Lying back, exhaling: usually the last thing a person does before leaving this world. Only in this case it is the world that is leaving me.

A few minutes ago I smoked a pipe of Salvia divinorum, a powerful hallucinogenic herb that I bought openly and legally from a shop near my home. Of the £25 I handed over, more than £4 will find its way into government coffers in the form of sales tax. And salvia was just one of dozens of powerful but entirely legal psychoactive substances that I could have chosen.

All that was far from my mind as the salvia took me on a consciousness-expanding journey unlike any other I have ever experienced. My body felt disconnected from “me” and objects and people appeared cartoonish, surreal and marvellous. Then, as suddenly as it had began, it was over. The visions vanished and I was back in my bedroom. I spoke to my “sitter” – the friend who was watching over me, as recommended on the packaging – but my mouth was awkward and clumsy. When I attempted to stand my coordination was off. Within a couple of minutes, however, I was fine and clear-headed, though dripping with sweat. The whole experience had lasted less than 5 minutes.

My salvia trip was part of a journey into the world of “legal highs”, a new generation of powerful mind-altering substances that are growing in popularity across the world (see Table). Accurate figures are hard to come by, as these substances are rarely monitored by drug-enforcement agencies. But the proliferation of online and high-street retailers suggest they are an increasingly …