Levomethamphetamine is an active metabolite of the antiparkinson's drug selegiline. Selegiline, a selective monoamine oxidase B (MAO) inhibitor at low doses, is also metabolized into levomethamphetamine and levoamphetamine. This has caused users to test positive for amphetamines.
Selegiline itself has neuroprotective and neuro-rescuing effects, but concern over the resulting  levomethamphetamine's neurotoxicity led to development of alternative MAO inhibitors, such as rasagiline, that do not produce toxic metabolites.