Pre-treatment with 600 mg of cannabidiol (CBD) reduced the likelihood of clinically significant psychotic symptoms and paranoia caused by intravenous delta-9-tetrahydrocannabinol (THC, 1.5 mg) in healthy volunteers. Participants who received CBD before THC had lower scores on the State Social Paranoia Scale and smaller declines in episodic memory compared with those who received placebo before THC. The odds of experiencing a clinically significant increase in positive psychotic symptoms were about 78% lower in the CBD group. These results support the view that cannabis products high in THC and low in CBD pose greater mental health risks.
Schizophrenia is best understood as a syndrome rather than a single disease, with high heritability and multiple genetic and environmental factors pushing individuals over a threshold into clinical expression. Evidence that certain drugs can induce schizophrenia-like psychosis has been neglected as an environmental factor. Over the past 60 years, understanding the link between drug abuse and psychosis has shaped the modern view that liability to psychosis, including schizophrenia, is distributed continuously through the general population, similar to hypertension and diabetes. This review examines hypotheses arising from the association between common psychotomimetic drugs (LSD, amphetamines, cannabis, phencyclidine) and schizophrenia.