This booklet, the third chapter of the World Drug Report 2019, provides an analysis of the market for substances that are broadly known as depressants of the central nervous system, which are primarily used to suppress, inhibit or decrease brain activity. The main classes of depressants discussed in this section include opioids, sedatives, tranquillizers and hypnotics. The sections on drug supply discuss both the depressants that have been diverted from licit sources and those that have been manufactured illicitly, while the sections on drug demand discuss the medical and non-medical use of depressants., To aid understanding of how depressants function in the human body, preliminary information is provided in the relevant sections.

While depressants of the central nervous system are used on their own for the psychoactive effect, they also figure prominently in the polydrug use patterns of people who use different drugs. One pattern of such use is the concurrent use of two or more depressants, such as the use of alcohol and benzodiazepines with opioids, to self-medicate or potentiate the effects of the opioid.1, 2 In other instances, people who use depressants such as opioids as their primary drug, in response to market dynamics such as changes in the availability, purity and price of a drug, may readily switch to another opioid (for example, from oxycodone to heroin or vice versa) in order to maintain the same level of psychoactive experience. Depressants are also used concurrently or sequentially with stimulants, either to overcome the side-effects of the other substance or to alleviate the adverse effects and severity of withdrawal symptoms.