Cigarette use associated with psychotic episodes

Cannabis use and cigarette use at age 16 were both associated, to a similar degree, with psychotic episodes at age 18 in a cohort study published in Psychological Medicine. I report this because it is commonly assumed cannabis can cause psychosis, but as the authors of the article say, there is no evidence that tobacco has a causal effect in psychosis. Of course, cannabis intoxication can cause transient psychotic-like experiences, but what is less clear is whether cannabis can cause psychosis not related to intoxification effects.

As I said in my BMJ letter, "Cannabis use is likely to be a proxy measure for poor premorbid adjustment associated with psychosis." I went on, "I am not saying that cannabis does not cause emotional problems; nor that people do not use it to deal with their emotional problems." The degree of correlation between the use of cannabis and cigarette smoking means it is difficult to disentangle independent effects. However, the likelihood is that cannabis use and cigarette smoking are both markers for socioeconomic and environmental factors associated with psychosis. It's illogical to interpret the association between cannabis and psychosis as causal, as it is to do the same for cigarette smoking.