In a groundbreaking study, Oxford University researchers found that pot can on occasion make you paranoid, in affirmation of the experiences of everyone who has ever smoked, talked to someone who smokes, or watched a movie about smoking.

"What?" the devoted pothead asks between hits, then passes the bowl to his two-faced, judgmental friends. "That's bullshit."

Not bullshit: science. Take it from here, scientists.

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.)

"However," the pothead slowly intones as two cops who can probably tell he's high walk by outside the window, "By your own admission, you're interested in exploring the relationship between early-age cannabis use and 'later severe mental health problems.' Even if I was feeling paranoid right now, which I'm definitely not, what does that have to do with larger questions about my psychological well-being?"

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.

"So all you're telling me is that pot makes some people paranoid while they're high, but that effect tends to wear off as they come down?" he asks. His phone starts to go off — probably mom wanting to chide him for being such a bad son; he should really call more — but he continues. "I'm not convinced. Can you at least tell me how THC is supposedly freaking me out right now?"

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.

Now the SWAT team is after our pothead, and he doesn't have much time to argue. He climbs out the fire escape, shouting over his shoulder as he goes: "Sounds pretty fuckin' tautological to me."

[Image via PathDoc/Shutterstock]