A stable-labeled internal standard for promethazine testing or isotope dilution methods by GC/MS or LC/MS for clinical toxicology, urine drug testing, or forensic analysis. Promethazine is a phenothiazine analog prescribed for treatment of motion sickness, nausea, vomiting and allergies. Sold under the trade names Phenergan® and Phenadoz®, promethazine has antihistamine, sedative, and mild antipsychotic effects. Illicit drug users have been reported to combine promethazine with other drugs such as opioids and benzodiazepines to enhance feelings of sedation.