However, research ended after 1967, when the drugs were prohibited, and legally classified as schedule 1 class A drugs – that is, as having “no accepted medical use and the greatest potential for harm, despite the research evidence to the contrary”.

As a result, the drugs are more legally restricted than heroin and cocaine. In his opinion piece, Prof Rucker says: “No evidence indicates that psychedelic drugs are habit forming; little evidence indicates that they are harmful in controlled settings; and much historical evidence shows that they could have use in common psychiatric disorders.”

In fact, recent studies indicate that psychedelics have “clinical efficacy in anxiety associated with advanced cancer, obsessive compulsive disorder, tobacco and alcohol addiction, and cluster headaches,” he suggests. The classification makes it almost impossible to carry out studies on the drugs, he says, with licenses too expensive for most institutions to afford.

In the UK, to hold a schedule 1 drug, institutions require a license, which costs about £5,000. Only four hospitals currently hold such licenses, which come with regular police or home office inspections and onerous rules on storage and transport. Dr Rucker said the rules mean that clinical research involving psychedelics costs at least five times as much as studies involving drugs such as heroin, which are more harmful, yet less restricted.

This article first appeared in The Telegraph, 27 May, 2015.