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.
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.