"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 public health importance to consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties", said Di Forti.
The research involved 901 patients aged 18 to 64 who have first-episode psychosis and have used mental health services in major sites across Europe between 2010 to 2015.
People reported the names of weed strains they used, such as skunk in the United Kingdom or the Dutch Nederwiet, which allowed the researchers to identify the THC content in each product through data gathered by the European Monitoring Center for Drugs and Drug Addiction and national data from the different countries.
Patients were twice as likely to report using skunk compared to healthy people, with nearly 40 percent of the patients admitting to using high-potency cannabis. For those who used high-potency marijuana daily, the risk jumped to almost five times. In the Netherlands, THC content can be as high as 67 percent, while 94 percent of highly potent, "skunk" pot in London averages 14 percent THC, the researchers said. Psychotic disorders, including schizophrenia and delusional disorder, are often triggered by genetic factors, trauma, and other environmental stresses. So the more people who used the drug daily; and the more who used high-potency marijuana, the higher the rate of psychosis.
Scientists from the Institute of Psychiatry, Psychology and Neuroscience at King's College London compared the assessment results of psychosis patients with that of 1,237 matched non-patients recruited for the study.
The research, which was published online Tuesday, adds to previous studies that have shown causal links between the drug and the onset of severe mental health disorders. Additionally, 30 percent of new psychosis cases diagnosed in London were also associated with high strains of the drug.
Di Forti and her team also identified and categorized the low potency and high potency tetrahydrocannabinol or THC - the psychoactive component of cannabis.
Overall, cannabis use was found to be more common among patients experiencing psychosis.
Use of high potency cannabis was a strong predictor of psychotic disorder in Amsterdam and London, where high potency cannabis is widely available.
And they said that even medicinal cannabis oil - available in the United Kingdom for a very limited number of people - should come with a warning of psychosis as a possible side effect.
It's important to note that the findings don't necessarily mean that strong cannabis causes psychosis, simply that it appears to increase a person's risk. Incidences of psychosis were higher in Amsterdam than most other locations studied, with only south London surpassing it. They should be aware that using high-potency cannabis comes with a risk, she says.