Methamphetamine is an illegally manufactured drug known by a variety of street names, including meth, crystal meth, ice, crank and speed. Once it enters the bloodstream, the drug drastically alters normal function inside a part of the brain called the limbic system. One result of methamphetamine-related change in this system is a feeling of intense euphoria, and meth addiction typically begins when users repeatedly seek this euphoric state. However, use of the drug also alters normal function in a specific part of the limbic system that processes emotions such as anger and fear. As a result of this alteration, people using methamphetamine can easily develop paranoid, aggressive, or violent states of mind. The Limbic System The limbic system is a collection of brain structures that includes the hippocampus, hypothalamus and amygdala. It sits between the upper portion of the brain, called the cortex, and the lower part of the brain, called the brainstem. The hippocampus plays a vital role in normal consciousness by converting unstable short-term memories into stable long-term memories. In addition to a wide array of other functions, the hypothalamus acts as the origin point for a number of different emotions and sensations, including pleasure, thirst, hunger, anger, aggression and varying degrees of sexual satisfaction. The amygdala shares in tasks performed by the hippocampus and hypothalamus, including storage of long-term memories and the generation of pleasure, fear, anger and other emotional states. Pleasure levels inside the limbic system are regulated by a chemical messenger (neurotransmitter) inside the brain called dopamine. Generally speaking, relatively high levels of this chemical translate into an increased experience of pleasure, while relatively low levels translate into a decreased experience of pleasure. Like most other commonly abused drugs, methamphetamine triggers euphoria by boosting the limbic system's dopamine levels. However, while some drugs produce relatively modest dopamine increases (two to four times above normal), methamphetamine produces an extreme dopamine boost (12 to 13 times above normal). This extreme effect largely accounts for the highly addictive nature of the drug. Methamphetamine-Driven Changes Inside the Amygdala Normal function inside the amygdala is controlled by another portion of the brain, called the pre-frontal cortex, or PFC. Much of what we think of as our "selves" comes from activities inside this brain region, including specific expression of personality, the ability to make complex plans, the ability make judgments or decisions, and the ability to act in accordance with established social norms for behavior. In effect, the links between the PFC and the amygdala form a bridge between rational thinking and emotional response. When people habitually use methamphetamine, the UCLA Brain Research Institute explains, they destabilize normal function in the pre-frontal cortex. In turn, this destabilization disrupts the pre-frontal cortex's control over the amygdala; it is this loss of control that triggers the erratic emotional states often found in chronic meth users. The Onset of Paranoia Methamphetamine-related changes in amygdala function commonly produce an increased sense of paranoia in an affected individual. Characteristics of this emotional state center on an untrue or exaggerated belief that "someone is out to get you." Specific threats that a paranoid person may mistakenly perceive include a belief that someone is spreading false rumors; a belief that someone intends to steal money, or damage or steal property; and a belief that someone intends to cause serious or fatal physical harm. Depending on the individual and the type of paranoid thinking in progress, the outcome of these beliefs can be volatile emotional states such as fear, terror, panic, or anxiety, or a combination of two or more such states. Aggression and Violence Together with decreased behavioral control inside the pre-frontal cortex, the presence of anxiety, fear, terror or panic set the stage for unpredictable episodes of aggression and violence in habitual meth users. The potential for these behaviors is also tied to the onset of a disorder called methamphetamine-induced psychosis. People with this disorder develop psychotic symptoms that can include auditory (sound-based) or visual hallucinations, as well as various forms of delusional thinking. In some cases, this delusional thinking involves paranoid fixations, and people in the grips of psychosis have a clear capacity to turn aggressive or violent. Methamphetamine-induced psychosis is an officially recognized psychiatric disorder that can last for a period of days, months or (in relatively rare cases) years, even in the absence of continued meth use. However, the brain changes associated with habitual meth use also appear in people who don't develop clinical psychosis, and any long-term methamphetamine user can experience bouts of paranoia-fueled aggressive or violent behavior.