Cannabis and mental health

While the BMJ editorial on the whole analyses the data well, Two most
glaringly obvious observations from the original Swedish conscript study
have been overlooked. That is that most of the people taking cannabis did
not develop psychosis, and conversely most people who developed
schizophrenia (197/41280) did not use cannabis. While the incidence in
cannabis users increases, and there is a dose-response relationship (the
higher the dose the greater the chance) the fact remains that out of those
2836 using 1-10 occasions only 18 developed schizophrenia (RR 1.3) 10/702
using cannabis 11-50 times developed schizophrenia (RR 3.0), and 21/752
using >50 times (RR 6.0).

Clearly there is a seed-soil phenomenon and cannabis may contribute
to the web of causation in some susceptible people, but cannabis by itself
is not the cause of schizophrenia.

Alan Gijsbers.

Competing interests:  
None declared