Methamphetamine is a powerful stimulant that activates certain systems in the brain. It is closely related chemically to amphetamine, but the central nervous system effects of methamphetamine are greater. Both drugs have some medical uses, primarily in the treatment of obesity, but their therapeutic use is limited. The central nervous system (CNS) actions that result from taking even small amounts of methamphetamine include increased wakefulness, increased physical activity, decreased appetite, increased respiration, hyperthermia, and euphoria. Other CNS effects include irritability, insomnia, confusion, tremors, convulsions, anxiety, paranoia, and aggressiveness. Hyperthermia and convulsions can result in death.

Illicit methamphetamine, which is almost exclusively methamphetamine hydrochloride, is sold in powder, ice, and tablet forms. Powder methamphetamine, the most common form available in the United States, is produced domestically and also is smuggled into the country from Mexico. Traditionally, Asian-produced ice was almost exclusively found in Guam, Hawaii, and parts of California; however, increased domestic production of this form of methamphetamine has increased availability to several other areas of the country previously untouched by exposure to ice. Methamphetamine tablets, primarily manufactured in Burma, have been smuggled into the United States, especially to northern California and the Los Angeles area.

Methamphetamine is also known as "speed" or "crystal" when it is swallowed or sniffed; as "crank" when it is injected; and as "ice" or "glass" when it is smoked. Ice is clear chunky crystal form of meth that resembles "ice."

YABA: Thai for "crazy medicine," Yaba, a methamphetamine tablet, has been appearing in the United States. Yaba tablets are sometimes flavored (grape, orange, and vanilla): tasting like candy, the tablets are obviously marketed to a young audience, particularly at rave’s or parties where Ecstasy (a similar looking drug) has been well established. The tablets are commonly reddish-orange or green, and fit inside the end of a drinking straw. They have a variety of logos, with “WY” the most common. Methamphetamine pills are normally ingested orally, although they can be crushed into powder and administered.

All forms of methamphetamine are extremely dangerous and induce long-lasting, debilitating effects.