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 release dopamine rapidly in reward regions of the brain produces the intense euphoria, or “rush,” that many users feel after snorting, smoking, or injecting the drug.

Chronic methamphetamine abuse significantly changes how the brain functions. Noninvasive human brain imaging studies have shown alterations in the activity of the dopamine system that are associated with reduced motor skills and impaired verbal learning. Recent studies in chronic methamphetamine abusers have also revealed severe structural and functional changes in areas of the brain associated with emotion and memory, which may account for many of the emotional and cognitive problems observed in chronic methamphetamine abusers.

Repeated methamphetamine abuse can also lead to addiction—a chronic, relapsing disease characterized by compulsive drug seeking and use, which is accompanied by chemical and molecular changes in the brain. Some of these changes persist long after methamphetamine abuse is stopped. Reversal of some of the changes, however, may be observed after sustained periods of abstinence

Meth reduces the amount of protective saliva around the teeth. Meth users also consume excess sugared, carbonated soft drinks, tend to neglect personal hygiene, grind their teeth and clench their jaws, leading to what is commonly called “meth mouth.” Teeth can eventually fall out of users’ mouths—even as they do simple things like eating a sandwich.

Taking even small amounts of meth can result in increased wakefulness, increased physical activity, decreased appetite, rapid breathing and heart rate, irregular heartbeat, increased blood pressure, and hyperthermia (overheating). High doses can elevate body temperature to dangerous, sometimes lethal, levels as well as cause convulsions and even cardiovascular collapse and death. Meth abuse may also cause extreme anorexia, memory loss, and severe dental problems.

A fairly common hallucination experienced by meth users is the so-called crank bug. The user gets the sensation that there are insects creeping on top of, or underneath, her skin. The user will pick at or scratch her skin trying to get rid of the imaginary bugs. This scratching can create open sores that may become infected. The image to the right shows sores associated with "crank bug."