Introduction: emerging psychoactive substances generically known as new drugs, designer drugs or "legal highs" are available in the market. Objective: this review presents the main features of these compounds, the effects they produce and what is known about their mechanisms of action in order to inform clinicians and researchers that work in the field of addiction. Method: search of available literature on the development, mechanisms of action, clinical effects and market of synthetic cathinones and cannabinoids, and krokodil. Results: it is estimated that there are over 450 such substances; most of them are amphetamine-like drugs, in particular synthetic cathinones (powerful stimulants known as "bath salts") or synthetic cannabinoids (sold as "herbal mixtures" or "environmental incense"); there are also some opioids and hallucinogens. These substances have in common that they avoid regulatory systems, are difficult to detect in drug-screening tests, and are sold as "legal" alternatives to known drugs, research compounds or alleged dietary supplements. The sources for epidemiological surveillances are, among others, data from emergency services and seizures of new psychoactive substances in different countries. Marketing strategies include the sale in packages labeled "not for human consumption", which are used to evade sanitary control systems. They are sold in small shops and by electronic means. Discussion and conclusions: there is insufficient research about their effects but, when available, it shows that they are clearly deleterious.
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