ACTIONS
These drugs appear to act primarily by inhibiting the CTZ or by depressing the sensitivity of the vestibular apparatus of the inner ear. Those that act on the CTZ are more effective for vomiting caused by stimulation of the CTZ, whereas those that act on the vestibular apparatus of the inner ear are more effective for vertigo
associated with motion sickness and middle and inner ear surgeries.
USES
Antiemetic Drugs
An antiemetic is used to prevent (prophylaxis) or treat nausea and vomiting. An example of prophylactic use is the administration of an antiemetic before surgery to prevent vomiting during the immediate postopera-tive period when the patient is recovering from anes-thesia. Another example is giving an antiemetic before administration of one or a combination of antineo-plastic drugs (drugs used in the treatment of cancer; which have a high incidence of causing vomiting.Dronabinol is the only currently available derivative of THC, which is a derivative of the active substance found in marijuana. Dronabinol is a second-line antiemetic and is used after treatment with other antiemetics has failed.
Other causes of nausea and vomiting that may be treated with an antiemetic include radiation therapy for a malignancy, bacterial and viral infections, nausea and vomiting caused by drugs, Ménière’s disease and other ear disorders, and neurological diseases and disorders. Some of these drugs also are used to trea
the nausea and vomiting seen with motion sickness Some antiemetics also are antivertigo drugs.
Antivertigo Drugs
An antivertigo drug is used to treat vertigo, which is usually accompanied by light-headedness, dizziness,and weakness. The individual often has difficulty walk-ing. Some of the causes of vertigo include high alcohol consumption during a short time, certain drugs, inner ear disease, and postural hypotension. Motion sickness (seasickness, carsickness) has similar symptoms but is caused by repetitive motion (eg, riding in an airplane,boat, or car). Both vertigo and motion sickness may result in nausea and vomiting.It is important to note that antivertigo drugs are essentially antiemetics because many of these prepara-tions, whether used for motion sickness or vertigo, also have direct or indirect antiemetic properties. They prevent the nausea and vomiting that occur because of stimulation of the vestibular apparatus in the ear.Stimulation of this apparatus results in vertigo, which is often followed by nausea and vomiting.
ADVERSE REACTIONS
The most common adverse reactions seen with these drugs are varying degrees of drowsiness. Additional adverse reactions for each drug
CONTRAINDICATIONS
The antiemetic and antivertigo drugs are contraindi-cated in patients with known hypersensitivity to these drugs, those in a coma, or those with severe central nervous system (CNS) depression. In general ,these drugs are not recommended during pregnancy,lactation, or for uncomplicated vomiting in young children.
Metoclopramide is contraindicated in patients with a seizure disorder, breast cancer,pheochromocytoma, or gastrointestinal obstruction.Prochlorperazine is contraindicated in patients with bone marrow depression, blood dyscrasia,
Parkinson’s disease, or severe liver or cardiovascular disease.Thiethylperazine is classified as Pregnancy Category X and is contraindicated during pregnancy.
PRECAUTIONS
Severe nausea and vomiting should not be treated with antiemetic drugs alone. The cause of the vomiting must be investigated. Antiemetic drugs may hamper the diagnosis of disorders such as brain tumors, appendicitis, intestinal obstruction, or drug toxicity (eg, dig-italis toxicity). Delayed diagnosis of any of these dis-orders could have serious consequences for the patient.Antiemetics and antivertigo drugs are used cau-tiously in patients with glaucoma or obstructive disease of the gastrointestinal or genitourinary system, those with renal or hepatic dysfunction, and in older men with possible prostatic hypertrophy. Promethazine is used cautiously in patients with hypertension, sleep
apnea, or epilepsy. Trimethobenzamide is used cau-tiously in children with a viral illness because it may increase the risk of Reye’s syndrome.Perphenazine, prochlorperazine, promethazine,scopolamine, chlorpromazine, and trimethobenzamide
are Pregnancy Category C drugs. The pregnancy cate-gory of diphenidol is unknown. Other antiemetics and antivertigo drugs are classified as Pregnancy Category B
(except for thiethylperazine, which is classified as Pregnancy Category X).
INTERACTIONS
The antiemetics and antivertigo drugs may have addi-tive effects when used with alcohol and other CNS depressants such as sedatives, hypnotics, antianxiety drugs, opiates, and antidepressants. There may be addi-tive anticholinergic effects when admin-istered with drugs that have anticholinergic activity such as the antihistamines, antidepressants, pheno-thiazines, and disopyramide. The antacids decrease absorption of the antiemetics.When ondansetron is administered with rifampin,blood levels of ondansetron may be reduced, decreas-ing the antiemetic effect. Dimenhydrinate may mask the signs and symptoms of ototoxicity when adminis-tered with ototoxic drugs, such as the aminoglycosides, causing irreversible hearing damage.When lithium is administered with prochlorperazine,the risk of extrapyramidal reactions increases
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