Methamphetamine is a psychomotor stimulant. In the brain it mimics a neurotransmitter ("messenger chemical") at serotonin and dopamine receptor sites. Receptors and neurotransmitters function like a lock (the receptor) and key (the neurotransmitter.) A given receptor will interact only with the specific neurotransmitter that "fits" it. Sympathomimetics like methamphetamine mimic particular neurotransmitters so well that they can "unlock" the actions that those neurotransmitters and their receptors usually have.
The neurotransmitters that meth mimics are monoamines: serotonin, epinephrine, norepinephrine, and dopamine. The drugs stimulate the release of these neurotransmitters, which leads to elevated levels of the neurotransmitters in the synapses (the gaps between neurons.) In addition, methamphetamine inhibits monoamine oxidase, the enzyme responsible for the destruction of serotonin, epinephrine, norepinephrine, and dopamine, further increasing their levels in the brain.
Among dopamine’s and serotonin’s functions are influencing aggressive, defensive, social and sexual behaviors, so meth users often display exaggerations in these behaviors.
Methamphetamine also stimulates locomotor activity, and produces stereotypic behaviors. Stereotypic behaviors are the random, meaningless, repetitive, and compulsive actions that "tweakers" display: twitches, jerks, patterned actions, picking at themselves, etc. Stereotypic behaviors have been related to the norepinephrine, dopamine, and serotonin systems. Stereotypic behaviors may also be called punding. Punding is performing a useless task compulsively again and again. Users report being aware of their punding behavior, but they say they are unable to stop it. More harmless punding or tweaker habits include coloring, writing, playing cards, or taking apart and putting back together items in a prolonged and bizarre manner.