The classical Greek term pharmakon indicates that a substance can be a remedy as well as a poison, and in many traditional societies cannabis was considered to have both of these properties. However, as the twentieth century progressed, cannabis gradually lost its status as a useful remedy, and fewer and fewer people regarded it as harmful. Indeed, by the 1990s, the prevailing medical wisdom held that smoking cannabis did not cause long-term harm to health. Recreational use became normalized to the extent that use of cannabis was seen, like that of alcohol, nicotine and caffeine, as a culturally acceptable lifestyle choice .
Recently, however, cannabis has reemerged as both a potential medicine and a potentially harmful drug. On the one hand, cannabis-based drugs have been shown to be of value to people with chronic pain, and to control spasticity in patients with multiple sclerosis. On the other hand, a number of reports have claimed that heavy use of cannabis increases the risk of psychotic illnesses, such as schizophrenia. If the latter is true for even a small minority of cannabis users, this would be of considerable public health importance, because cannabis is the world’s third most-popular recreational drug, after alcohol and tobacco.
In this Perspective, we briefly outline recent research into the endocannabinoid system3–8 and discuss how exogenous cannabinoids might disrupt interneuronal signalling and information processing in the brain. We then consider the evidence as to whether cannabis can induce acute and chronic psychosis, whether it is addictive and whether its use leads on to the use of hard drugs, such as heroin and cocaine.
Finally, we discuss how different societies are attempting to deal with cannabis use.
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Copyright: Nature Reviews: Neuroscience