After taking promethazine, approximately half of the drug is excreted in five to 14 hours, according to Drugs.com. The amount of time it takes for promethazine to completely exit the body depends on the route of administration and the individual patient.
When administered intravenously, promethazine begins working in about five minutes. The drug's onset of action is 20 minutes when it's given orally or by intramuscular injection, notes Drugs.com. One dose of promethazine usually lasts four to six hours, but its effects sometimes persist for up to 12 hours. Promethazine is a medication used to treat certain allergic reactions, relieve nausea and vomiting, prevent motion sickness, and achieve sedation.