Promethazine will be mostly out of your system in two to four days . The elimination half-life of a medication is the time it takes for blood levels of the medication to be reduced by half. For Promethazine the half-life is approximately 10 to 19 hours. This means that every 10 to 19 hours, the level in your blood will drop by 50 percent. It takes on average 5.5 x elimination half-life for a drug to be eliminated from your system. For Promethazine this is (5.5 x 10 hours) two days to (5.5 x 19) four days approximately to be eliminated from your system.

Metabolism, age, health, weight and the amount and frequency of the drug taken, are all factors that can affect the rate at which the body can clear the drug from your system.