Schizophrenia

Cannabis is the most widely used illicit recreational drug in the UK, and whilst many users smoke the drug without incurring much harm, for a minority it is known to precipitate serious negative health consequences such as schizophrenia and addiction. A MRC -funded project set out to identify what might determine whether a person would experience more negative consequences from smoking cannabis. One key finding emerged: harm was related to the type of cannabis smoked. The ‘type’ of cannabis is determined by the levels of ‘cannabinoids’ in the plant. Cannabinoids are the 70-odd chemicals unique to the cannabis sativa plant, the two most abundant of which are THC (delta-9-tetrahydracannibinol) and CBD (cannabidiol).

The MRC-funded research found three key related findings:

1) Smoking cannabis is associated with increased risk of schizophrenia but smoking high THC / low CBD cannabis increases your risk of experiencing schizophrenia-like symptoms still further.

2) When intoxicated, smoking cannabis high in THC and low in CBD will cause even greater memory impairment than other varieties of cannabis and is more addictive.

3) THC is present at high levels in the ‘skunk’ variety of cannabis, whereas CBD is absent. THC is the chemical which produces some of the dangerous effects of smoking cannabis, against which CBD is partially protective, therefore skunk is a preparation of cannabis that is particularly harmful.

The public health relevance of these findings is enormous. Skunk currently dominates in the UK with over 80% of the cannabis market. At present, it is estimated that 14% of people who have schizophrenia would not have the disorder if they had not smoked cannabis. Cannabis dependence is also now the primary reason for referral to drug services in Europe in 22% of cases making it second only to heroin. And this figure is rising. The burden of care of cannabis addiction and cannabis-related schizophrenia treatment on the NHS and society as a whole is considerable. On top of this many individuals suffer depression and fail to achieve at school, university or in the workplace as a result of their cannabis use. Young people are unaware of the highly harmful profile of skunk. Increasing awareness of this harm, and consequently changing cannabis smoking behaviour may reduce the negative consequences of the drug that afflict a minority of individuals.