Methamphetamine (/mɛθæmfɛtəmiːn/, also known as methylamphetamine, N-methylamphetamine or desoxyephedrine) is a psychostimulant and sympathomimetic drug. It is a member of the family of phenylethylamines. The dextrorotatory (S-isomer) dextromethamphetamine can be prescribed to treat attention-deficit hyperactivity disorder, though unmethylated amphetamine is more commonly prescribed. Narcolepsy and obesity can also be treated by the aforementioned isomer under the brand name Desoxyn. It is considered a second line of treatment, used when amphetamine and methylphenidate cause the patient too many side effects. It is only recommended for short term use (~6 weeks) in obesity patients because it is thought that the anorectic effects of the drug are short lived and produce tolerance quickly, whereas the effects on CNS stimulation are much less susceptible to tolerance. It is also used illegally for weight loss and to maintain alertness, focus, motivation, and mental clarity for extended periods of time, and for recreational purposes.

Methamphetamine enters the brain and triggers a cascading release of norepinephrine, dopamine and serotonin. To a lesser extent methamphetamine acts as a dopaminergic and adrenergic reuptake inhibitor and in high concentrations as a monamine oxidase inhibitor (MAOI). Since it stimulates the mesolimbic reward pathway, causing euphoria and excitement, it is prone to abuse and addiction. Users may become obsessed or perform repetitive tasks such as cleaning, hand-washing, or assembling and disassembling objects. Withdrawal is characterized by excessive sleeping, eating, and depression-like symptoms, often accompanied by anxiety and drug-craving. Users of methamphetamine sometimes take sedatives such as benzodiazepines as a means of easing their "come down".

Methamphetamine is extremely addictive. Its mechanism of operation is similar to that of cocaine, but (unless it is smoked or injected) it acts more slowly, and therefore the addiction takes longer to develop. A typical sequence is that a person begins to use the drug as a stimulant, because of its enhancing effects on pleasure and sex, alertness and ability to concentrate. Over time, however, the effectiveness decreases, and users find that they need to take higher doses to get the same results; also that they have great difficulty functioning effectively without the drug. This leads to a cycle of steadily increasing doses that can end in catastrophe. Common nicknames for methamphetamine include "crank", "meth", "jib", "ice", "crystal", "tina", "glass", "P", and "yaa baa" (Thailand). Methamphetamine is sometimes referred to as "speed", but this term is usually used for regular amphetamine or dextroamphetamine.