I don't smoke cigarettes and if I were asked to take part in a Cannabis study I would ask not to be provided the placebo but the real thing, as I do not wish to pollute myself unecessarily. This reflects responsible behaviour.

Perhaps asymptomatic psychotic behaviour is demonstrated while under the influence of Cannabis, but the psycho-active effects wear off. Chronic use like in tobacco smoking no doubt results in psychosis, but not many people are chronic users and their addiction underlines other serious psychological issues.

People should not have their personal choice decisions made for them. Regardless of the results of laboratory studies, people's choices of what is in effect a harmless behaviour in moderation should not be legislated.

What a bunch of CRAP!! Currently, I'm over 50 years old. I've been using pot since I was 18 years old. Since then, I went to school & earned a bachelors degree in mechanical engineering. I've been married to the same woman for over 22 years. We've bought & paid for our house. I've worked at the same company for over 13 years. I play tennis twice a week. I ride a bicycle in organized rides up to 65 miles. Many of my friends use pot and are even more successful in life than I am. NONE of us are what I would call psychotic. The language that these "studies" use drives me crazy. They use vague terms like "maybe" & "seems to" which causes people to overreact. "This suggests that 14% of all psychotic illness in Britain is caused by cannabis use." How can someone write that in a journal of medicine? It would be more appropriate in a supermarket tabloid.

I have the long-held opinion that people who use cannabis chronically are consciously or (more likely) unconsciously self-medicating to overcome some latent psychiatric disorder. The study presented here acknowledges that the causal link between psychosis and cannabis is not established nor controlled for in the experiment. Accordingly, the study results may be interpreted either way with equal validity. Particularly in view of the preposterous conclusion that a single instance of cannabis usage may raise one's risk of psychosis by 41%, doesn't it make more sense to suggest that latent psychiatric issues drive a significant number of people to try cannabis at least once?

In your study, what was the placebo. Were the subjects given another type of drug that would give them the same high? Also, most people that suffer from depression and other mental illnesses will of course self medicate. Would this not exclude them from the study since they may have been ill before use? Thanks.

Out of context never gives you a solid answer. I would ask:

Has there ever been a test about alcohol causing mental illness?

If so, of course the answer is yes. Any mind altering drug causes problems. I'll bet alcohol causes many more problems than cannabis, including road rage, partner murder, etc.

LadyLFAB

Since THC-receptors are also elsewhere in the body, did the study undercover any other abnormalilies? I also would like to know if a baseline for physcological behavior was charted.

Hi,
Your reports regarding the relationship between cannabis and schizophrenia are very interesting and probable true. I run a guesthouse in the north Dutch municipality of Zandvoort. Unfortunately we have a number of coffee shops in the village. I noticed changes in people (guests) who regularly consume both marijuana and cannabis, an almost psychotic change. We may be discovering a misconception of the use of light (harmless) drugs. The mind-blowing component of both marijuana and cannabis is THC (delta-9-tetrahydrocannabinol) The amount of THC in marijuana determines the drug’s strength, and THC levels are affected by a great many factors, including plant type, weather, soil, and time of harvest. Sophisticated cannabis cultivation of today produces high levels of THC and marijuana that is far more potent than pot of the past. THC content of marijuana, which averaged less than 1 percent in 1974, rose to an average 4 percent by 1994. The use of marijuana, cannabis as a safe light drug must be reassessed.
Regards Dr. Terence Hale Zandvoort

Following statement is quite clear:-
".. people who have taken cannabis are 41% more likely to develop schizophrenia or other psychotic problems than those who have never used it"

But what to make of following? Maybe somebody kindly explain it to me?

".. increased risk of psychosis, particularly in long-term cannabis users, is worrying, despite the actual risk of developing schizophrenia only being 1% of the population overall"

Thanks

Could we please get a list of who peer reviewed this research? I've always regarded "Nature" as a very credible publication. This article was vague at best.

I think that if I lived in a village with a number of Mary Jane Cafe's I would indeed be seeing a lot of spaced-out people acting quite out of the ordinary. But psychotic? Drinking has become so accepted and understood that one turns a blind eye to the drunks staggering out of the bars at closing time. But, my word! What's wrong with those people? Pot is still a novelty and "high" smokers are still a curiosity until the human compulsion for inebriation to detach from the stresses of the daily grind becomes accepted as a fact of life.

Too many people are suffering needlessly from the legal ramifications inherent in prohibition. I am seeing that alchohol is actually more harmful.

Does psychosis proceed primarily from physical effects of smoking the plant, or might it result from the stress associated with legal dangers or derogatory social interactions (akin to "post-abortion psychosis", perhaps), or might it even reflect some underlying difference in thought processes? It is important to resist knee-jerk legal reactions until it is determined whether legal enforcement action reduces or contributes to the observed symptoms. If marijuana smoking can be shown to cause psychosis, this lends urgency to the observation that tobacco smokers also report a doubled rate of self-reported psychotic symptoms (see PMID: 16738341). Could something as simple as carbon monoxide exposure be contributing to this phenomenon?

Cannabis has increased in use and strength over the last 30 years.

There is a correlation between cannabis and schizophrenia.

There has been no increase in Schizophrenia in the lasy 30 years, in fact a slight decrease.

Surely the only conclusion from the above facts is that cannabis either has no effect or in fact is of some benefit to the Schizophrenic.

I am suprised to see such a poor conclusion passing peer review in the lancet and this nature article.

“Despite a thorough review the authors admit that there is no conclusive evidence that cannabis use causes psychotic illness. Their prediction that 14 per cent of psychotic outcomes in young adults in the UK may be due to cannabis use is not supported by the fact that the incidence of schizophrenia has not shown any significant change in the past 30 years.” - Dr Leslie Iverson. from
http://www.timesonline.co.uk/tol/life_and_style/health/article2148315.ece

The greater danger is the innapropriate prescription of psychiatric medication to those who are simply intoxicated because the intoxication is concealed by the patient.

Clearly greater prohibition entails greater harm.

Campaigning against patients self-medicating when the patient percieve benefits is vile.

In fact, brain images of cannabis addicts are similar to schizophrenia patients. Everyone is the owner of their lifes, but health is the more important thing. To whom feel confortable with the situation, should be aware that psychosis risk improve (schizophrenia, dementia and others diseases).

This is complete nonsense. I've covered it in detail here at scienceblogs.

Briefly, the study shows no real causality. The argument is ultimately post hoc ergo propter hoc. Repeating the nonsense that your risk increases by 40% with just one joint is especially irresponsible, since they didn't study people who just had one joint and stopped, but had used on or more occasions vs, chronic use.

Take cigarettes for example. 90% of schizophrenics smoke, 68% smoke heavily - about 6 times more than the normal population. 90% of schizophrenics start smoking before their diagnosis of schizophrenia. Further, you show the same relative risk for psychotic disorders with cigarette smoking, alcohol, cocaine etc., and no one ever interprets those studies as causative.

It isn't the drugs causing the problem. These findings show, if anything, the well-known and long-studied tendency towards drug abuse in the schizophrenic population, both before and after diagnosis.

Having seen the same sort of claims appear for the last several decades, only to be discredited, I take this cum grano salis. We toxicologists are perpetually plagued by covariables. The wet side-walk does not make it rain, nor does the rooster's crow make the sun come up.

Meta-analyses are notoriously vulnerable to selection bias. Something just does not smell right here. e.g., where is the increase in the overall incidence of schizopherenia? "Drugs" is almost as politicised as "Guns". So anything that looks fishy probably is.

"Frequent use more than doubles psychosis risk" implies causality, yet this study does not in any way show causality. It only shows that, when the researchers looked at the data for pot smokers, they were twice as likely to find psychosis. That's like discovering that wife-beaters are twice as likely to abuse alcohol and concluding that wife-beating causes alcoholism. This is not only bad science, it is irresponsible journalism.

This same story in my local paper (San Diego Union-Tribune) included the information that these researchers were employed and this study was financed by drug companies with a financial interest in the reported outcome, so this should be taken with a truckload of salt.

I'm surprised and disappointed that such shoddy and biased research is reported as straight science by such a prestigious magazine as Nature. My brother is a medical marijauna user and is one of the most stable, non-psychotic people you'll ever meet. Over the course of my lifetime I have seen people on just about every kind of drug there is, and I can assure you that none of the pot smokers I've known have ever exhibited psychotic behavior due to marijauna. I have however, seen an awful lot of psychotic behavior due to alcohol, nicotine deprivation, and legally prescribed drugs used as recommended (i.e., not abused).

Before accepting such articles for publication and thereby lending credence to the results, I expect this publication to determine if the study and financers and authors were more motivated by profit than by impartial science, and to so inform the readership if that is the case. Sadly, this report diminishes my trust in this publication. Apparently the editors have no problem with predetermined "scientific" results.

The study proves nothing, and more or less admits as much. We don't know any more about the relationship between pot and psychosis now then we did before the study.

What is clear is that attributing causality to cannabis is really bad science writing.

It saddens me to see science mis-used this way. Oh well, anything for a grant huh?

I am elated to see that there may be statistical evidence of a link between the chronic use of cannabis and some type of mental illness. As a substance abuse counselor, I have to "stretch" most of time to give my clients valid reasons not to use, other than health risks, educational and occupational problems, and the mere effects of intoxication in general.

Having worked in mental health and substance abuse, I find that users are quite often both mentally ill and substance dependent to a certain degree. Here, we have the proverbial "chicken and egg" scenario -- was it mental illness that preceeded substance abuse or was it the other way around.

I certainly hope that more research will be done in this area, because we as a society need the facts rather than mere statements of users that they haven't suffered any ill effects from the use of cannabis.

While it is refreshing to see so much sense being put forth here to debunk this ridiculous study, I am appalled at one "Dr"'s assertation that since the THC content of pot has risen from 1% to 4%, therefore it is much more dangerous. This is ridiculous! Instead of smoking 1-2 joints of 1% pot, we now smoke 1-2 "hits" of the 4%, ingesting the same amount as before. What about hashish? It's higher than 4% THC content, so we smoke even less. Better keep this doctor away from distilled spirits since their alcohol content is 4 times that of wine, therefore he cannot handle it.

It truly amazes me that so-called "doctors" and "scientists" cannot distinguish correlation from causality, one of the first things one learns in a university or research lab. Have their emotions clouded their judgements, or did they get their degrees for $125 by writing to an address found in a match book cover?
Since opinions are like certain body parts-- every body has one, I truly wish these misinformed pundits would keep their erroneous opinions to themselves.

Such results need to be evaluated carefully, rather than stimulating moral panic. I offer one extended experience and a conclusion:

I used mainly hashish, for four or five extended periods, each of a number of years, spanning ~25 years overall, and experienced the same cycle each time: Early in the period the experience was delightful, but with increasing use, often 2-3 times daily in the later years of each period, it would induce anxiety for the first 20 mins, before relaxation. This was what would lead me to terminate each period.

However, a month *after* the 2nd such voluntary termination, I did experience a brief "flashback" episode that might be classified as psychotic (I walked across town at night to a hospital and remained for a few hours until I'd had a calming conversation with a psychiatrist; and I kept the tablets he gave me for many years - "just in case", though I never used one.)

There were at least a couple more such extended periods of use, before ceasing finally in order to stop smoking tobacco. (Conversely, I used it for ~25 years but took a few extended breaks to relieve the anxiety response.)

I work in a university,
having earned my science-based PhD in the early 70s. Now aged 60, I've never been hospitalised (aside the brief incident mentioned above).

With informed use, I still believe it can be a positive experience, and acknowledge that a few of my life's most joyful moments have been while mildly intoxicated.

I conclude that cannabis can be dangerous, but not notably more so than many other aspects of life. Use it judiciously, and be aware that life always has its outliers, those who are exceptionally vulnerable.

Cannabis use has a far weaker association with schizophrenia than tabacco use. This has been known for many years. See:-
Kelly & McCredie :Advances in Psychiatric Treatment (2000) 6: 327-331

In summary;
1.) The rate of smoking in people with schizophrenia is at least two to three times that in the general population. (four to six-fold greater than cannabis use.)
2.)Patients who smoke, smoke at heavier rates than in the general population
3.) Most patients start smoking in their teens, before the illness begins

It seems there is another agenda being played out in the reporting of the scientific data on cannabis and schizophrenia. It seems quite wrong to build a public expectation that schizophrenia can be prevented by stopping cannabis use, it can't. We should instead focus on finding better treatments rather than a discussion on recreational drug use.

I have been using cannabis since i was 12. It makes me feel good!!!