Twenty years ago cannabis was generally perceived to be a benign drug with few significant adverse effects. As outlined elsewhere in this book, evidence has since mounted in the scientific literature for a range of harms associated with the use of cannabis, including the development of dependence and health-related harms (see also Hall and Solowij, 1998; Hall and Degenhardt, 2009). As the overall theme of this book indicates, an association between cannabis use and the development of psychotic symptoms or overt psychosis has grown to be recognized as a significant potential harm, and investigating the mechanisms by which cannabis may trigger psychosis is a priority. This includes understanding the effects of cannabis on brain structure, biology and function. We recently highlighted a similarity between the cognitive impairment that has been reported in cannabis users and the deficits observed in schizophrenia (Solowij and Michie, 2007), suggesting common underlying neuropathology. Few would argue that cognition is not impaired to some degree during acute intoxication with cannabis. That impaired cognition persists beyond the period of acute intoxication is more contentious. Despite objective appraisals of the literature in interpreting the evidence, it is inevitable that researchers will be influenced by the weight of their own data in formulating scientific opinion. Accordingly, and on the basis of the accumulating evidence, this review will come to some rather different conclusions from those made in the first edition of this book (Pope and Yurgelun-Todd, 2004).
The goal of this chapter is to update our knowledge of the short- and long-term effects of cannabis on cognition based on integrating evidence from the most recent literature on this topic. We acknowledge the weight of evidence from our own studies that must inevitably guide us to the conclusions that we draw, while also aiming objectively to assess the evidence from multiple sources. We consider evidence from preclinical research, studies of acute administration of cannabinoids to humans, studies of long-term or heavy cannabis users tested in the unintoxicated state, including adults and adolescents and patients with schizophrenia, and we evaluate the evidence for recovery of function after prolonged abstinence.