Methamphetamine is a potent CNS stimulant metabolized to amphetamine. Most urinary positives reflect illicit use, but some result from legitimate medications, and routine LC-MS/MS does not distinguish the d- (illicit or prescription stimulant) from the l- (some decongestants, selegiline) isomer without dedicated isomer testing.1
Stimulant-involved overdose deaths, often methamphetamine combined with fentanyl or heroin, have risen sharply over the past decade.2 Because a positive methamphetamine result can trigger serious consequences such as discontinuation of prescribed controlled substances, accurate interpretation, including isomer analysis when the source is in question, is critical.1
Amphetamine-class immunoassays are prone to cross-reactivity and false positives; LC-MS/MS specifically identifies methamphetamine and amphetamine. Because standard LC-MS/MS does not separate isomers, d/l isomer analysis helps distinguish illicit d-methamphetamine from l-methamphetamine sources such as selegiline or some decongestants.1 The l-form converts to amphetamine slowly, so an l-methamphetamine positive may show little or no amphetamine.3
Confirms stimulant exposure. LC-MS/MS detection of methamphetamine and/or amphetamine indicates use within the window of detection.
Source may need isomer testing. Routine testing does not separate illicit d- from medicinal l-methamphetamine; isomer analysis and the table below can.1
Amphetamine may be low or absent. With l-methamphetamine, slow conversion can yield little amphetamine.3
Timing and cutoff. Absence may reflect timing of last use, low dose, or levels below the cutoff.
Screen limitations. Amphetamine immunoassays are class-based and imperfect; use LC-MS/MS to confirm.
Does not exclude other stimulants. A methamphetamine-negative result says nothing about cocaine or synthetic cathinones.
| Source | Detected as | Isomer | Context |
|---|---|---|---|
| Illicit methamphetamine | d-MAMP, d-AMP | d (≥20%) | Illicit use |
| Desoxyn (d-methamphetamine) | d-MAMP, d-AMP | d (≥20%) | Rx ADHD or weight loss (rare) |
| Didrex (benzphetamine) | d-MAMP, d-AMP | d (≥20%) | Schedule III weight loss |
| Selegiline (Emsam, Zelapar) | l-MAMP, l-AMP | l (<20% d) | Parkinson disease, depression |
| OTC l-methamphetamine inhaler | l-AMP (slow) | l (<20% d) | Nasal decongestant |
Federal programs use a 20% d-isomer threshold: at or above 20% d indicates illicit or prescription d-methamphetamine; below 20% d indicates an l-methamphetamine product or selegiline.