“I would not feel so alone,” sang Bob Dylan back in 1966, “everybody must get stoned.” Millions of individuals have obliged: cannabis is now far and away the world’s favorite illicit drug, with approximately 180 million people having taken it in the past year. According to the National Institute on Drug Abuse, almost one third of American 18 to 25-year-olds have taken it in the past year. Among adults aged 26 and over, around one in ten have used cannabis in the previous twelve months, and 45 percent have taken it at least once during their lifetime.

Given that a significant chunk of the population are consuming the stuff, it is perhaps not surprising that most people believe the risks involved in getting stoned are more or less equivalent to the risks from getting drunk. President Obama seems to be one of them: “I smoked pot as a kid, and I view it as a bad habit and a vice, not very different from the cigarettes that I smoked as a young person up through a big chunk of my adult life," he explained to the New Yorker. "I don't think it is more dangerous than alcohol."

Certainly many people use cannabis without adverse effects, and indeed with plenty of very pleasant ones. Moreover, there is evidence that cannabis can bring real medical benefits, for example in alleviating chronic pain. But there is also known to be a link between cannabis and paranoid thoughts.

“Paranoid” in this context means the unfounded or excessive fear that other people are trying to harm us. It’s a feeling that is far more common than previously thought. That is understandable given that we are all constantly compelled to interpret social situations, weighing up the attitudes and intentions of the people we meet. Because it is impossible to know for sure what other people are thinking, there is ample scope for our anxiety to get the better of us.

Like most psychological experiences, there is a spectrum of paranoia within the population: many people have a few, relatively mild paranoid thoughts, while for a few people those thoughts are numerous, persistent, and profoundly unsettling. Cannabis users are more likely to be at the problematic end of that spectrum. For instance, our study of the population of England found that the belief that people are deliberately trying to harm you is three times as common among cannabis users as it is among non-users. The belief that people are trying to cause you serious injury or harm is five times as common among cannabis users.

However, what we see here is an association between cannabis and paranoia. Experts generally agree that regular use of cannabis starting from an early age is an accurate predictor of later severe mental health problems, but what hasn’t been established is whether the drug causes paranoid thoughts. Maybe people suffering from paranoia are more likely to start taking cannabis; or perhaps the drug use and the suspicious thoughts are independent consequences of another factor entirely.

This question of the tangled interrelationship of paranoia and cannabis use was at the heart of a study we conducted with colleagues from the University of Oxford, the Institute of Psychiatry at King's College London, and the University of Manchester, published this week in Schizophrenia Bulletin. Why did we focus on paranoia rather than mental health in general? Other studies tend to lump all such problems together under the heading of “psychosis” or “schizophrenia”, but as we’ve argued previously on this blog such experiences frequently occur independently: having paranoid thoughts doesn’t mean, for example, that someone will also hear imaginary voices.

To discover whether cannabis really does cause paranoia in vulnerable individuals, we carried out the largest ever study of the effects of THC (∆9-tetrahydrocannabinol, the drug’s principal psychoactive ingredient). We recruited 121 volunteers, all of whom had taken cannabis at least once before, and all of whom reported having experienced paranoid thoughts in the previous month (which is typical of half the population). None had been diagnosed with a mental illness. The volunteers were randomly chosen to receive an intravenous 1.5mg dose of either THC (the equivalent of a strong joint) or a placebo (saline). To track the effects of these substances, we used the most extensive form of assessment yet deployed to test paranoia, including a virtual-reality scenario, a real-life social situation, self-administered questionnaires, and expert interviewer assessments.

The results were clear: THC caused paranoid thoughts. Half of those given THC experienced paranoia, compared with 30% of the placebo group: that is, one in five had an increase in paranoia that was directly attributable to the THC. (Interestingly, the placebo produced extraordinary effects in certain individuals. They were convinced they were stoned, and acted accordingly. Because at the time we didn’t know who had been given the drug, we assumed they were high too.)

THC also produced other unsettling psychological effects, such as anxiety, worry, lowered mood, and negative thoughts about the self. Short-term memory was impaired. And the THC sparked a range of what psychologists call “anomalous experiences”: sounds seemed louder than usual and colours brighter; thoughts appeared to echo in the individuals’ minds; and time seemed to be distorted.

Why is cannabis such a potent trigger for paranoia? Our statistical analysis showed that in our experiment the culprits were THC’s negative effects on the individual’s mood and view of the self, and the anomalous sensory experiences it can produce. Negative emotions leave us feeling down and vulnerable. Worry leads us to the worst conclusions. So when we try to make sense of the anomalous experiences – when we try, in other words, to understand what’s happening to us – the world can appear a weird, frightening and hostile place. Hence the paranoia. Our analysis suggests that the impairments in short-term memory did not increase the paranoia.

Clearly cannabis doesn’t cause these problems for everyone. And the suspiciousness wore off as the drug left the bloodstream. But the study does show that paranoia isn’t tenuously linked to THC: for a significant number of people, it’s a direct result.

Perhaps most importantly, the research shines a light on the psychological processes underlying paranoia in general. When we worry, think negatively about ourselves and experience perceptual disturbances, it’s much more likely that we will feel needlessly suspicious of others.

Daniel and Jason Freeman are the authors of Paranoia: the 21st-Century Fear. On Twitter they are @ProfDFreeman and @JasonFreeman100. A version of this blog first appeared in The Guardian: http://www.theguardian.com/science/2014/jul/16/cannabis-paranoia-psychoa...

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