Science Interviews


Mon, 23rd Feb 2015

Skunk and schizophrenia

Suzi Gage, University of Bristol

Listen Now    Download as mp3 from the show Marijuana: Risk or Remedy?

Smoking a spliffOn the streets, cannabis is one of the most commonly used illegal drugs. Recent news headlines announced that 1 in 4 cases of mental psychosis might be due to the super-strong “skunk” form of cannabis and that the risk of psychosis is five times higher for those who use it every day compared with non-users. But does cannabis cause mental illness, or are people who are destined to become mentally ill just more likely to use it in the first place? Suzi Gage from Bristol University works on the link between cannabis and psychotic episodes, and explained to Chris Smith why the association between cannabis and schizophrenia is more complicated than many headlines imply... 

Suzi - Psychosis is one aspect of Schizophrenia but you can also just have psychosis on its own and it refers to things that are present that shouldn’t be there. So, you might have hallucinations. So, you could hear voices or see things, but hallucinations can affect any of the senses as well. You might experience delusions. So, you might feel like someone is out to get you, feelings of persecution or you might feel like your're a very important person, like you're sort of a king or something but no one realises those kind of thoughts that don't really have any real world truth to them. There's also sort of thought insertion or broadcaster withdrawal where you think that someone else is interfering with your thoughts or other people can hear your thoughts, that sort of thing.

Chris - So, why might cannabis – let’s not say that it does or it doesn’t at this stage – but why might exposure to the chemicals in cannabis provoke those sorts of symptoms in a patient?

Suzi - Well, the reason that it was first investigated was because intoxication effects of cannabis can induce  transient psychotic-like experiences. This has been shown in randomised trials as well that if you give people THC – the compound that we’ve been talking about a lot through the show that this will induce transient psychotic experiences. But of course, that's not the same as saying that long term use will cause something like Schizophrenia. That's much harder to research because you can't randomly assign a group of teenagers, half of them to take cannabis for however, many years, and half of them not to and see what happens. So, you can only look observationally and the people who choose to smoke cannabis are going to be different from the people who choose not to smoke cannabis for all sorts of reasons other than the cannabis use.

Chris - Including potentially because they're already feeling in some way mentally unwell or unstable. And they find perhaps in their case that cannabis helps to put them on a more, even kill or helps them to cope better the symptoms they're experiencing.

Suzi - Well absolutely. That's obviously not necessarily the case. But it’s something that when people are conducting these observational studies, they have to be really careful to try and exclude anyone who already had any kind of psychotic experiences before they started using cannabis in order to try and get a bit of a better handle and whether it’s actually the cannabis causing the psychosis.

Chris - So, what work have you done to try to disentangle those two and try to work out which it is whether the cannabis comes first and causes the Schizophrenia or whether the predisposition to mental illness comes along and causes people to use more cannabis?

Suzi - So, I've been using a data set based in Bristol where I work which is called Children of the ‘90s. This is a longitudinal birth cohort. So, lots of pregnant women in Bristol and the surrounding areas were recruited in the 1990s. And them and their kids have been followed up ever since. So, the kids are now not kids anymore. They're in their 20s. So, I'm not actually looking at Schizophrenia as an outcome. I'm looking at psychotic experiences. So,  non-clinical unusual experiences that people might have that are sort of akin to psychosis.

What I found was that although we see an association between cannabis and later, psychotic experiences, like lots and lots of other studies have shown as well, what I found was after I took into account a lot of the other things that might have also been impacting on this, the size of the association actually got quite a lot smaller until we couldn’t be really sure that it was actually any different from no effect.

Chris - So, where do these headlines from researchers elsewhere in the UK that surfaced in the last month saying that maybe 1 in 4 cases – that's a lot isn’t it – 1 in 4 of mental cases and presentations might be due to exposure to super strong forms of cannabis? And that in fact, people are 5 times more likely to develop these mental illnesses if they smoke this stuff.

Suzi - So, a lot of these headlines came from one particular paper and the paper itself is absolutely brilliant. But the headlines slightly overegged it as  can often be the case when it’s such on an emotive issue. So, this particular study was looking at sort of hospitalisation or initial hospitalisation for first episode of psychosis. So, this is a more severe outcome than the one I've been looking at. And what they found was that people who’d smoked skunk every day had a much higher likelihood to have been a psychosis case compared to a control group that they also sampled.

But they're very clear  in their journal article that they can't be sure that the association seen in their study is causal. They also split their sample into people who reported using skunk and people who reported using hash. So, they kind of used these terms as shorthand for skunk being very high in THC and very low in cannabidiol which we’ve been talking about earlier. Whereas hash being slightly less high in THC but having a sort of equivalent amount of cannabidiol. But this was only people self-reporting what type of cannabis they were using. They didn’t collect samples or anything, so they can't actually be really sure. But there does seem to be evidence that cannabidiol might be anti-psychotic. That's something that's being actively researched at the moment. Which if was the case, would certainly – their results would be well on the way to helping, to try and tease these apart whether it might be that cannabis, that's now grown under hydroponic lights which seems to be much higher in THC but much lower in cannabidiol could be what's driving this link that we see between cannabis and psychosis.



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Cannabis do not induce schizophrenia. This is a myth developed by pharmaceutical industries
to sell anti-psychotics for cannabis-induced psychosis. Personally, I believe 
cannabis is a anti-depressant and neuroprotective medication which can be used to treat neuronal
disorders like depression and epilepsy.
tkadm30, Tue, 8th Dec 2015

I don't think there is any causative relationship established between cannabis and schizophrenia. There are many studies that have found some degree of correlation, but I think many have been poorly designed or poorly interpreted, and none has shown any conclusive causation.

Pure THC is known to cause psychotic episodes, but the effects are modulated by CBD, which is also naturally found in cannabis. Only when the THC is extracted or synthesized and administered in a pure form is this a problem.

Excessive doses of cannabis can induce panic attacks, hallucinations and delirium, but these are all short-term effects, and have not been shown (as far as I know) to increase the risks of becoming schizophrenic.

Schizopherina is a very poorly understand disorder (actually probably multiple types of disorder that have been lumped together.) Cannabis may exacerbate (or mimic) the positive effects of schizophrenia (paranoia, hallucinations), but as far as I know, cannot elicit any of the negative effects of schizophrenia (poor ability to communicate, disinterest, loss of pleasure...) chiralSPO, Tue, 8th Dec 2015

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