Fear, which invariably gets called 'paranoia', is the most common bad consequence of smoking cannabis, normally stopping some way short of the fully dramatized hallucinations of a bad LSD trip. It's the downside of the unpredictability that makes Freek Polak classify pot as 'psychodysleptic', and which means that despite his plainly addictive personality, Karl McCullogh doesn't feel at risk from dope. 'It isn't like any other drug. The reason cannabis isn't addictive is that it doesn't necessarily improve things. It enhances perception - but that can be a bad perception. So if you're feeling low it's really pointless taking Cannabis. I do think it increased the paranoia after my heart attack.' (On the other hand many MS patients, who have plenty to worry about, find that cannabis gives them more equanimity.)

Pro-cannabis experts claim that inexperienced smokers are more at risk. Dr Freek Polak says that unfamiliarity in itself is unsettling: 'When you become aware that the way you think or the way you see the world is changing or has changed - that can be a frightening experience in itself' In Hashish! Robert e. Clarke says that feelings of anxiety or even panic are 'rarely a strong effect' for experienced users. But even if you've been smoking on and off you can get into trouble if you break too many of Professor Tart's suggestions for ensuring a good experience.

I remember being petrified with fear during my first visit to the Rif in the mid-1970s. A young guy had met me and my then girlfriend in Chef chao en and invited us out to his farm, where we'd stayed one or maybe two nights. The views over the hills were exhilarating, and it was fascinating to see how hashish was made, with an oldish colleague permanently stoned from beating powder from dried plants over a sieve. But it was a claustrophobic social situation: we were imprisoned in our role as guests; our host didn't want to talk or go for a walk or do anything except smoke relentlessly. So I sat mutely beside my girlfriend accepting joints which soon tipped us from being ill at ease to feeling grossly anxious.

The set-up wasn't ideal, but the main thing that made us miserable was simply excess. There is such a thing as too much cannabis, even for enthusiasts. Two of the peoples of the Congo, the Kassai and Balaba, this century formed a cannabis cult called the Beni Riamba, using the herb as a medicine and a symbol of peace, but also as a means of punishing criminals by forcing them to smoke to the point of unconsciousness. On the face of it you'd expect long-term cannabis smokers like Karl McCulloch to be immune from this problem and be Paranoia-free. Yet in the world of what might be called 'cannabis professionals' - people who sell pot, or raise seeds, or write on the subject - it's surprisingly common to hear that so-and-so is chronically 'paranoid'. An acquaintance lived for several months in Amsterdam with friends who, unlike her, all smoked a lot of pot. 'There was always this paranoia under the surface,' she told me. In Amsterdam I also heard a lot about the paranoia of various cannabis celebrities: how one wouldn't venture from home for fear of DE A agents, another was so given to feelings of persecution that he felt he was insane to risk further anxiety in search of journalistic exclusives, and so on ... In fairness, though, people who make their living from selling or writing about an illegal substance have good reason to feel threatened. Although Howard Marks tells me that he is not troubled by paranoia, I would not like to have been in his shoes during the months in which the Drug Enforcement Administration was closing the net around his smuggling operation.

Of course, 'paranoia' is a word which in psychiatry means something more precise than a well-founded fear of prison. The Royal College of Psychiatrists reported to the House of Lords committee that the phrase 'cannabis psychosis' should be avoided, because of its lack of precision. Instead they describe a 'toxic confusional state', with confusion and memory impairment and an 'acute functional psychosis', resembling the symptoms of acute schizophrenia. Both of these were to be distinguished from true psychosis in that they cleared up within a week of abstaining from cannabis.

As well as being a pro-legalization activist, Dr Polak has spent many years treating emergency psychiatric admissions to Dutch hospitals. 'There used to be a large general hospital near the Vondelpark - it's now been removed to the south-east part of town. But the Emergency Room got very experienced with these cases: many tourists, but also Dutch people who first tried it. If you have people who are very frightened that they're going mad combined with the specific experiences of cannabis you can have something that looks like an acute psychosis. The classic medical approach was to give them heavy sedation and even anti-psychotic drugs, or to hospitalize them. But all over the world this approach has been replaced by simply reassuring them. You hope they have a friend or a confidant with them and say that if they remain quiet for half an hour it will get better, or if it's bad you offer a little tranquillizer.

'A very small percentage of these cases are really psychotic, and it may even prove to be the beginning of schizophrenia. Probably no one gets schizophrenic because of this, and they would have become schizophrenic anyway. We have no way of knowing that. What is reassuring is that the incidence of schizophrenia - that is, the number of new cases each year - has been getting somewhat lower for the past few years in Holland. It certainly isn't increasing, and given the large number of young people trying marijuana I think that is significant.'