Methamphetamine - Pharmacokinetics

Pharmacokinetics

Following oral administration, methamphetamine is readily absorbed into the bloodstream, with peak plasma concentrations achieved in approximately 3.13 to 6.3 hours post ingestion. The amphetamine metabolite peaks at 10 to 24 hours. Methamphetamine is also well absorbed following inhalation and following intranasal administration. It is distributed to most parts of the body. Methamphetamine is known to produce central effects similar to the other stimulants, but at smaller doses, with fewer peripheral effects. Methamphetamine's high lipophilicity also allows it to cross the blood brain barrier faster than other stimulants, where it is more stable against degradation by monoamine oxidase (MAO).

Methamphetamine is metabolized in the liver with the main metabolites being amphetamine (active) and 4-hydroxymethamphetamine (pholedrine); other minor metabolites include 4-hydroxyamphetamine, norephedrine, and 4-hydroxynorephedrine. Other drugs metabolized to amphetamine and methamphetamine include benzphetamine, furfenorex, and famprofazone. Selegiline (marketed as Deprenyl, EMSAM, and others) is metabolized into the less active L-isomer of amphetamine and the inactive L-isomer of methamphetamine. Although only the D-Isomer of selegiline will metabolize into active metabolites, both isomers may cause a positive result for methamphetamine and amphetamine on a drug test, in certain cases.

It is excreted by the kidneys, with the rate of excretion into the urine heavily influenced by urinary pH. Between 30-54% of an oral dose is excreted in urine as unchanged methamphetamine and 10-23% as unchanged amphetamine. Following an intravenous dose, 45% is excreted as unchanged parent drug and 7% amphetamine. The half-life of methamphetamine is variable with a mean value of between 9 and 12 hours.