Inaugural Mark Gold Lecture - “The rise and rise of drug-induced psychosis”

There has been an important change in the presentation of schizophrenia-like psychoses over the past 30 years.  Reports started coming from Japan and Taiwan in the 1990s of the emergence of methamphetamine psychosis as a common illness,  This then spread through Thailand, Australia, South Africa to North America. In a number of countries 25%-50% of cases of psychosis are due to methamphetamine.  Experimental studies have long shown amphetamines and other drugs that increase synaptic dopamine can induce psychosis. Our experimental studies administering the active ingredient of cannabis, THC, intravenously to normal volunteers also demonstrate that a transient psychosis can be readily induced. Cannabis-induced psychosis has become much more common with the relaxation of legal constraints on its use, and the increasingly high potency of the substance in many countries. Furthermore, studies have shown that a considerable proportion of people (up to 50%) initially diagnosed with methamphetamine or cannabis psychosis proceed to receive a diagnosis of schizophrenia in the next 5 years. Evidence has emerged that the impact of cannabis-induced psychosis is sufficiently large to effect the incidence of psychosis. Thus, our EU-GEI study demonstrated that daily use of high potency cannabis is associated with an increased incidence of psychosis, with rates of both much higher in large Northern European cities than in Southern Europe. Similarly our recent INTREPID study of sites in rural India (near Chennai), Nigeria and Trinidad has shown that the incidence of psychosis was low in rural India and Nigeria (comparable to Southern Europe) but reached nearly 80 per 100,000 in Trinidad (similar to Northern European Cities).  As in the EU-GEI study the incidence of psychosis tracked the frequency of high potency cannabis use. Studies in the same country over time also show that as cannabis use and potency has increased, so the incidence of psychosis has increased; this is the case across Portugal, Scandinavia, Canada, USA and UK. The worldwide trend to increased use and potency, suggests that the incidence of psychosis is likely to increase in more and more countries