Question: What are your views on Cannabis?

Joseph Finlayson answered on 13 Jun 2011:

Obviously, as with any illegal drug this is going to be a controversial issue but what I’ll briefly do is examine the science behind claims that are often made about cannabis. I don’t think it’s especially appropriate to list my personal views on cannabis, as it’s far better to be dispassionate about these issues.

I’ll look at the claims over 3 separate answers that:
– Cannabis causes schizophrenia
– Cannabis is addictive
– Cannabis is a gateway drug, that makes users crave bigger highs.

I’d love other zone scientists to chip in on this discussion.

So first, schizophrenia. The largest study to date was the “Cannabis use and risk of psychotic or affective mental health outcomes: a systematic review” in the Lancet (probably the most famous medical journal – you can read it here – http://www.thelancet.com/journals/lancet/article/PIIS0140-6736(07)61162-3/fulltext ) which said that cannabis use is statistically associated – depending on how much you smoke – with mental illness. This does not mean that cannabis causes schizophrenia, or mental illness. ‘Association’ or correlation, does not imply causation.

A good way to illustrate this concept is as follows: it is often the case that if you have more firemen around larger fires. Now, if you didn’t know anything about fires and firemen, you might take that observation and think that firemen cause fires. However, we know that that is not the case, it is larger fires that cause more firemen to cluster round the fire.

So, we know that cannabis use is associated with mental illness but what we do not know is if cannabis causes mental illness. It could be the case that those with mental illness, or pre-clinical (mild) psychotic symptoms self medicate with a drug like cannabis. So, in fact, mental illness could cause cannabis use.

Or perhaps there might be a third factor. Those in the at risk categories for long term cannabis use (often from lower income backgrounds, who live in larger cities) might be in the at risk group for psychosis, or other mental illness. So it is these third factors that cause both cannabis use and mental illness separately.

These debates are problematic as they are highly politicised and polarising. But what I can tell you is that the best studies that have occured to date do not assert a causal link between cannabis and mental illness. However, the jury is still out, and it’s probably better to be interested in cannabis as a purely academic matter rather than making yourself into an experimental guinea pig.

Is this the kind of answer you were after?

– Joseph