Function of Stimulants

Stimulants are psychoactive drugs that induce temporary improvements in mental and/or physical functions. Occasionally referred to as "uppers," stimulants are the functional opposites of depressants, or "downers," which decrease mental and/or physical function. Stimulants are widely used throughout the world as prescription medicines and as illicit substances of recreational use or abuse.

Stimulants increase the activity of the central nervous system (the brain and spinal cord), the sympathetic nervous system (part of the peripheral nervous system that controls the fight-or-flight response), or both. Common effects, which vary depending on the substance in question, may include enhanced alertness, awareness, wakefulness, endurance, productivity, and motivation. Effects can also include an increase in arousal, locomotion, heart rate, and blood pressure, and the perception of diminished requirements for food and sleep. Some stimulants produce a sense of euphoria, especially those that exert influence on the central nervous system.

Neurological Mechanisms of Stimulants 

Stimulants exert their effects through a number of different mechanisms. Some stimulants facilitate the activity of certain neurotransmitters, specifically norepinephrine and/or dopamine. Others block the action of certain receptors (such as the adenosine receptors) in a process known as receptor antagonism. Still others cause action in other receptors (such as nicotinic acetylcholine) in a process known as receptor agonism.

Use and Abuse of Stimulants

Therapeutically, stimulants are used and/or prescribed for a variety of reasons. They are used to increase or maintain alertness; to boost endurance or productivity; to counteract fatigue and lethargy throughout the day; to counteract abnormal states that diminish alertness or consciousness (such as in narcolepsy); to decrease appetite and promote weight loss; and to enhance concentration (especially for those with attentional disorders such as ADHD). Many stimulants are also capable of improving mood and relieving anxiety, and are occasionally used to treat symptoms of depression.

The euphoria produced by some stimulants leads to their recreational use, so many stimulants are either illegal or very carefully controlled in the United States. Some may be legally available only by prescription. Addiction to some central-nervous-system stimulants can quickly lead to medical, psychiatric, and psychosocial deterioration. Drug tolerance, dependence, sensitization, and withdrawal can occur after repeated use. Over time, stimulants can disrupt the functioning of the brain's dopamine system, dampening users' ability to feel any pleasure at all.

Types of Stimulants

Examples of well-known stimulants include amphetamines, MDMA, NDRIs, cocaine, caffeine, and nicotine.

Amphetamines 

Amphetamines (such as ephedrine and methamphetamine) are a group of stimulants that increase the levels of norepinephrine and dopamine in the brain through reuptake inhibition—meaning they block these neurotransmitters from being reabsorbed back into the neural networks. Amphetamines are known to cause elevated mood and euphoria, and are often used for their therapeutic effects. Physicians occasionally prescribe amphetamines to treat major depression, and numerous studies have demonstrated the effectiveness of drugs such as Adderall in controlling symptoms associated with ADHD. Due to their availability and fast-acting effects, amphetamines are prime candidates for abuse.

MDMA

Methylenedioxymethamphetamine (MDMA), known by its common street names Ecstasy and Molly, had a medical application as a treatment for depression and a psychotherapy aid until 1985, when it became a controlled substance. The stimulant effects of MDMA include appetite loss, euphoria, social disinhibition, insomnia, improved energy, increased arousal, and increased perspiration. MDMA also has many physical side effects: since MDMA increases the activity of serotonin, the brain becomes depleted of serotonin, causing a rebound sadness or depression in the days following use. MDMA differs from most stimulants in that its primary pharmacological effect is on the neurotransmitter serotonin rather than dopamine, epinephrine, or norepinephrine. MDMA also decreases the release of dopamine. The increase in seratonin is caused because the MDMA prevents seratonin from entering the reuptake site, causing a continued flow, and eventually excess seratonin. 

NDRIs

Norepinephrine and dopamine reuptake inhibitors (NDRIs) (such as the antidepressant Wellbutrin) inhibit the uptake of dopamine and norepinephrine, effectively increasing their amounts in the brain and causing a stimulating effect. Many of these compounds are effective ADHD medications and antidepressants. These medicines have an extended release mechanism, and are typically less popular for recreational use.

Cocaine

Cocaine is made from the leaves of the coca shrub, which grows in the mountain regions of South America. In Europe and North America, the most common form of cocaine is a white crystalline powder. Most cocaine use is recreational and its abuse potential is high, and so its sale and possession are strictly controlled in most jurisdictions.

Cocaine

Cocaine, in powder form, is a commonly abused stimulant that produces a sense of euphoria in the user.

Caffeine and Nicotine

Caffeine is a drug that is found naturally in coffee, tea, soft drinks, and cocoa. Caffeine stimulates the body, increases heart rate and blood pressure, and facilitates alertness and concentration. The vast majority (over 80%) of people in the United States consume caffeine on a daily basis. In very low concentrations, nicotine also acts as a stimulant, and it is one of the main factors responsible for the dependence-forming properties of tobacco.