Methamphetamine (methylamphetamine or desoxyephedrine), popularly shortened to meth, is a psychostimulant drug. It is prescribed for attention-deficit hyperactivity disorder or narcolepsy under the brand name Desoxyn. It is also used illegally for recreational purposes. "Crystal meth" refers to the crystalline, smokeable form of the drug and is not used for the drug in pill or powdered form.
Methamphetamine acts as a dopaminergic and adrenergic reuptake inhibitor and as a sympathomimetic. Since it stimulates the mesolimbic reward pathway, causing euphoria and excitement, it is prone to abuse and addiction. Methamphetamine rapidly enters the brain and triggers a cascading release of norepinephrine, dopamine and serotonin. 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 often take one or more benzodiazepines as a means of "coming down".
Methamphetamine is a potent central nervous system stimulant which affects neurochemical mechanisms responsible for regulating heart rate, body temperature, blood pressure, appetite, attention, mood and responses associated with alertness or alarm conditions. The acute effects of the drug closely resemble the physiological and psychological effects of an epinephrine-provoked fight-or-flight response, including increased heart rate and blood pressure, vasoconstriction (constriction of the arterial walls), bronchodilation, and hyperglycemia (increased blood sugar). Users experience an increase in focus, increased mental alertness, and the elimination of fatigue, as well as a decrease in appetite.
Users must also take caution and avoid being showered by cold water, riding high-speed roller coasters, consuming caffeinated drinks, or exercising and weight lifting, as these actions can trigger hypertension, nervousness, extreme rapid heartbeat, dilated heartbeat, or sudden death.
As with other amphetamines, tolerance to methamphetamine is not completely understood, but known to be sufficiently complex that it cannot be explained by any single mechanism. The extent of tolerance and the rate at which it develops varies widely between individuals, and even within one person it is highly dependent on dosage, duration of use and frequency of administration. Many cases of narcolepsy are treated with methamphetamine for years without escalating doses or any apparent loss of effect.
Short term tolerance can be caused by depleted levels of neurotransmitters within the vesicles available for release into the synaptic cleft following subsequent reuse (tachyphylaxis). Short term tolerance typically lasts 2-3 days, until neurotransmitter levels are fully replenished. Prolonged overstimulation of dopamine receptors caused by methamphetamine may eventually cause the receptors to downregulate in order to compensate for increased levels of dopamine within the synaptic cleft. To compensate, larger quantities of the drug are needed in order to achieve the same level of effects.
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