Methamphetamine (contracted from N-methylamphetamine) is a strong central nervous system (CNS) stimulant that is mainly used as a recreational drug and less commonly as a second-line treatment for attention deficit hyperactivity disorder and obesity. Methamphetamine was discovered in 1893 and exists as two enantiomers: levo-methamphetamine and dextro-methamphetamine. Methamphetamine properly refers to a specific chemical, the racemic free base, which is an equal mixture of levomethamphetamine and dextromethamphetamine in their pure amine forms. It is rarely prescribed due to concerns involving human neurotoxicity and potential for recreational use as an aphrodisiac and euphoriant, among other concerns, as well as the availability of safer substitute drugs with comparable treatment efficacy. Dextromethamphetamine is a much stronger CNS stimulant than levomethamphetamine.

Both methamphetamine and dextromethamphetamine are illicitly trafficked and sold owing to their potential for recreational use. The highest prevalence of illegal methamphetamine use occurs in parts of Asia, Oceania, and in the United States, where racemic methamphetamine, levomethamphetamine, and dextromethamphetamine are classified as schedule II controlled substances. Levomethamphetamine is available as an over-the-counter (OTC) drug for use as an inhaled nasal decongestant in the United States. Internationally, the production, distribution, sale, and possession of methamphetamine is restricted or banned in many countries, due to its placement in schedule II of the United Nations Convention on Psychotropic Substances treaty. While dextromethamphetamine is a more potent drug, racemic methamphetamine is sometimes illicitly produced due to the relative ease of synthesis and limited availability of chemical precursors.

In low doses, methamphetamine can elevate mood, increase alertness, concentration and energy in fatigued individuals, reduce appetite, and promote weight loss. At higher doses, it can induce psychosis, breakdown of skeletal muscle, seizures and bleeding in the brain. Chronic high-dose use can precipitate unpredictable and rapid mood swings, stimulant psychosis (e.g., paranoia, hallucinations, delirium, and delusions) and violent behavior. Recreationally, methamphetamine’s ability to increase energy has been reported to lift mood and increase sexual desire to such an extent that users are able to engage in sexual activity continuously for several days. Methamphetamine is known to possess a high addiction liability (i.e., a high likelihood that long-term or high dose use will lead to compulsive drug use) and high dependence liability (i.e. a high likelihood that withdrawal symptoms will occur when methamphetamine use ceases). Heavy recreational use of methamphetamine may lead to a post-acute-withdrawal syndrome, which can persist for months beyond the typical withdrawal period. Unlike amphetamine, methamphetamine is neurotoxic to human midbrain dopaminergic neurons. It has also been shown to damage serotonin neurons in the CNS. This damage includes adverse changes in brain structure and function, such as reductions in grey matter volume in several brain regions and adverse changes in markers of metabolic integrity.

Methamphetamine belongs to the substituted phenethylamine and substituted amphetamine chemical classes. It is related to the other dimethylphenethylamines as a positional isomer of these compounds, which share the common chemical formula: C10H15N1.

See also: Party and play and the Recreational routes of methamphetamine administration
Methamphetamine is often used recreationally for its effects as a potent euphoriant and stimulant as well as aphrodisiac qualities.

According to a National Geographic TV documentary on methamphetamine, an entire subculture known as party and play is based around sexual activity and methamphetamine use. Participants in this subculture, which consists almost entirely of homosexual male methamphetamine users, will typically meet up through internet dating sites and have sex. Due to its strong stimulant and aphrodisiac effects and inhibitory effect on ejaculation, with repeated use, these sexual encounters will sometimes occur continuously for several days on end. The crash following the use of methamphetamine in this manner is very often severe, with marked hypersomnia (excessive daytime sleepiness). The party and play subculture is prevalent in major US cities such as San Francisco and New York City.

The psychological effects of methamphetamine can include euphoria, dysphoria, changes in libido, alertness, apprehension and concentration, decreased sense of fatigue, insomnia or wakefulness, self-confidence, sociability, irritability, restlessness, grandiosity and repetitive and obsessive behaviors. Peculiar to methamphetamine and related stimulants is “punding”, persistent non-goal-directed repetitive activity. Methamphetamine use also has a high association with anxiety, depression, amphetamine psychosis, suicide, and violent behaviors

A methamphetamine overdose may result in a wide range of symptoms. A moderate overdose of methamphetamine may induce symptoms such as: abnormal heart rhythm, confusion, difficult and/or painful urination, high or low blood pressure, high body temperature, over-active and/or over-responsive reflexes, muscle aches, severe agitation, rapid breathing, tremor, urinary hesitancy, and an inability to pass urine. An extremely large overdose may produce symptoms such as adrenergic storm, methamphetamine psychosis, substantially reduced or no urine output, cardiogenic shock, bleeding in the brain, circulatory collapse, hyperpyrexia (i.e., dangerously high body temperature), pulmonary hypertension, kidney failure, rapid muscle breakdown, serotonin syndrome, and a form of stereotypy (“tweaking”). A methamphetamine overdose will likely also result in mild brain damage due to dopaminergic and serotonergic neurotoxicity. Death from methamphetamine poisoning is typically preceded by convulsions and coma.

Main section: Stimulant psychosis § Substituted amphetamines
Abuse of methamphetamine can result in a stimulant psychosis which may present with a variety of symptoms (e.g., paranoia, hallucinations, delirium, and delusions). A Cochrane Collaboration review on treatment for amphetamine, dextroamphetamine, and methamphetamine abuse-induced psychosis states that about 5–15% of users fail to recover completely. The same review asserts that, based upon at least one trial, antipsychotic medications effectively resolve the symptoms of acute amphetamine psychosis. Amphetamine psychosis may also develop occasionally as a treatment-emergent side effect.

Signaling cascade in the nucleus accumbens that results in psychostimulant addiction

The image above contains clickable links This diagram depicts the signaling events in the brain’s reward center that are induced by chronic high-dose exposure to psychostimulants that increase the concentration of synaptic dopamine, like amphetamine, methamphetamine, and phenethylamine. Following presynaptic dopamine and glutamate co-release by such psychostimulants, postsynaptic receptors for these neurotransmitters trigger internal signaling events through a cAMP-dependent pathway and a calcium-dependent pathway that ultimately result in increased CREB phosphorylation. Phosphorylated CREB increases levels of ?FosB, which in turn represses the c-Fos gene with the help of corepressors; c-Fos repression acts as a molecular switch that enables the accumulation of ?FosB in the neuron. A highly stable (phosphorylated) form of , one that persists in neurons for 1–2 months, slowly accumulates following repeated high-dose exposure to stimulants through this process. ?FosB functions as “one of the master control proteins” that produces addiction-related structural changes in the brain, and upon sufficient accumulation, with the help of its downstream targets (e.g., nuclear factor kappa B), it induces an addictive state.
Current models of addiction from chronic drug use involve alterations in gene expression in certain parts of the brain, particularly the nucleus accumbens. The most important transcription factors that produce these alterations are ?, cAMP response element binding protein (CREB), and nuclear factor kappa B (NF?B). plays a crucial role in the development of drug addictions, since its overexpression in D1-type medium spiny neurons in the nucleus accumbens is necessary and sufficient for most of the behavioral and neural adaptations that arise from addiction. Once ?FosB is sufficiently overexpressed, it induces an addictive state that becomes increasingly more severe with further increases in  expression. It has been implicated in addictions to alcohol, cannabinoids, cocaine, methylphenidate, nicotine, opioids, phencyclidine, propofol, and substituted amphetamines, among others.

?JunD, a transcription factor, and G9a, a histone methyltransferase enzyme, both directly oppose the induction of ?FosB in the nucleus accumbens (i.e., they oppose increases in its expression). Sufficiently overexpressing ?JunD in the nucleus accumbens with viral vectors can completely block many of the neural and behavioral alterations seen in chronic drug abuse (i.e., the alterations mediated by ? ?FosB also plays an important role in regulating behavioral responses to natural rewards, such as palatable food, sex, and exercise. Since both natural rewards and addictive drugs induce expression of ? (i.e., they cause the brain to produce more of it), chronic acquisition of these rewards can result in a similar pathological state of addiction. ?FosB is the most significant factor involved in both amphetamine addiction and amphetamine-induced sex addictions, which are compulsive sexual behaviors that result from excessive sexual activity and amphetamine use. These sex addictions (i.e., drug-induced compulsive sexual behaviors) are associated with a dopamine dysregulation syndrome which occurs in some patients taking dopaminergic drugs, such as amphetamine or methamphetamine.

Treatment and management
Further information: Addiction § Research
Cognitive behavioral therapy is currently the most effective clinical treatment for psychostimulant addictions in general. As of May 2014, there is no effective pharmacotherapy for methamphetamine addiction.Methamphetamine addiction is largely mediated through increased activation of dopamine receptors and co-localized NMDA receptors in the nucleus accumbens. Magnesium ions inhibit NMDA receptors by blocking the receptor calcium channel.

Dependence and withdrawal
Tolerance is expected to develop with regular methamphetamine use and, when used recreationally, this tolerance develops rapidly. In dependent users, withdrawal symptoms are positively correlated with the level of drug tolerance. Depression from methamphetamine withdrawal lasts longer and is more severe than that of cocaine withdrawal.

According to the current Cochrane review on drug dependence and withdrawal in recreational users of methamphetamine, “when chronic heavy users abruptly discontinue [methamphetamine] use, many report a time-limited withdrawal syndrome that occurs within 24 hours of their last dose”. Withdrawal symptoms in chronic, high-dose users are frequent, occurring in up to 87.6% of cases, and persist for three to four weeks with a marked “crash” phase occurring during the first week. Methamphetamine withdrawal symptoms can include anxiety, drug craving, dysphoric mood, fatigue, increased appetite, increased movement or decreased movement, lack of motivation, sleeplessness or sleepiness, and vivid or lucid dreams.

History, society, and culture
Main article: History and culture of substituted amphetamines
A methamphetamine tablet container
Pervitin, a methamphetamine brand used by German soldiers during World War II, was dispensed in these tablet containers.
Amphetamine, discovered before methamphetamine, was first synthesized in 1887 in Germany by Romanian chemist Lazar Edeleanu who named it phenylisopropylamine. Shortly after, methamphetamine was synthesized from ephedrine in 1893 by Japanese chemist Nagai Nagayoshi. Three decades later, in 1919, methamphetamine hydrochloride was synthesized by pharmacologist Akira Ogata via reduction of ephedrine using red phosphorus and iodine.

During World War II, methamphetamine was sold in tablet form under the brand name Pervitin (not to be confused with Perviton, which is a synonym for Phenatine), produced by the Berlin-based Temmler pharmaceutical company. It was used extensively by all branches of the combined Wehrmacht armed forces of the Third Reich, and was popular with Luftwaffe pilots in particular, for its performance-enhancing stimulant effects and to induce extended wakefulness. Pervitin became colloquially known among the German troops as “Stuka-Tablets” (Stuka-Tabletten) and “Herman-Göring-Pills” (Hermann-Göring-Pillen). Side effects were so serious that the army sharply cut back its usage in 1940. Historian Lukasz Kamienski says “A soldier going to battle on Pervitin usually found himself unable to perform effectively for the next day or two. Suffering from a drug hangover and looking more like a zombie than a great warrior, he had to recover from the side effects.” Some soldiers turned very violent, committing war crimes against civilians; others attacked their own officers.

Obetrol, patented by Obetrol Pharmaceuticals in the 1950s and indicated for treatment of obesity, was one of the first brands of pharmaceutical methamphetamine products. Due to the psychological and stimulant effects of methamphetamine, Obetrol became a popular diet pill in America in the 1950s and 1960s. Eventually, as the addictive properties of the drug became known, governments began to strictly regulate the production and distribution of methamphetamine. For example, during the early 1970s in the United States, methamphetamine became a schedule II controlled substance under the Controlled Substances Act. Currently, methamphetamine is sold under the trade name Desoxyn, trademarked by the Danish pharmaceutical company Lundbeck. As of January 2013, the Desoxyn trademark had been sold to Italian pharmaceutical company Recordati.