Corcoran et al. [41] prospectively followed 32 cases of prodromal psychosis for up to 2 years and found that these cases had significantly more perceptual disturbances and worse functioning during epochs of increased cannabis use. They concluded that the use of cannabis was a risk factor for the exacerbation of subthreshold psychotic symptoms (perceptual aberrations) in these high-risk cases. Similarly, Cadenhead et al. [119] reported that in a sample of individuals with a high risk for developing psychosis, those individuals with cannabis use were ten times more likely to convert to psychosis than individuals without cannabis use. This interaction of psychosis-proneness and cannabis exposure has also been observed in an experimental approach—in a controlled laboratory study, Henquet [94] showed that psychosis-proneness influenced the effects of Δ9-THC on cognition and psychosis.
