Tim Lott: I took LSD when I was 15 and my life will never be the same again

Dr Sessa is right to call for the end of this clinical ostracism. Of all the experiences I have had in my life, there is one that far surpasses everything else in terms of the profundity of its effect. For those people who have had the same experience this statement will come as no surprise, but for those who haven't, it will make no sense at all.

This experience was swallowing a tiny blue pill - microdot acid, or LSD. I have many times tried to describe how simply taking a drug can change your whole perception of life, but it is rather like trying to explain colour to the blind. There are no terms of reference in ordinary life to help you to understand. And actually, I am torn between evangelising for the drug and warning everyone not to go within a million miles of it.

I suppose the most simple and incredible fact about LSD is also the one that is hardest to believe: that what it reveals to you is not, as is popularly supposed, a hallucination, but an awe-inspiring glimpse of reality. Other drugs distort, but LSD gives you a reality far beyond words, or visual representation, or language.

It is quite the reverse of seeing something that isn't there. LSD disables some chemical filter in the brain that, in order to keep the world manageable, limits the amount of reality you can experience with your senses. An LSD trip allows "reality" - and if you have never questioned what that is, you would after taking LSD - to flood in untrammelled. The result may be terrifying and it may be wonderful, but it will be more "real" than anything you experience in everyday life.

LSD shows you that ordinary life is the hallucination. Or to put it another way, ordinary life is like listening to a record with fluff on the needle, and LSD removes the fluff. Psychotherapeutically speaking, it releases your subconscious into the conscious mind (or vice versa).

I know that this is the sort of thing one might expect an unreconstructed Sixties acid casualty to say, but in fact I was never a hippy, and have taken LSD only twice in my life, some 35 years ago. On those two occasions, both when I was 15 years old, it was in the one case indescribably lovely and in the other, unbearably horrific.

Ecstasy or horror, it is intensely real. If you have not taken LSD, you may defend yourself against the threat of this truth by defining it simply as temporary madness - even if you have read similar statements from other LSD experimenters as various as Cary Grant, Aldous Huxley, Bill Gates, William Burroughs, Larry Hagman and Jonathan Aitken. But actually, I've been mad - or at least severely mentally ill. And taking LSD is as different an experience as you can imagine.

All the same, we are probably right to try to keep a taboo on LSD. It is not a recreational drug; it is a revolution in how you see the world.

And that is why Dr Sessa is unquestionably right in calling for a debate to be started once more into its therapeutic uses. It can take you to heaven or it can take you to hell - but above all it can make you question, and possibly redraw, the maps of who you are as a person. And if that is not the principal aim of psychiatry, I don't know what is.

Tim Lott describes his LSD experiences in his memoir, 'The Scent of Dried Roses'