Henry Cockburn was diagnosed with schizophrenia in 2002 at the age of 20. Before that he was a heavy cannabis user.

His father, Patrick Cockburn, The Independent's award-winning foreign correspondent, has long wondered whether the two were linked and spent months speaking to the world's leading experts in the field. In a four-part series prompted by his son's condition, he will examine the medical evidence linking sustained cannabis use with schizophrenia, before going on to look at the way the mentally ill have been let down by the health service and stigmatised by public opinion, and concluding on Thursday with his manifesto for a more humane and effective system – accompanied each day by Henry's account of his journey from pyschosis to a normal life.

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For cannabis it is the "tobacco moment". The long-suspected link between consuming cannabis and developing schizophrenia has been repeatedly confirmed by recent studies. Observers say that for cannabis the present moment is similar to that half a century ago when scientific proof of a connection between smoking tobacco and cancer became so strong that no serious doctor or scientist could deny it.

Popular perception of the risks involved for the 2.3 million people taking cannabis in Britain over the last year has lagged behind evidence of its toxicity as shown in a mounting number of scientific studies. One recent expert survey of the evidence published by different scientists in different countries says that research "has consistently found that cannabis use is associated with schizophrenia outcomes later in life".

Sir Robin Murray, Professor of psychiatric research at the Institute of Psychiatry in London, says that studies show that "if the risk of schizophrenia for the general population is about one per cent, the evidence is that, if you take ordinary cannabis, it is two per cent; if you smoke regularly you might push it up to four per cent; and if you smoke 'skunk' every day you push it up to eight per cent".

The great majority of those taking cannabis suffer no ill-effects and may regard warnings about the drug's dangers as exaggerated and alarmist. This includes those taking "skunk", which today contains at least three times more THC, the major psychoactive ingredient in the cannabis plant, than it did in the 1960s. A survey of cannabis confiscated by the police in 2008 showed that ordinary cannabis had about 4-5 per cent THC and "skunk" about 16 per cent. Less potent varieties are becoming more difficult to buy on the street while ever more concentrated ones are available on the internet.

For cannabis smokers diagnosed years later with schizophrenia the outcome is a lifetime battling with psychosis, including symptoms such as paranoid delusions, hostile voices and unexplained waves of terror and guilt. Many sufferers end up isolated, jobless, impoverished and with their lives ruined.

Pro- and anti-cannabis campaigners have furiously disputed the dangers of taking cannabis. Proponents of decriminalisation claim it is no more risky to health than junk food. But doctors and nurses treating the mentally ill in Britain have long noticed that a very high proportion of their patients are serious users of the drug, often starting to take it at a young age. Dr Humphrey Needham-Bennett, medical director and consultant psychiatrist of Cygnet Hospital, Godden Green in London, says that among his patients "cannabis use is so common that I assume that people use or used it. It's quite surprising when people say 'no, I don't use drugs'."

A psychiatrist leading an Early Intervention in Psychosis team in a large inner city area, who did not want to be identified, likewise says "it was not fashionable to say so in the 1990s, but any practising mental health professional would agree that if you smoked a lot of cannabis, particularly in your teenage years, there is a risk of psychosis. Studies coming out over the last five years have confirmed this."

Until very recently conventional wisdom was that while cannabis might have a toxic effect on the life of a minority of users, most people who took it would be unaffected. Consumption was a form of Russian roulette in which a live round was only occasionally fired, though when this did happen the effect could be ruinous.

It is this comforting belief that only a minority of cannabis users is at risk which is now in doubt according to new research showing that the danger to public health may be much wider than previously supposed. The study, published in August 2012, examines 1,000 people in Dunedin, New Zealand, who are one of the most intensively researched groups in the world from the point of view of their mental health.

All were born in 1972-73 and they took IQ tests and other mental functioning tests at the age of 13 and again at the age of 38. Every few years they were also asked about their use of marijuana. Those who smoked significant amounts as teenagers before the age of 18, while their brains were still developing, showed a significant 8-point drop in their IQ levels (between 90 and 110 is considered average) compared to people who were non-consumers. People starting to smoke cannabis after 18 have some reduction in their IQ, but nothing like as large as the younger group. Experts suggest the results may explain why teenage heavy cannabis users are frequently under-achievers.

Until about 10 years ago, cannabis was often seen as a "harmless" recreational drug, the effects of which should be sharply distinguished from heroin and cocaine. There was little research into cannabis as a cause of schizophrenia or as a factor exacerbating the condition once it was diagnosed, leading to "revolving door" cases that clog up such in-patient facilities that exist.

Zerrin Atakan, formerly head of the National Psychosis Unit at the Maudsley Psychiatric Hospital and now a researcher at the Institute of Psychiatry, recalls: "I got interested in cannabis because I was working in the 1980s in an intensive care unit where my patients would be fine after we got them well. We would give them leave and they would celebrate their new-found freedom with a joint and come back psychotic a few hours later."

Dr Atakan became intensely interested in investigating the link between cannabis and mental illness, though this turned out to be easier said than done. She says: "I was astonished to discover that cannabis, which is the most widely used illicit substance, was hardly researched in the 1990s and there was no research on how it affected the brain." She intended to study how the brain was affected by THC using neuro-imaging. "We did eight grant applications and got nothing," she says. "We carried out studies without grants, which is generally unheard of, because you have to pay for imaging sessions using brain scanners."

Prior to about 2002, scientific evidence that cannabis could be risky for some was often discounted or ignored. One such piece of research was a study of some 50,000 Swedish army conscripts who joined the army in 1969-70. They were interviewed then and later about their drug consumption. The evidence from this very large sample was that heavy consumers of cannabis at the age of 18 were six times more likely than non-consumers to be diagnosed with schizophrenia over the next 15 years.

Epidemiological studies like this are now being confirmed by new studies of the brain through neuro-imaging. Dr Paul Morrison, a psychiatrist specialising in cannabis research, says: "Pretty well all the studies have been consistent in linking the use of cannabis and having a chronic [mental] disorder."

But the connection is not straightforward. Susceptibility to schizophrenia depends on genetic inheritance, but this in turn is highly complex since 62 genes so far have been identified as contributing to vulnerability to psychosis. Social and personal stress play a role, with immigrants and people from areas which are socially fragmented more likely to suffer from schizophrenia. Professor Murray says that there are few reliable figures on this, but "the incidence of schizophrenia in south London since 1964 has doubled".

Cannabis consumption has been falling in Britain as well as the rest of northern Europe since 2004, but the age when teenagers start taking the drug is also going down. There has been a nearly 20-fold increase in first-time use by under-18s, with 40 per cent of under 15-year-olds in the UK having used the drug. This is a dangerous trend. Dr Morrison says "adolescence seems to be the critical variable when the neuro-circuits are being sculpted and the personality is emerging". In fact, for the children of cannabis smokers the first impact of cannabis starts before birth according to post-mortems on the aborted foetuses of mothers who admit to taking cannabis.

Debate over the risks or lack of them stemming from cannabis has traditionally been rancorous and embittered, often revolving around the separate issue of decriminalisation. There is limited reference to long-term mental illness. The pro-cannabis lobby says that the so-called "war on drugs" has failed and legalisation or regulation should be tried, though critics argue that no government would ever license a drug that sends at least two per cent of its consumers insane.

Knowledge of the risks stemming from cannabis use as revealed by recent studies may be spreading, particularly as consumers become more aware of the greater toxicity of skunk. Professor Murray says that the average doctor may not know much about the dangers of cannabis, but "I think that the average 19-23-year-old knows more because they have a friend who has gone paranoid. People know a lot more about bad trips than they used to."

Schizophrenia: The shame of silence, the relief of disclosure

Henry Cockburn: If I say I'm schizophrenic people reply, 'So you've got a split personality'

The stigma of the hidden schizophrenia epidemic

Editorial: We are failing sufferers of mental illness

Henry Cockburn: I like to be liked – and finally I’ve found friends who really like me

Henry Cockburn: 'I can hear what other people can't'

Fashion advice from the shrink’s sofa

The demise of the asylum and the rise of care in the community

Henry Cockburn: I was trying to listen to my shoes. I thought I was going to be put in a straitjacket