Many countries are experimenting with legalising previously illegal substances, including marijuana and heroin. The rationale for the legalisation movement includes concerns about the financial and social burden of the so-called ‘war on drugs’, which is widely considered to be a failure, anyway. But what drug legalisation advocates forget is why certain substances were made illegal in the first place: taking them leads to behaviours that are harmful both to users and to the people around them. For evidence of such effects, we only need to look to Colorado in the USA. Following the state’s recent legalisation of marijuana, the rates of murder and of violent crime overall increased. This example shows that the drug legalisation movement is as dangerous as it is misguided.