A recent and dramatic increase in the emergence of novel psychoactive substances ('legal highs') has left many governments unable to provide a timely response to an increasing number of potentially harmful drugs now available to the public. In response to this rapid increase in lawful drug use, the UK government intends to implement temporary class drug orders, whereby substances with a potential for misuse and harm can be regulated for a 12 month period. During this period an investigation of the potential for harms induced by these drugs will take place. However, the short time-frame in which information must be gathered, and the paucity of data available on novel psychoactive substances, means that robust pharmacological and toxicological analyses may be replaced by extrapolating data from illegal drugs with similar chemical structures. This review explores the potential pharmacology and toxicology of past and present 'legal highs' and discusses the risks of failing to carry out in-depth scientific research on individual substances.