Amid growing decriminalisation of cannabis use, a new study warns that daily cannabis use, especially of high potency, is strongly linked to the risk of developing psychosis, a mental disorder characterised by a disconnection from reality.
The findings, published in the journal the Lancet Psychiatry, are consistent with previous studies showing smoking pot with a high concentration of THC -- over 10 per cent of the psychoactive substance within cannabis -- has more harmful effects on mental health than the use of weaker forms.
"As the legal status of cannabis changes in many countries and states, and as we consider the medicinal properties of some types of cannabis, it is of vital importance that we also consider the potential adverse effects that are associated with daily cannabis use, especially of high potency varieties," said lead author of the study Marta Di Forti from King's College London.
The new study looked at 11 sites across Europe and Brazil. First, the researchers estimated the prevalence of psychosis by identifying individuals with first episode of psychosis, presented to mental health services between 2010 and 2015.
Second, they compared 901 patients with first episode of psychosis with 1,237 healthy matched controls to understand the risk factors associated with psychosis.
The researchers collected information about participants' history of cannabis use and other recreational drugs.
Across the 11 sites, people who used cannabis on a daily basis were three times more likely to have a diagnosis of first episode psychosis, compared with people who had never used cannabis, the findings showed.
This increased to five times more likely for daily use of high potency cannabis.