

Cannabis and Psychosis - Gatsky
http://www.bmj.com/content/342/bmj.d738.full

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gjm11
Summary:

They looked at people aged 14-24 ("time T1", which they actually call
"baseline"), and followed them up 3.5 years later ("time T2") and 5 years
after that ("time T3"). They looked at whether they'd used cannabis five times
or more by T1, or between T1 and T2. And they looked at whether they'd had any
"psychotic symptoms" -- quite broadly defined, so that ~10% had such symptoms
between T2 and T3 -- between between T2 and T3.

They excluded from their sample anyone who'd had psychotic symptoms by time
T2; that was 23% of their sample! (They wanted to avoid finding people who
had, e.g., already had psychological problems and taken cannabis in the hope
that that would help.)

They also excluded anyone who'd taken cannabis before T1.

(The results without those exclusions were similar to the results with them.)

They found a substantial increase -- about a factor of 2 -- in incidence of
"psychotic symptoms" between T2 and T3 among people who used cannabis between
T1 and T2. Remember that "psychotic symptoms" doesn't mean actually being
psychotic.

They also looked for evidence of that "self-medication" scenario, by looking
to see whether people with psychotic symptoms by T2 were more likely to use
cannabis between T2 and T3. Apparently not.

They found some evidence that using cannabis over a longer period is
correlated with (and probably causes, given their findings about self-
medication, but that's harder to be sure of) larger risk increases for
psychotic symptoms.

There are a bunch of caveats; for instance, they're relying on self-reported
data for both cannabis use and psychotic symptoms. (They give some reasons for
thinking that this doesn't hurt their analysis too much.)

I think their investigation would have been improved by introducing an extra
delay, and looking at whether cannabis use between T1 and T2 is associated
with more psychotic symptoms between T3 and T4. (Compare the two propositions
"Taking cannabis now will make your brain a bit weird for a year or two" and
"Taking cannabis now will make your brain a bit weird for the rest of your
life".)

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stjohn
TL;DR:

 _Conclusion: Cannabis use is a risk factor for the development of incident
psychotic symptoms. Continued cannabis use might increase the risk for
psychotic disorder by impacting on the persistence of symptoms._

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pandeiro
Why is it always cannabis and psychosis? How about a longitudinal study on
cannabis and creativity?

~~~
kunley
You mean: cannabis and discreativity. Lots of ideas, no will to implement
them.

~~~
nfg
Speak for yourself...

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extension
This is a correlative study, so nothing can be concluded from it other than
"voluntary cannabis use and psychotic symptoms have some direct or indirect
covariant relationship".

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cryofan
bought and paid for by the pharma/booze/police state industries

~~~
MaxGabriel
Is that the case?

"Funding: The EDSP study is funded by grants of the German Ministry of
Research, Education and Technology (01EB9405/6 and 01EB9901/6) and the
Deutsche Forschungsgemeinschaft (DFG), and this paper is part of NIH grant
RO1DA016977-01, PL. Competing interests: All authors have completed the
Unified Competing Interest form at www.icmje.org/coi_disclosure.pdf (available
on request from the corresponding author) and declare: no support from any
organisation for the submitted work; no financial relationships with any
organisation that might have an interest in the submitted work in the previous
three years; and no other relationships or activities that could appear to
have influenced the submitted work."

