Robin Lefever regards himself as an addict, yet neither alcohol, nicotine nor any other recreational drug has entered his body for eight years. He is 31, and although he has been "in recovery" since the age of 23, he is not out of the woods yet. The physical symptoms of withdrawal may have subsided long ago, but he still attends group therapy meetings. To him, recovering from addiction is like walking up a down escalator: if he stops, he'll descend back into his "madness".

Lefever started taking drugs at the age of 12, and has dabbled in most mind-altering substances you'd care to mention. But the drug on which he became most dependent was cannabis. He dropped out of school because of it, enrolled in a tutorial college, took more drugs and dropped out again. From there, he says, the move into London's financial centre was natural. In the mid-eighties you needed no serious qualifications, and there was the attraction of quick, easy money. As an addict in the City, he was far from alone.

For six years he worked as a futures trader -- gambling in a high-risk environment where the hours were long and the stakes and adrenaline high. "I got off on the intensity of the markets," he says. "The job I was doing was addictive itself, and it paid for all my other addictions beautifully." He took speed and cocaine on the job, drank in the evenings, and smoked cannabis throughout the day, preferring resin to grass because it was cheaper, stronger and more readily available.

"When I drove into the City I would roll up going down the Mall because it's a nice, straight road. I could then spark up on the Embankment and put it out as I parked my car." Knowing he had five or ten minutes before the drug kicked in, he would walk into the office looking relatively alert, then "zonk out" while the analysts gave the rundown on what was happening in Tokyo, and come round again in time for the morning business.

During his stint in the City, Lefever made one attempt to give up cannabis. It lasted eighteen months. Now he believes he was doomed to fail because he only dealt with the physical component of his addiction without addressing the underlying psychological problem: the urge to anaesthetise all feeling. "I don't think being stoned is about happiness or unhappiness," he explains, "It's about not having to feel at all."

Eight years ago, when he gave up for good, "detox-ing" was the easy part. Unlike heroin, for which recovering addicts are often prescribed methadone to ease withdrawal, there is no substitute. You simply stop smoking. But once he had made up his mind he found the experience exciting rather than frightening: "I found that I would wake up maybe fifteen per cent a day, and I kept thinking, this is extraordinary. I'm becoming more and more alert." His short-term memory improved and visually things began to appear with greater clarity. For the first time in years he could make out detail in the leaves on a tree.

Physically, Lefever suffered no withdrawal symptoms. Others have reported cold-like symptoms such as a stuffy head or runny nose. But while the withdrawal is typically mild, the predominantly psychological symptoms suffered by some heavy cannabis users can be more distressing.

Sammy, 38, smoked cannabis every day for 20 years. When he gave up two years ago, he found that for the first few nights he was unable to sleep at all. Then, for the next couple of weeks, his dreams were so vivid and action-packed that in the morning he would feel exhausted despite having slept for a good eight hours: "I would wake up having gone round the world twice, had three affairs and got to number one in the charts." When you go to sleep stoned, he says, you don't dream.

According to Wayne Hall, director of the National Drug and Alcohol Research Centre at the University of New South Wales, Sydney, cannabis is not generally regarded as a drug of dependence because it does not have a clearly defined withdrawal syndrome. But that, he says, is an old-fashioned definition of addiction. "While there may be debate about whether there is a cannabis withdrawal syndrome there is no doubt that some users want to stop or cut down their cannabis use and find it difficult or impossible to do so, and they continue to use cannabis despite the adverse effect it has on their lives."

Of course, most habitual users never report for treatment, particularly since the withdrawal is mild. And Lefever, who now runs the Promis Recovery Centre, a rehabilitation service near Canterbury, Kent that was set up by his father 11 years ago, says people come to the centre all the time to be treated for cannabis dependence. It is rarely their only addiction, and of all the drug dependences he sees it represents the smallest proportion. But that is partly because public funding is reserved for those whose dependences are conventionally regarded as more serious. Besides, very few addicts are dependent on a single substance or behaviour -- which supports his argument that the problem lies in the person, not the substance.

Since it opened, Promis has treated around 1700 people for dependences ranging from alcohol, heroin, cannabis and other drugs to obsessive behaviours such as gambling, sex, shopping, exercise and eating disorders. All patients are treated with the same programme -- the 12-step Minnesota model adopted by Alcoholics Anonymous, which is based on group therapy -- and the average stay there is three months.

"We lose hardly anybody in the detox process," says Lefever. "Getting off drugs is a doddle. It's being able to live life and stay off them which is the hard part."