A new study of THC, the chemical which causes the high people get from marijuana, has shown that marijuana does cause a chemically-induced paranoia in some people.

This is the first study that provides scientific evidence to support a long-suspected belief: that marijuana can cause paranoia.

Other studies have been done that correlate regular cannabis use from a young age with poor mental health in the future. However, it has not been shown conclusively before that the drug causes paranoia.

The study, which was funded by the Medical Research Council (MRC), is the most comprehensive research study ever done on the effects of THC on paranoia.

In the new research study the effects of THC was measured on 121 participants, age 21 to 50, to measure its effect on paranoid thoughts. All participants were in good mental health. One third of the participants was used as a control group, and injected with a placebo.

Participants were injected with the same amount of THC, equivalent to the amount found in a strong joint, into their bloodstream. The effects lasted for about 90 minutes. Participants were then subjected to a range of tests of testing them for excessive suspiciousness, including real-life social situations, a virtual reality simulation, self-report questionnaires and clinical interviews.

Of the two thirds of the group that received THC, about 50% said that they felt paranoid thoughts, much more than the 30% who received the placebo who said that they felt paranoid. After the THC passed through their system the participants said that they felt noticeably less paranoid.

The researchers said that these statistics "very convincingly" show that marijuana can cause short-term paranoia in some people.

Other negative side effects associated with THC use in the participants included anxiety, negative thoughts, and changes in the way the participants heard and saw things. They said that noises seemed louder than usual.

The study attributed the induced paranoia to cognitive changes caused by THC consumption. Consuming THC is more likely to make users feel anxious and think negatively about themselves, which is a breeding ground for paranoia in the mind.

Freeman said in the paper that the new research "shines a light on the way our mind encourages paranoia. Paranoia is likely to occur when we are worried, think negatively about ourselves, and experience unsettling changes in our perceptions."

In the paper, Freeman wrote, "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. Just small differences in our perception can make us feel that something strange and even frightening is going on."

The research was published in the journal Schizophrenia Bulletin.