Cannabis use during pregnancy – no link to schizophrenia in offspring found by study

A study with the snappy title of “Maternal tobacco, cannabis and alcohol use during pregnancy and risk of adolescent psychotic symptoms in offspring” was published recently  in BJpsych (abstract) (full text)  by researchers at Cardiff, Bristol, Warwick and Nottingham universities which threw another spanner in the claims that cannabis causes schizophrenia. As the title suggests this study looked at the effects of tobacco, alcohol and cannabis use by the mother during pregnancy on the offspring.

Needless to say it didn’t hit the headlines although the BBC did quietly report it with the  headline “Pregnant smoking ‘psychosis link'” (BBC news).

Of perhaps special interest is that the researcher at Cardiff was one Prof Stanley Zammit who featured in the cannabis and mental health debate a few years ago with his analysis of the famous Swedish study into military conscripts which Dr Zammit claimed to demonstrate a causal link between cannabis and serious mental illness. He was also behind the study which was widely reported as warning

The study found that those who had used the drug were 41 per cent likelier to experience an episode of this kind than people who never smoked.

Or as the Daily Mail put it

Smoking just one cannabis joint raises danger of mental illness by 40%

Of course we don’t blame Sr Zammit for the Mail’s excess, but it would be reasonable to assume from his past work that Dr Zammit was looking for, and expected to find, a link between cananbis use during pregnancy and later mental illness. Indeed, on his university profile page he writes of the study:

Approximately 15% of the population report psychotic-like experiences not meeting criteria for clinical disorders. These occur more commonly than schizophrenia, and are likely to be closer to underlying aetiological pathways. Studies of PLIKS (sub-clinical, psychosis-like symptoms) may increase understanding of schizophrenia aetiology, and help focus prevention and intervention strategies. All the genes above, as well as cannabis and tobacco, are thought to affect glutamatergic transmission. Examination of gene-environment interplay may provide further insights into aetiological mechanisms.

So it’s reasonable to conclude that Dr Zammit did not go into this study with the mindset of someone trying to disprove a relationship between cannabis use during pregnancy and later psychotic symptoms in offspring.  That is a good thing and makes his results all the more interesting.

What the study found was that tobacco and alcohol use by pregnant women was associated with the development of serious mental illness in the offspring, but cannabis was not. The BBC reported this finding thus:

The group was also studied for their mother’s use of cannabis and alcohol. No link was found for the drug, while only those whose mothers drank more than 22 units had a higher chance of psychotic episodes.

Notice how cannabis is referred to as “the drug”, as if alcohol and tobacco are something else. But that sort of subtle bias is only to be expected sadly.

Now of course demonstrating a link does not mean proving a causal mechanism – a point which has often not been fully appreciated by sections of the media when covering the cannabis and mental health debate, but showing no link would seem to go some way to disproving any such causal role. After all, if there are no ill people, there isn’t a cause.

Now of course no one study is going to give a clear cut answer like that and it has to be noted that although the study sample was large, the number of cannabis users who had claimed not to have used tobacco during pregnancy was relatively small

So what of the study? It reports this:

Cannabis use during pregnancy
Maternal cannabis use was not associated with any suspected or definite PLIKS in the crude analysis (OR for linear trend 1.22, 95% CI 0.83–1.79). The odds ratio was reduced after adjusting for confounders (Table 2), with adjustment for maternal tobacco use having the greatest impact on attenuation of this estimate (adjusted OR = 0.94, 95% CI 0.62–1.41, P = 0.755). Of the 157 women with PLIKS data who used cannabis during pregnancy, 51 (32.5%) claimed not to have smoked tobacco during their pregnancy. There were insufficient numbers of women using cannabis to examine trimester-specific effects of cannabis use.

However, you can bet that had a link been found, the headlines would have screamed the message of cannabis use during pregnancy causes schizophrenia because it would have fitted the media’s agenda.

What the study did find was a link between tobacco use and later mental health problems in the offspring and that should be ringing those alarm bells for mental health campaigners. Throughout the cannabis and mental health campaign of the middle of this decade, no reference was ever made to the use of tobacco by the people supposedly damaged by their cannabis use, yet every one probably smoked tobacco filled joints, the nature of which give huge doses of tobacco along with the cannabis. It’s also more than possible that the parents of these ill people also smoked tobacco of course.

The evidence is still only circumstantial, but it’s looking ever more likely that tobacco has a role in the development of mental illness. That isn’t to claim that cannabis is always harmless in this respect, but  it should give us cause for concern and to perhaps encourage those interested take a more critical look at the role of tobacco.

Non of the above is intended to imply that cannabis use – or the use of any drug come to that – during pregnancy is safe and the advice as always is not to drink, smoke, toke, snort or spike when pregnant for the good of your kid.