A depressant (also called a central depressant) is a chemical compound that manipulates neurotransmission levels, thereby reducing arousal or stimulation in various parts of the brain. Depressants are also occasionally referred to as "downers" because they lower the level of arousal in the brain when taken. Stimulants, or "uppers," which increase mental and/or physical function, are the functional opposites of depressants.
Effects of Depressants
Depressants are widely used throughout the world as prescription medicines and as illicit substances. Effects often include ataxia, anxiolysis, pain relief, sedation or somnolence, and cognitive/memory impairment; in some instances, effects include euphoria, dissociation, muscle relaxation, lowered blood pressure or heart rate, respiratory depression, anticonvulsant effects, and even complete anesthesia or death. Calming effects of depressants may be beneficial to those suffering from anxiety, sleep disorders, or pain; however, the more extreme effects can be dangerous and even life-threatening.
On the neural level, most depressants act on the brain by affecting the neurotransmitter gamma-aminobutyric acid (GABA), which is responsible for regulating (specifically, decreasing) neuronal excitability throughout the nervous system. Neurotransmitters are brain chemicals that facilitate communication between brain cells. Depressants inhibit the next neuron from sending impulses by binding to receptor molecules. Although the different classes of depressants work in unique ways, it is through their ability to increase GABA—and thereby inhibit brain activity—that they produce a drowsy or calming effect.
Types of Depressants
Alcohol
Though initially a stimulant, alcohol ultimately depresses the brain, resulting in relaxation and impaired judgment.
Alcohol as a depressant
Alcohol depresses the brain, resulting in relaxation and impaired judgment.
Barbiturates
Barbiturates are effective as anxiolytics, hypnotics, and anticonvulsants, and also have an analgesic (painkiller) effect. However, they are commonly misused, physically addictive, and have serious potential for overdose. In the late 1950s, it became clear that the social cost of barbiturates was beginning to outweigh the medical benefits, which sparked a serious search for a class of replacement drugs. Most people still using barbiturates today do so to prevent seizures or for relief of migraine symptoms.
Barbiturates have been largely replaced by benzodiazepines, because the latter have fewer side effects and less potential for lethal overdoses. However, barbiturates are still used as anti-convulsants (e.g., phenobarbital, an anti-seizure medication), as sedatives (e.g., sodium thiopental), and analgesics for cluster headaches and migraines (e.g., Fioricet).
Benzodiazepines
Benzodiazepines enhance the effect of the neurotransmitter gamma-aminobutyric acid (GABA) at the GABA receptor, resulting in sedative, hypnotic (sleep-inducing), anxiolytic (anti-anxiety), anticonvulsant, and muscle-relaxant properties. Amnesic-dissociative actions are also seen in the applied pharmacology of high doses of many shorter-acting benzodiazepines. These properties make benzodiazepines useful in treating anxiety, insomnia, agitation, seizures, muscle spasms, alcohol withdrawal, and as a premedication for medical or dental procedures. Some of the most common benzodiazepines are anti-anxiety medications, such as clonazepam (Klonopin), alprazolam (Xanax), diazepam (Valium), and zolpidem (Ambien).
Cannabinoids
Although cannabis or marijuana is often considered either in its own unique category or as a mild psychedelic, the drug—notably the chemical compound cannabidiol that it contains—nevertheless has many depressant effects such as muscle relaxation, sedation, decreased alertness, and tiredness. There are at least 85 different cannabinoids isolated from cannabis, the most common of which is THC, the active ingredient in marijuana. Cannabinoids can be administered by smoking, vaporizing, oral ingestion, transdermal patch, intravenous injection, sublingual absorption, or rectal suppository. Once in the body, most cannabinoids are metabolized in the liver.
Opioids
An opioid is any psychoactive chemical that resembles morphine or other opiates in its pharmacological effects. One of the oldest known drugs, opioids induce an analgesic (painkiller) effect by decreasing perception of pain, decreasing reaction to pain, and increasing pain tolerance. Known for their highly addictive quality, opioids work by binding to opioid receptors, which are found principally in the central and peripheral nervous system and the gastrointestinal tract. Opioids are among the world's oldest known drugs; therapeutic use of the opium poppy predates recorded history. The most common opioids in modern history are morphine, heroin, and codeine, which are known as incredibly effective, albeit addictive, painkillers. As opposed to these naturally occurring opioids that are derived directly from the resin of the poppy plant, synthetic opioids are synthesized chemically in the laboratory and are agents commonly used in pain relief, treating drug dependence and anesthesia. Some examples include hydrocodone and oxycodone.
A Note on the Term "Narcotics"
The term "narcotic" originally referred medically to any psychoactive compound with sleep-inducing properties. In the United States, it has since become associated with opioids, commonly morphine and heroin and their derivatives. The term is, today, imprecisely defined and typically has negative connotations. When used in a legal context in the United States, the term "narcotic drug" refers to a substance that is completely prohibited, or one, such as codeine or morphine, that is used in violation of governmental regulation. From a medical standpoint, it is no longer a useful term.