One of the largest-ever studies of THC, the main ingredient in marijuana, has revealed that cannabis does indeed cause paranoia among vulnerable users.
A team of researchers led by Oxford University Professor Daniel Freeman have also determined the key cognitive mechanisms which cause paranoid anxiety: negative affect and anomalous experiences.
According to the researchers, the study has finally provided hard evidence of the long-suspected causal link between cannabis and paranoia.
While experts generally agree that regular use of cannabis from an early age is an accurate predictor of later mental health problems, what has not previously been established is whether the drug causes paranoid thoughts.
It is known that THC, full name delta-9-tetrahydrocannabinol, is responsible for the majority of the drug's psychological effects, such as hallucinations and delusions.
For the study, the team tested the effects of THC on 121 participants ages 21 to 50 to see whether the compound triggers paranoid feelings and how.
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 chose to inject the participants with the compound as it ensured they all had similar levels of THC in their bloodstream. The effects of THC on participants lasted for approximately 90 minutes.
Of those participants who were injected with THC, around half reported paranoid thoughts, compared with 30% of participants who received the placebo. The participants noted that as the compound left the bloodstream, feelings of paranoia reduced.
Using a statistical analysis, they found these findings "very convincingly" show that cannabis can cause short-term paranoia in some users.
The team found that THC also induced anxiety, encouraged participants to think negatively about themselves and induced changes in perception, with participants reporting that noises seemed louder and clouds brighter.
According to the researchers, it is these cognitive mechanisms which encourage paranoid feelings.
"The study identifies a number of highly plausible ways in which our mind promotes paranoid fears," Freeman said. "Worry skews our view of the world and makes us focus on perceived threat. Just small differences in our perception can make us feel that something strange and even frightening is going on."
"Clearly cannabis doesn't cause these problems for everyone," the scientist told the Guardian. "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."
The research was published in the journal Schizophrenia Bulletin.