Since early 2009, over 80 illicitly produced synthetic cannabinoid receptor agonists have been notified to the European Monitoring Centre for Drugs and Drug Addiction (EMCDDA). Yet more have been reported in other countries or offered for sale on websites. These cannabinoids typically act as agonists at CB1 receptors, and mimic the effects of Δ9-tetrahydrocannabinol (THC), the principal psychoactive constituent of Cannabis L. As with other ‘new psychoactive substances’, they have shown the limitations of current drug control procedures. First, the regular appearance of new compounds makes specific listing impractical and overwhelms any attempt to create risk assessments on a substance-by-substance basis. Secondly, the lack of human pharmacological and toxicological data hinders any objective attempt to show that synthetic cannabinoids are harmful. The UK has had a long experience of using generic legislation to control groups of compounds. However, cannabimimetic activity arises in a large number of distinct chemical families, and it is clear that generic control can no longer cope with this diversity whilst remaining intelligible to non-chemists. Analogue control presents further difficulties, and does not appear to offer an acceptable solution. For these reasons, legislatures in many countries are creating novel forms of regulation separate from domestic drug laws. The generic definition of classical cannabinoids, introduced into the UK Misuse of Drugs Act in 1971, also shows signs of weakness. Thus the growing interest in the clinical potential of tetrahydrocannabivarin (THCV) is inhibited, at least in the UK, by its unintended status as a Schedule 1 substance. Copyright © 2013 John Wiley & Sons, Ltd.