Methamphetamine (also known as speed, meth, crystal, crank, tina, and sometimes confusingly called ice) is a chemical widely known for its stimulant properties on the human body. It is frequently confused with other drugs that share similar effects, including amphetamine, 4-methyl-aminorex, ephedrine, caffeine, and other chemicals, both legal and illegal.

Methamphetamine can be taken orally, snorted, smoked or injected, in approximately increasing order of immediacy of onset. Onset can be immediate (in the case of injection), or can take as long as 30-40 minutes if ingested orally. Duration is subjective, but is probably on the order of 4 - 8 hours. Delayed absorption (for example, due to oral ingestion) can prolong the effects relative to time of administration. Of course, larger doses last longer due to the fact that it is removed from the blood at a finite rate. The length of time that methamphetamine will stay in the plasma (blood) is between 4 to 6 hours. It can be detected in the urine one hour after use and up to 48 hours after use.

A toxic reaction (or overdose) can occur at relatively low levels, 50 milligrams of pure drug for a non-tolerant user. Different peoples' metabolisms work at different rates, and drug strengths vary, so there is no way of stating a "safe" or "unsafe" level of use. Effects include euphoria, hyper-excitability, extreme nervousness, accelerated heartbeat, sweating, dizziness, restlessness, insomnia, tooth grinding, incessant talking, and other effects.

Methamphetamine and other CNS stimulants have strong bronchodilation effects. Vasoconstriction (tightening of blood vessels) and pupil dilation are also common, as are elevated blood pressure, heart rate, and other general symptoms of increased sympathetic nervous activity.

The physical effects are almost assuredly due to interactions between the amphetamine structure and human physiology, probably due to the similarity to adrenaline (epinephrine).

Mental capacity is not diminished directly by the drug. In fact, some studies have shown slight increases in mental capacity on simple tasks. It has been prescribed for attention deficit disorder, among other things. Reports tend to center around the effects of fatigue on mental capacity.

Emotional responses may range from euphoria to anger and paranoia. Preliminary doses tend to produce the former, while continued use (e.g. for three or more days) tends to produce the latter.