Action A phenothiazine that acts as an antihistamine, antiemetic, and CNS-antipsychotiC typical hypnotic. As an antihistamine, inhibits histamine at histamine receptor sites. As an antiemetic, diminishes vestibular stimulation, depresses labyrinthine function, and acts on the chemoreceptor trigger zone. As a sedative-hypnotic, produces CNS depression by decreasing stimulation to the brainstem reticular formation. Therapeutic Effect Prevents allergic responses mediated by histamine, such as rhinitis, urticaria, and pruritus. Prevents and relieves nausea and vomiting. Pharmacokinetics ... 

Diphenhydramine (Benadryl) [OTC] [Antihistamine/ Antitussive/Antiemeticj Uses Tx prevent allergic Rxns, motion sickness, potentiate narcotics, sedation, cough suppression, Tx of extrapyramidal (dystonic) Rxns Action Antihistamine, antiemetic Dose Adults. 25-50 mg IV/EM Feds. > 2 y. 2—5 mg/kg IV/EM Caution [B, —] Contra Acute asthma Disp Tabs,... 

Antihistamine antiemetics such as diphenhydiinate (Dramamine), meclizine hydrochloride (Antivert), and diphenhydramine hydrochloride (Benadryl) are over-the-counter medications that prevent nausea, vomiting, and dizziness (vertigo) caused by motion by inhibiting stimulation in the middle ear. They also cause drowsiness, dryness of the mouth, and constipation. 

Since there was some evidence that these compounds owe their action to interference with the action of histamine, this class has earned the soubriquet "antihistamines." This class of drugs is further characterized by a spectrum of side effects which occur to a greater or lesser degree in various members. These include antispasmodic action, sedative action, analgesia, and antiemetic effects. The side effects of some of these agents are sufficiently pronounced so that the compounds are prescribed for that effect proper. Antihistamines, for example, are used as the sedative-hypnotic component in some over-the-counter sleeping pills. 

The antiemetics and antivertigo drug may have additive effects when used with alcohol and other CNS depressants such as sedatives, hypnotics, antianxiety drugp, opiates, and antidepressants. There may be additive anticholinergic effects (see Chap. 25) when administered with drag s that have anticholinergic activity such as the antihistamines, antidepressants, pheno-thiazines, and disopyramide The antacids decrease absorption of the antiemetics. 

Profuse or prolonged vomiting can lead to complications of dehydration and metabolic abnormalities. Patients must have adequate hydration and electrolyte replacement orally (if tolerated) or intravenously to prevent and correct these problems. Some pharmacologic treatments work locally in the GI tract (e.g., antacids and prokinetic agents), whereas others work in the central nervous system (e.g., antihistamines and antiemetics).1... 

Hydration, heating pads, relaxation, and distraction Stool softener and/or stimulants for constipation Antihistamine for itching Antiemetics for nausea or vomiting... 

Antiemetic drugs from the antihistaminic-anticholinergic category may be appropriate in the treatment of simple symptomatology. 

Medications for symptomatic relief from vertigo consist of antiemetics, benzodiazepines and antihistamines. They are all mostly aimed at the psychological consequences of dizziness and can all have highly unfavourable side effects, for example, sedation, anticholinergic effects and insomnia. The psychological consequences of dizziness in elderly should rather be treated with information about the condition, supportive help actions and increased social activities, than with drugs. 

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. 

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-... 

Pregnancy vomiting is prone to occur in the first trimester thus pharmacotherapy would coincide with the period of maximal fetal vulnerability to chemical injury. Accordingly, antiemetics (antihistamines, or neuroleptics if required) should be used only when continuous vomiting threatens to disturb electrolyte and water balance to a degree that places the fetus at risk. 

Antihistamine Sedative effects Antihistaminic activity Anticholinergic activity Antiemetic effects... 

Pharmacoiogy Cyclizine and meclizine have antiemetic, anticholinergic, and antihistaminic properties. 

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. 

Bronchodilator activity, antihistaminic, and analgesic effects have been confirmed. Hydroxyzine has antispasmodic properties, and an antiemetic effect has been demonstrated. 

Hydroxyzine hydrochloride Atarax, Vistarit) is the antihistamine with the greatest use in the treatment of anxiety. It is often used to reduce the anxiety that is associated with anesthesia and surgery. It also produces sedation, dries mucous membranes (via an anticholinergic mechanism), and has antiemetic activity. A more extensive discussion of the pharmacology of the Hj-receptor antagonists is found in Chapter 38. 

---