Methamphetamine and Dopamine
"Methamphetamine increases the release and blocks the reuptake of the brain chemical (or neurotransmitter) dopamine, leading to high levels of the chemical in the brain, a common mechanism of action for most drugs of abuse. Dopamine is involved in reward, motivation, the experience of pleasure, and motor function. Methamphetamine’s ability to rapidly release dopamine in reward regions of the brain produces the intense euphoria, or 'rush,' that many users feel after snorting, smoking, or injecting the drug."
National Institute on Drug Abuse, InfoFacts: Methamphetamine (Rockville, MD: US Department of Health and Human Services), Revised: July 2009.