Methamphetamine has a half-life of approximately 10 hours, depending on urinary pH, and can be detected in urine for about two days after consumption.
Because methamphetamine is metabolized to amphetamine, both methamphetamine and amphetamine are detected in patients taking methamphetamine.
Some drugs are metabolized to amphetamine and methamphetamine. Selegiline is converted to l-methamphetamine and l-amphetamine. The drug benzphetamine is also converted to d-methamphetamine and d-amphetamine. Patients on these drugs should expect to have positive drug screens for amphetamine and/or methamphetamine.
To further confound interpretation, methamphetamine exists as two isomers, d-methamphetamine and l-methamphetamine. The d-isomer is the illicit form of the drug, while the l-isomer is not psychoactive. L-methamphetamine, which has no abuse potential but is an effective vasoconstrictor, is found in Vick's inhaler, an over-the-counter decongestant device. Heavy use of Vick's inhaler can cause positive results for methamphetamine, but only a chiral analysis would reveal that only the l-isomer was present.
Occasionally, when a patient has taken methamphetamine, the amphetamine metabolite will be present at a level below the cutoff of the laboratory amphetamine confirmation. For example, a patient screens positive for amphetamines by immunoassay, and an amphetamine drug confirmation is reflexed. Methamphetamine is detected at 975 ng/mL. And amphetamine at 190 ng/mL. If the laboratory's confirmation cutoff for amps is 200 ng/mL, they will report methamphetamine as positive and amphetamine as negative. This can be confusing to a physician expecting to see the amphetamine metabolite as well as methamphetamine, but this is the nature of using cutoffs in a qualitative assay.
