Most people who smoke pot enjoy it, but a smaller proportion experience psychotic-like symptoms, such as feeling suspicious or paranoid.

The question that polarises researchers is whether smoking cannabis is associated with a risk of developing psychotic problems, such as schizophrenia, in the long term.

Of course, cannabis use is common, while schizophrenia is relatively rare, affecting less than one per cent of the population.

Even if cannabis use were to double the risk, over 98 per cent of cannabis users would not develop schizophrenia. Researchers have to tread carefully in evaluating the evidence and avoiding scaremongering.

Although several studies suggest that cannabis users have a higher risk of developing schizophrenia, one key point remains hotly contested.

Since the 1960s, cannabis potency and rates of use have risen in many Western countries with high-potency strains now dominating the market.

If cannabis were a cause of psychosis, we would expect that, as this increased, rates of schizophrenia would increase alongside it. But this has not happened.

Most people who smoke pot enjoy it, but a smaller proportion experience psychotic-like symptoms, such as feeling suspicious or paranoid

Still not settled

Although this topic was debated by two eminent British psychiatrists, David Nutt and Robin Murray, in The Guardian and by others in Nature, it remains contested whether a cause-and-effect relationship between smoking cannabis and schizophrenia truly exists.

Perhaps we lack sufficient records of schizophrenia cases to show a robust correlation.

CAN CANNABIS MAKE YOU FEEL ALIENATED? Cannabis users are more likely to experience negative emotions, particularly feeling alienated from others, research suggested in January 2018. People who use marijuana are significantly more likely to feel that others wish them harm or are deceiving them, a study found. Brain scans also reveal the class-B drug increases signal connectivity in regions of the brain that have previously been linked to psychosis, the research adds, which is associated with severe depression. Teenage cannabis users are particularly affected as their brains are still developing, according to the researchers from the National Institute on Alcoholism and Alcohol Abuse in Bethesda. The scientists analyzed 60 people, half of which were cannabis dependent. The study's participants completed a questionnaire that asked them about their feelings of stress, aggression, reactivity and alienation. Brain scans were also taken of all of the participants. In the US, 44 percent of those aged 12 or over have used cannabis at some point in their lives. Around 2.1 million adults in the UK use marijuana every year. Advertisement

It has also been argued that not all effects follow causes. For example, although obesity in the West is increasing and is a known cause of heart disease, the risk of suffering fatal heart disease is going down.

The reason for this is a third factor: treatments for heart disease have improved and are saving more lives. If cannabis potency is increasing and rates of schizophrenia are not, a similar third factor may explain this.

Perhaps the answer is in those brief experiences we have when we use cannabis.

This week results from our online survey thecannabissurvey.com are published in Psychological Medicine.

We asked 1,231 cannabis users about their experiences when they used cannabis and calculated a 'pleasurable experiences score' and a 'psychotic-like experiences score'.

We then asked the participants if they were continuing to use cannabis, or if they were thinking of quitting in the future.

Those who reported the most pleasurable experiences continued to use the drug and had no intention of quitting.

Those with higher psychotic-like experiences had either stopped or were thinking of quitting in the future.

The experience you have with the drug determines whether you continue to use it or not, regardless of your age, sex, mental health history or other drugs you have used.

Interestingly, this might mean that the people at highest risk are the very ones who are quitting.

Other studies suggest that, compared with healthy controls, people with schizophrenia have more psychotic-like experiences when they use cannabis.

And those at higher risk of schizophrenia – that is, people with genetic or psychological risk factors for the disease – tend to have more psychotic-like experiences.

If these are the people who are stopping using cannabis, they may offset their risk of developing schizophrenia from cannabis use.

If cannabis were a cause of psychosis, we would expect that, as this increased, rates of schizophrenia would increase alongside it. But this has not happened (stock)

WHICH POLICE FORCES TURN A BLIND EYE TO CANNABIS SMOKING? The latest figures available show that more than two million people in England and Wales smoke cannabis. Crime Survey figures show a million of these are aged between 16 to 24. The substance is currently a Class B drug in the UK, and anyone found in possession can face up to five years in prison. Selling the drug can get someone 14 years in prison and an unlimited fine. The same punishment exists for giving it to friends for free. It is illegal to be under the influence of cannabis while driving, and can result in a prison sentence and disqualification. On the streets, the average price of an ounce of cannabis tends to be in the region of £200. This is often enough for 40 joints, depending on how strongly people roll them. Reports suggest that around 270 tons of cannabis is consumed every year in Britain - which would equate to around £8.6 million at the aforementioned pricing. Some 80 per cent of this is grown here and not imported. But during 2011/2012, in excess of 1.1million plants were seized by police - with an estimated street value of £207 million. But five police forces in the UK have relaxed their laws on smoking cannabis and growing it for personal use. Avon & Somerset Police, Derbyshire, Dorset and Surrey have taken a relaxed stance on cannabis smoking. A spokesman for Avon & Somerset Police said in 2016: 'We do not proactively seek out people who use cannabis but we do take a rigorous approach to dealing with anyone found in possession of it and they will always be dealt in one of a number of ways. 'They will either be offered intervention, such as educational programmes, given a caution, asked to voluntarily attend a police station for interview or they may be arrested. 'We regularly receive information from the public about suspected cannabis cultivation sites. As the public would expect we research that intelligence and act on what we're told.' Advertisement

The cannabis discontinuation hypothesis

DOES CANNABIS INCREASE THE RISK OF PSYCHOSIS? Going from being an occasional marijuana user to indulging every day increases the risk of psychosis by up to 159 percent, research revealed in July 2017. Marijuana is thought to cause psychosis-like experiences by increasing a user's risk of depression, a study found. The two mental health conditions have previously been linked. Frequently abusing the substance also significantly reduces a user's ability to resist socially unacceptable behavior when provoked, the research adds. Study author Josiane Bourque from the University of Montreal, said: 'Our findings confirm that becoming a more regular marijuana user during adolescence is, indeed, associated with a risk of psychotic symptoms. '[Psychosis symptoms] may be infrequent and thus not problematic for the adolescent, when these experiences are reported continuously, year after year, then there's an increased risk of a first psychotic episode or another psychiatric condition.' The researchers, from the University of Montreal, analyzed around 4,000 13-year-olds from 31 high schools in the surrounding area. Every year for four years, the study's participants completed questionnaires about any substance abuse and psychotic experiences. Psychotic symptoms included perceptual aberration - for example feeling that something external is part of their body - and thinking they have been unjustly badly treated. The participants also completed cognitive tasks that allowed the researchers to assess their IQ, memory and stimuli response. Advertisement

We could think of the experience as a warning sign to which they are responding.

This could be the third factor that explains why the link between cannabis potency and schizophrenia rates is not direct. We call this the 'cannabis discontinuation hypothesis' and propose it in more detail in our paper.

This hypothesis is more nuanced than simply being pro- or anti-cannabis. On the one hand, if you believe that cannabis causes psychosis, this may explain why the rates of cannabis and schizophrenia are not directly correlated.

On the other hand, you could argue that since those at highest risk heed the body's warning system, why does any of this matter. People who are at highest risk will stop in any case.

Of course, it is likely that not everyone does, and we need to make sure that we offer the right support to that small group at highest risk who continue to use.

It is important to remember that, at this stage, this is a hypothesis, not a fact. The survey was taken at a single point in time and the online sample we had may be different from the average cannabis user.

But this group were moderate to heavy users, drawn from activist sites and social media – those that we need to engage the most in this kind of work.

The best study to confirm the hypothesis would be a long-term study mapping cannabis experiences to schizophrenia risk, drawn from the general population, but this would be a long and expensive study to do.

In the meantime, we are continuing to work at thecannabissurvey.com looking at what causes the different experiences we have.

Improved knowledge of these factors will lead to more nuanced understanding in the future.