“I would not feel so alone,” sang Bob Dylan back in 1966, “everybody must get stoned.” It would seem lots of people have obliged: cannabis is now far and away the world’s favourite illicit drug, with approximately 180 million people having taken it in the past year. In England and Wales about 13.5% of 16 to 24 years olds have used cannabis in the past year, and almost one in three people will try it at least once during their lifetime. Of the 6.4% of adults aged 16-59 who reported using cannabis in the past year, over 40% said they used it at least once a month. 
 
 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. US president Barack 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 on Wednesday 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. 
 
 Follow @ProfDFreeman and @JasonFreeman100 on Twitter ||||| In what is deemed the largest and most in-depth study of the effects of the main ingredient in marijuana, researchers say they have identified the psychological factors that can lead to feelings of paranoia among users of the drug. 
 
 The research team, led by Prof. Daniel Freeman of the University of Oxford in the UK, recently published their findings in the journal Schizophrenia Bulletin. 
 
 Marijuana, also known as cannabis, is a drug that is produced from the plants Cannabis sativa or Cannabis indica. The main active ingredient in marijuana is delta-9-tetrahydrocannabinol (THC), which is responsible for the majority of the drug's psychological effects, such as hallucinations and delusions. 
 
 
 
 Researchers suggest that THC - the main active ingredient in marijuana - causes certain negative feelings and changes perceptions that induce paranoid feelings among users of the drug. Researchers suggest that THC - the main active ingredient in marijuana - causes certain negative feelings and changes perceptions that induce paranoid feelings among users of the drug. 
 
 Past research has indicated that marijuana use can induce paranoia - which Prof. Freeman describes as "excessive thinking that other people are trying to harm us." 
 
 "It's very common because in our day-to-day lives we have to weigh up whether to trust or mistrust, and when we get it wrong - that's paranoia," he explains." Many people have a few paranoid thoughts, and a few people have many paranoid thoughts." 
 
 For the study, Prof. Freeman and colleagues tested the effects of THC on 121 participants ages 21-50 in order to see whether the compound triggers paranoid feelings and how it does this. All participants had used cannabis at least once previously and had no history of mental health conditions. 
 
 Two thirds of participants were injected with THC at a dose equivalent to a strong joint, while a third of participants were injected with a placebo. The researchers note that they chose to inject the participants with the compound as it ensured they all had similar levels of THC in their bloodstream. The researchers report that the effects of THC on participants lasted for 90 minutes. 
 
 THC causes 'negative feelings and changes in perception that induce paranoia' 
 
 Results of the study revealed that among participants who were injected with THC, around 50% reported paranoid thoughts, compared with 30% of participants who received the placebo. The researchers note that as the compound left the bloodstream, feelings of paranoia reduced. 
 
 The team found that THC also induced anxiety, worry, reduced mood, negative thoughts about oneself, changes in perception - including the report of louder noises and clouds being brighter - and altered their perception of time. Using a statistical analysis, the researchers found that it may be these negative feelings and changes in perception that cause paranoid feelings among marijuana users. 
 
 The team says their findings not only "very convincingly" show that cannabis can cause short-term paranoia in some users, but they may also explain how our mind encourages paranoid feelings. 
 
 "Paranoia is likely to occur when we are worried, think negatively about ourselves, and experience unsettling changes in our perceptions," says Prof. Freeman, adding: 
 
 "The study identifies a number of highly plausible ways in which our mind promotes paranoid fears. Worry skews our view of the world and makes us focus on perceived threat. Thinking we are inferior means we feel vulnerable to harm. Just small differences in our perception can make us feel that something strange and even frightening is going on." 
 
 He notes that although the study - funded by the UK's Medical Research Council - provides more information about the immediate effects marijuana can have, it does not look at the effects of cannabis addiction and therefore "does not necessarily hold implications for policing, the criminal justice system or legislation." 
 
 "The implication is that reducing time spent ruminating, being more confident in ourselves, and not catastrophizing when unusual perceptual disturbances occur will in all likelihood lessen paranoia," adds Prof. Freeman. 
 
 This research is the latest in a line of studies that reveal potential negative implications of marijuana use. Earlier this year, Medical News Today reported on a study suggesting that marijuana use may increase the likelihood of sleep problems, while another study published in the Journal of the American Heart Association links cannabis use to cardiovascular complications and death.
– There has long been a known association between marijuana and paranoia, but UK researchers wanted to find out if cannabis actually causes paranoia in some people—and the answer is a resounding yes, write researchers Daniel Freeman and Jason Freeman in the Guardian. They studied 121 people who had taken cannabis at least once, reported paranoid thoughts sometime during the past month, and had not been diagnosed with a mental illness. Then, in a random and blind test, two-thirds received an intravenous dose of THC (equivalent to a "strong joint," Medical News Today notes) while the rest got a placebo. Then paranoia was comprehensively tracked, and half of the subjects who'd received THC experienced it, compared to 30% of those who got the placebo. "That is, one in five had an increase in paranoia that was directly attributable to the THC," the researchers write. The study shows that "paranoia isn’t tenuously linked to THC: For a significant number of people, it’s a direct result." The researchers also found that the THC produced other negative mental health effects, including anxiety, negative thoughts, and lowered mood. Oh, and it also caused "anomalous experiences" like echoing thoughts and a distorted sense of time, sounds, and colors. Trying to make sense of those experiences while already in a vulnerable state (thanks to the lowered mood) could lead to the paranoia, the researchers suggest. The good news? Subjects' paranoia decreased as the drug left their bloodstream. (Cannabis is also genetically linked to schizophrenia.)