Drugs Home > How Long Does Phenergan Stay in Your System?

The elimination half-life of Phenergan (the time it takes for blood levels of the medication to be reduced by half) is approximately 10 to 19 hours. This means that Phenergan will stay in your system for about two to four days. This medication could cause false-positive results on certain drug tests; however, it is not typically screened for.

Phenergan and Drug Testing

Phenergan® (promethazine hydrochloride) is a prescription medication commonly used to treat nausea and vomiting, although it is approved for other uses as well. It is also available in combination with other ingredients as various prescription cough syrups (such as Phenergan with Codeine).
Although drug screening tests usually do not check for Phenergan, since it is not usually considered to be a drug of abuse, there have been reports that the medication can cause false positives on urine drug tests for amphetamines. For this reason, people may wonder how long Phenergan stays in the body.

What Is the Half-Life of Phenergan?

The elimination half-life of a medication is the time it takes for blood levels of the medication to be reduced by half. For Phenergan, 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.
In other words, after the first 10 to 19 hours, the blood level will be 50 percent of the initial level. After the second 10 to 19 hours, it will be 25 percent of the initial level. After the next 10 to 19 hours, it will be 12.5 percent of the initial level, and so forth.
With an elimination half-life of 10 to 19 hours, Phenergan will be mostly out of your system in two to four days (at least for most practical purposes). However, this should not be interpreted to mean that a drug test will not "pick up" Phenergan after two to four days.
It has been suggested that a metabolite of Phenergan (after the medication is metabolized in the liver), not Phenergan itself, may cause the false positives. In addition, it is unknown exactly what blood level of might cause the problem, or if the level is different for different types of drug tests.
At this time, there is simply not enough information to accurately predict how long Phenergan would cause a false positive on a drug screen.
It should be noted that the medicine will not cause false positives for all types of drug tests.
Written by/reviewed by:
Last reviewed by: Kristi Monson, PharmD;
Last updated/reviewed:
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