04/03/2015 07:19 GMT | Updated 03/05/2015 06:59 BST

Cannabis Species Are Not Created Equal

Cannabis is in the limelight again with the current Channel 4 documentary Drugs Live: Cannabis on Trial, which is related to the recent Lancet publication that evaluates the distinction between high potency and other types of cannabis based on a study of South London cannabis users, published in last month.

That cannabis is linked to mental health problems is not really new news. For example, in 1987 a study of just over 45,000 Swedish conscripts showed with 95% confidence that cannabis smokers were six times more likely to develop schizophrenia. This, and other important subsequent studies, showed that the risks of cannabis smoking extends to other mental health, physical health and social problems in young people.

Another significant example is the longitudinal study from Australian and New Zealand, reported in the Lancet in September 2014. This study showed that teenage cannabis users, followed up over seven years, were five times more likely to be depressed and anxious. Furthermore there is a substantially increased rate - almost a sevenfold increase - of suicide in daily cannabis smokers younger than 17 when they were followed up at aged 30.

Despite the evidence cannabis is widely used; there are an estimated 2million people in the UK that smoke cannabis, there continues to be a lobby for its legalisation. Indeed, legalisation of cannabis has been realised in California for medicinal purposes, a place that is hailed as being at the forefront of technology and scientific understanding. For recreational purposes it is available further up the coast in Oregon and much closer to home in Dutch coffee shops. What is notable about the licensing in Holland is that in October 2011 the Dutch government released a proposal to prevent the sale of cannabis in coffee shops that contained more than 15% of the pro-psychotic chemical Δ-tetrahydrocannabinol (THC). This however was never enacted.

Many people who view cannabis as a type of benign, natural product have recognised that there are stronger forms of cannabis (e.g. skunk) that carry a risk but often believe this is because skunk has been somehow 'made unnatural' through being 'genetically modified' or 'sprayed' with man-made hallucinogens. The reality is that just as breeding dogs can naturally lead to different animal characteristics some cannabis strains have different amounts of the various naturally occurring cannabinoids (e.g. THC versus cannabidiol which may in fact be anti-psychotic) and so place users at different risk. Some samples of skunk contain high concentrations of THC and virtually no cannabidiol.

The most recent Lancet study on 410 patients in South London with first episode psychosis compares individuals who smoked skunk-like cannabis to never-smokers. It demonstrated that smoking skunk once a week increases your risk of psychosis by 2, more than once a week by almost 3 and more than five times a week by more than 5 times. Furthermore, skunk use appears to contribute to 24% of first episode psychotic disorders in South London.

What the study also shows is that hash smokers in their sample did not have a statistically significant increased risk of psychosis, so the Dutch government might have been on to something. However it is worth highlighting that the South London study sample were aged 18-65, unlike many other studies that focus on teenagers, where the results show a clear link between teenage cannabis use and adverse consequences.

Of course even the best informed cigarette smokers continue smoking and it is worth remembering that Doll and Harris' early seminal studies on the health risks of smoking were on doctors, a group of individuals who were in the best possible position to foresee the subsequently proven risks.