To discover whether cannabis really does cause paranoia in vulnerable individuals, we carried out the largest ever study of the effects of THC (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.)

It can trigger it in heavy users if they were that way inclined in the first place.