Sedative
A sedative or tranquilliser is a substance that induces sedation by reducing irritability or excitement. They are CNS depressants and interact with brain activity causing its deceleration. Various kinds of sedatives can be distinguished, but the majority of them affect the neurotransmitter gamma-aminobutyric acid (GABA). In spite of the fact that each sedative acts in its own way, most produce relaxing effects by increasing GABA activity.This group is related to hypnotics. The term sedative describes drugs that serve to calm or relieve anxiety, whereas the term hypnotic describes drugs whose main purpose is to initiate, sustain, or lengthen sleep. Because these two functions frequently overlap, and because drugs in this class generally produce dose-dependent effects (ranging from anxiolysis to loss of consciousness) they are often referred to collectively as sedative-hypnotic drugs.Sedatives can be used to produce an overly-calming effect (alcohol being the most common sedating drug). In the event of an overdose or if combined with another sedative, many of these drugs can cause deep unconsciousness and even death.

Terminology
There is some overlap between the terms "sedative" and "hypnotic".
Advances in pharmacology have permitted more specific targeting of receptors, and greater selectivity of agents, which necessitates greater precision when describing these agents and their effects:

Anxiolytic refers specifically to the effect upon anxiety. (However, some benzodiazepines can be all three: sedatives, hypnotics, and anxiolytics).
Tranquilizer can refer to anxiolytics or antipsychotics.
Soporific and sleeping pill are near-synonyms for hypnotics.

The term "chemical cosh"
The term "chemical cosh" (a club) is sometimes used popularly for a strong sedative, particularly for:

widespread dispensation of antipsychotic drugs in residential care to make people with dementia easier to manage.
use of methylphenidate to calm children with attention deficit hyperactivity disorder, though paradoxically this drug is known to be a stimulant.
See also Antipsychotic controversy

Types of sedatives
Therapeutic use
Doctors and veterinarians often administer sedatives to patients in order to dull the patients anxiety related to painful or anxiety-provoking procedures. Although sedatives do not relieve pain in themselves, they can be a useful adjunct to analgesics in preparing patients for surgery, and are commonly given to patients before they are anaesthetized, or before other highly uncomfortable and  invasive procedures like cardiac catheterization, colonoscopy or MRI.

Risks
Sedative dependence
Some sedatives can cause psychological and physical dependence when taken regularly over a period of time, even at therapeutic doses. Dependent users may get withdrawal symptoms ranging from restlessness and insomnia to convulsions and death. When users become psychologically dependent, they feel as if they need the drug to function, although physical dependence does not necessarily occur, particularly with a short course of use. In both types of dependences, finding and using the sedative becomes the focus in life.  Both physical and psychological dependence can be treated with therapy.

Misuse
Many sedatives can be misused, but barbiturates and benzodiazepines are responsible for most of the problems with sedative use due to their widespread recreational or non-medical use. People who have difficulty dealing with stress, anxiety or sleeplessness may overuse or become dependent on sedatives. Some heroin users may take them either to supplement their drug or to substitute for it. Stimulant users may take sedatives to calm excessive jitteriness. Others take sedatives recreationally to relax and forget their worries. Barbiturate overdose is a factor in nearly one-third of all reported drug-related deaths. These include suicides and accidental drug poisonings. Accidental deaths sometimes occur when a drowsy, confused user repeats doses, or when sedatives are taken with alcohol.
A study from the United States found that in 2011, sedatives and hypnotics were a leading source of adverse drug events (ADEs) seen in the hospital setting: Approximately 2.8% of all ADEs present on admission and 4.4% of ADEs that originated during a hospital stay were caused by a sedative or hypnotic drug.  A second study noted that a total of 70,982 sedative exposures were reported to U.S. poison control centers in 1998, of which 2310 (3.2%) resulted in major toxicity and 89 (0.1%) resulted in death.  About half of all the people admitted to emergency rooms in the U.S. as a result of nonmedical use of sedatives have a legitimate prescription for the drug, but have taken an excessive dose or combined it with alcohol or other drugs.There are also serious paradoxical reactions that may occur in conjunction with the use of sedatives that lead to unexpected results in some individuals.  Malcolm Lader at the Institute of Psychiatry in London estimates the incidence of these adverse reactions at about 5%, even in short-term use of the drugs.   The paradoxical reactions may consist of depression, with or without suicidal tendencies, phobias, aggressiveness, violent behavior and symptoms sometimes misdiagnosed as psychosis.

Dangers of combining sedatives and alcohol
Sedatives and alcohol are sometimes combined recreationally or carelessly. Since alcohol is a strong  depressant that slows brain function and depresses respiration, the two substances compound each others actions and this combination can prove fatal.

Worsening of psychiatric symptoms
The long-term use of benzodiazepines may have a similar effect on the brain as alcohol, and are also implicated in depression, anxiety, posttraumatic stress disorder (PTSD), mania, psychosis, sleep disorders, sexual dysfunction, delirium, and neurocognitive disorders (including benzodiazepine-induced persisting dementia which persists even after the medications are stopped). As with alcohol, the effects of benzodiazepine on neurochemistry, such as decreased levels of serotonin and norepinephrine, are believed to be responsible for their effects on mood and anxiety. Additionally, benzodiazepines can indirectly cause or worsen other psychiatric symptoms (e.g., mood, anxiety, psychosis, irritability) by worsening sleep (i.e., benzodiazepine-induced sleep disorder). Like alcohol, benzodiazepines are commonly used to treat insomnia in the short-term (both prescribed and self-medicated), but worsen sleep in the long-term. While benzodiazepines can put people to sleep, they disrupt sleep architecture: decreasing sleep time, delaying time to REM sleep, and decreasing deep slow-wave sleep (the most restorative part of sleep for both energy and mood).

Dementia
Sedatives and hypnotics should be avoided in people with dementia, according to the medication appropriateness tool for co‐morbid health conditions in dementia criteria. The use of these medications can further impede cognitive function for people with dementia, who are also more sensitive to side effects of medications.

Amnesia
Sedatives can sometimes leave the patient with long-term or short-term amnesia.
Lorazepam is one such pharmacological agent that can cause anterograde amnesia.  Intensive care unit patients who receive higher doses over longer periods, typically via IV drip, are more likely to experience such side effects.
Additionally, the prolonged use of tranquilizers increases the risk of obsessive and compulsive disorder, where the person becomes unaware whether he has performed a scheduled activity or not, he may also repetitively perform tasks and still re-performs the same task trying to make-up for continuous doubts. Remembering names that were earlier known becomes an issue such that the memory loss becomes apparent.

Disinhibition and crime
Sedatives — most commonly  alcohol but also GHB, Flunitrazepam (Rohypnol), and to a lesser extent, temazepam (Restoril), and midazolam (Versed) — have been reported for their use as date rape drugs (also called a Mickey) and being administered to unsuspecting patrons in bars or guests at parties to reduce the intended victims defenses. These drugs are also used for robbing people.
Statistical overviews suggest that the use of sedative-spiked drinks for robbing people is actually much more common than their use for rape. Cases of criminals taking rohypnol themselves before they commit crimes have also been reported, as the loss of inhibitions from the drug may increase their confidence to commit the offence, and the amnesia produced by the drug makes it difficult for police to interrogate them if they are caught.

See also
Hypnotic
Antidepressants
Benzodiazepine withdrawal syndrome
Tranquilizer
Tranquilizer gun

Notes
References
Further reading
Tone, Andrea. The Age of Anxiety: A History of Americas Turbulent Affair with Tranquilizers (Basic Books, 2009) 288 pp.; ISBN 978-0-465-08658-0 excerpty and text search