Trevor Grice's opening remark on cannabis was a prediction that one day it would be the source of wonderful medicines - and among pro-pot activists: FreeRob Cannabis told me that it was a substance naturally found in the body, so how could it be bad for you? This isn't quite right; researchers are starting to appreciate how different the cannabinoids in the cannabis plant are from the substances in the brain that switch on the same brain receptors." Put more accurately in the words of Professor Trevor Robbins, giving evidence to the Lords on behalf of the Medical Research Council: 'Cannabinoid pharmacology has exploded in the past decade ... opening up all sorts of exciting possibilities.'

Put simply, the old theory was that THC had its effect by dissolving in the membranes which surround body cells and 'perturbing' them: disrupting the flow of signals between cells. This is the way that both alcohol and general anaesthetics work. But the theory didn't explain why if THC could be effective at such extraordinarily low doses - one-thousandth of that needed for alcohol - its action on cells was essentially the same.

The anomaly was explained in 1990 when Tom Bonner and Liza Matsuda of the US National Institute of Health in Bethesda, Maryland proved the existence in certain cells of a 'receptor' activated by THc. A receptor is a single protein, made up of about 500 molecules, which sits on the cell wall and can create alterations in the cell's activity - but only if it is triggered by certain chemical substances which 'bind' to it. An analogy often used is with a lock that can only be opened by certain chemical keys.

It had been a chance discovery for Bonner and Matsuda, molecular biologists with no special interest in cannabis - unlike the man who now, at the age of sixty, picked up the baton. For a drug to act on a receptor it has to have certain structural similarities to a chemical used in the transmission of the body's own nerve impulses. Heroin and morphine act by mimicking the action of the 'neurotransmitter' named endorphin, which produces its pleasurable effects when, for example, we take exercise or go to sleep. Professor Raphael Mechoulam of the Hebrew University of Jerusalem renewed his quest for what he calls 'the chemistry of emotion', and began to hunt for a neurotransmitter chemically similar to THc.

The professor, whose family escaped to Palestine from Nazi occupied Bulgaria, had still been in his early thirties when he transformed our understanding of cannabis by discovering THc.

This substance wasn't the first of the cannabinoids (the chemicals found only in cannabis) to be isolated; this had been cannabinol, which was found in 1895 by a team at the Cambridge University Medical School. The end of the previous century had seen a burst of work led by Walters Ernest Dixon, the Salters Research Fellow, who took a benign view of smoking cannabis, which he said could be placed 'in the same category as coffee, tea and cola. It is not dangerous and its effects are never alarming, and I have come to regard it in this form as a useful and refreshing stimulant and food accessory.f In fact cannabinol is a degradation product - none is found in fresh cannabis - and it's only a tenth as psychoactive as the new substance Raphael Mechoulam isolated in 1964.

'We were the first to obtain THC in a pure form and to demonstrate its structure. I thought cannabis was a good topic to work on because it was the most prevalent drug in the world, even then in the 1960s, but the active compound had never been isolated. People had worked on opium and discovered morphine about 200 years ago, and cocaine was discovered in the mid nineteenth century. They are both easy to obtain using nineteenth-century techniques. But the cannabinoids form a horrible complex mixture. There are about eighty, and we've identified about twelve major compounds. They have to be separated by techniques that weren't available many years ago.'

Mechoulam's skills of analysis were called on ten years ago by archaeologists excavating a Roman site twenty kilometres south of Jerusalem. They had found the remains of a fourteen-year old girl who had died in childbirth, together with something that had been burnt on a brazier beside her bed. 'When we analysed the ashes we found traces, not of Delta 9 THC, which you find in cannabis, but of Delta 8 THc. When you burn cannabis part of it is converted to Delta 8, which is extremely stable. In fact it had been there for 1,600 years. They had burnt cannabis next to the bed: at that time people had used cannabis to reduce pain or help in childbirth.' The discovery strengthened Mechoulam's belief in the medical potential of cannabis - a belief that would create some coolness with his old friend Gabriel Nahas.

Then came the discovery of the receptors that were 'switched on' by THC, found largely in cells in the brain and the spinal cord. Mechoulam's team began looking for a substance in the brain that would bind to them. The original experimental material was cows' brains, but it wasn't until the team moved over to the un-kosher alternative of pigs' brains that they were able to collect measurable amounts of a promising-looking candidate.

This substance bound to the receptors, but this was not proof that it switched them on. (,Ligands' - chemicals that bind to receptors - can be 'agonists' which create changes in the cell, or 'antagonists' which have no action but stop the receptor being triggered by an agonist.) Mechoulam turned to a British colleague, Dr Roger Pertwee of Aberdeen University Medical School, for help.