Teenagers and young adults who use cannabis face increased
risk of psychosis, research published in the British Medical Journal.
Cannabis is the most commonly used illicit drug in the
world, particularly among adolescents, and is consistently linked with an
increased risk for mental illness. However, it is not clear whether the link
between cannabis and psychosis is causal, or whether it is because people with
psychosis use cannabis to self medicate their symptoms.
So a team of researchers, led by Professor Jim van Os from Maastricht University
in the Netherlands,
set out to investigate the association between cannabis use and the incidence
and persistence of psychotic symptoms over 10 years.
The study took place in Germany and involved a random
sample of 1,923 adolescents and young adults aged 14 to 24 years.
The researchers excluded anyone who reported cannabis use or
pre-existing psychotic symptoms at the start of the study so that they could
examine the relation between new (incident) cannabis use and psychotic
The remaining participants were then assessed for cannabis
use and psychotic symptoms at three time points over the study period (on
average four years apart).
Incident cannabis use almost doubled the risk of later
incident psychotic symptoms, even after accounting for factors such as age,
sex, socioeconomic status, use of other drugs, and other psychiatric diagnoses.
Furthermore, continued use of cannabis over the study period increased the risk
of persistent psychotic symptoms.
There was no evidence for self medication effects as
psychotic symptoms did not predict later cannabis use.
These results "help to clarify the temporal association
between cannabis use and psychotic experiences," say the authors. "In addition,
cannabis use was confirmed as an environmental risk factor impacting on the
risk of persistence of psychotic experiences."
The major challenge is to deter enough young people from
using cannabis so that the prevalence of psychosis is reduced, say experts from
in an accompanying editorial.
Professor Wayne Hall from the University of Queensland
and Professor Louisa Degenhardt from the Burnet Institute in Melbourne,
question the UK's decision to retain criminal penalties for cannabis use,
despite evidence that removing such penalties has little or no detectable
effect on rates of use. They believe that an informed cannabis policy "should
be based not only on the harms caused by cannabis use, but also on the harms
caused by social policies that attempt to discourage its use, such as criminal
penalties for possession and use."