A designer drug is a structural or functional analog of a controlled substance that has been designed to mimic the pharmacological effects of the original drug, while avoiding classification as illegal and/or detection in standard drug tests. Designer drugs include psychoactive substances that have been designated by the European Union as new psychoactive substances (NPS) as well as analogs of performance-enhancing drugs such as designer steroids. Some of these were originally synthesized by academic or industrial researchers in an effort to discover more potent derivatives with fewer side effects and were later co-opted for recreational use. Other designer drugs were prepared for the first time in clandestine laboratories. Because the efficacy and safety of these substances have not been thoroughly evaluated in animal and human trials, the use of some of these drugs may result in unexpected side effects.
The development of designer drugs may be considered a subfield of drug design. The exploration of modifications to known active drugs—such as their structural analogues, stereoisomers, and derivatives—yields drugs that may differ significantly in effects from their “parent” drug (e.g., showing increased potency, or decreased side effects). In some instances, designer drugs have similar effects to other known drugs, but have completely dissimilar chemical structures (e.g. JWH-018 vs THC). Despite being a very broad term, applicable to almost every synthetic drug, it is often used to connote synthetic recreational drugs, sometimes even those which have not been designed at all (e.g. LSD, the psychedelic side effects of which were discovered unintentionally).