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What is Promethazine- AKA Phenergan®
Promethazine, known to us as Phenergan® is an antiemetic, that is, an anti- nausea/vomiting medication. Many Migraineurs have experienced this safe treatment at an emergency room visit. Usually administered with Demerol® as a "rescue" treatment to abort an aggressive Migraine. This is a very safe and effective treatment for the non-pain symptoms of nausea and vomiting associated with Migraines.
Promethazine is a phenothiazine however it is not used clinically as a neuroleptic. It is an H1-antagonist with considerable anticholinergic, sedative, and antiemetic effects and some local anesthetic properties. Promethazine is a versatile drug but is used predominantly as an antiemetic or to prevent motion sickness. Promethazine was approved by the FDA in 1951.
Mechanism of Action:
The predominant action of promethazine is antagonism of H1-receptors. Although promethazine is classified as a phenothiazine, its ability to antagonize dopamine is approximately one-tenth that of chlorpromazine. For this reason, promethazine is not used as a neuroleptic.
Like other H1-antagonists, promethazine does not prevent the release of histamine, as do cromolyn and nedocromil, but competes with free histamine for binding at H1-receptor sites. Histamine receptors in the GI tract, uterus, large blood vessels, and bronchial muscle are blocked. The relief of motion sickness and nausea/vomiting appear to be related to central anticholinergic actions and may implicate activity on the medullary chemoreceptor trigger zone. Other CNS receptor sites can also be affected, since promethazine is believed to indirectly reduce stimuli to the brain stem reticular system. Sedation is significant at concentrations achieved from therapeutic dosages. Mild antitussive activity has been attributed to promethazine, but this effect probably results from anticholinergic and sedative actions. Local anesthetic activity requires higher concentrations than those required to antagonize histamine receptors.
Promethazine is administered orally, rectally, intramuscularly, and intravenously. Onset of action occurs within 15-60 minutes after oral or rectal administration and within 20 minutes after intramuscular administration. Following intravenous administration, onset of action occurs within 3-5 minutes. Antihistaminic and sedative effects are sustained for 4-6 hours and 2-8 hours, respectively. Promethazine is highly protein-bound (80-93%). It is widely distributed in body tissues and fluids, and it crosses the placenta and is excreted into breast milk. Metabolism occurs in the liver, producing inactive metabolites such as promethazine sulfoxide and other glucuronides. The elimination half-life is 10-14 hours, with excretion of metabolites in the urine and the feces.
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