Adverse effects of heavy cannabis use: even plants can harm the brain

The spread of laws legalising cannabis for medicinal or recreational use has been accompanied by more relaxed attitudes towards cannabis. Data from the United States show that in states that have legalised cannabis, prevalence of daily, weekly, and monthly cannabis use was 11.3%, 18.3%, and 25.0% respectively, whereas in countries where it is still illegal, it was lower (7.4%, 11.6%, and 16.8% respectively). Evidence indicates a trend of increase among adolescents, a particular vulnerable category for the initiation of substance use. In parallel, we have seen the concentration of THC (D-9-tetrahydrocannabinol) in the cannabis sold both in the United States and in Europe rising and those types of cannabis with high THC, and a corresponding decrease of cannabidiol (CBD) content, becoming more widely available. Most commonly, cannabis is used for its enjoyable effects, the “high” feeling. In addition, in those countries where its use has been legalised, many people smoke cannabis for medical use, anxiety, depression, and pain relief, with those suffering from chronic pain being at higher risk of developing cannabis use disorder (CUD). This review aims to challenge the widespread view that cannabis being a “plant” does not carry adverse effects, and review the evidence concerning the effects of cannabis use on mental health and cognition