The researchers found that compared with never users, daily cannabis use correlated with increased odds of psychotic disorder (adjusted odds ratio, 3.2); for daily use of high-potency types of cannabis, the odds were increased almost fivefold (adjusted odds ratio, 4.8).
"If you are going to legalise cannabis, unless you want to pay for more a lot more psychiatric beds and a lot more psychiatrists, then you need to devise a system where you would legalise in a way that wouldn't increase the consumption and increase the potency", study author Sir Robin Murray told The Guardian.
So how common is high-potency weed, anyway?
Cannabis use is associated with increased odds of psychotic disorder, according to a study published online March 19 in The Lancet Psychiatry.
All participants provided information about how frequently they had used marijuana and other drugs over a period of years. And 37 percent of people with first psychosis events used high-potency cannabis every day.
Those who used ultra-potent cannabis daily were five times more likely to have a first psychotic episode - leading researchers to believe that THC may be the last straw for otherwise mentally healthy individuals. They compared these patients with more than 1,200 random folks who'd never been diagnosed with psychosis. Psychosis is a mental health problem that causes people to become unable to distinguish things. To be precise, the study used the term "psychotic disorder" to refer to the symptoms post-marijuana use, such as delusions and hallucinations lasting one week. On the other hand, if a person suffers regular bouts of psychosis for at least six months, they are open to a potential diagnosis of schizophrenia.
Although inconclusive, the study has significant implications for mental health services and public health. But in Europe, the drug is roundly legalized - so that's where the research team at King's College London began.
Cannabis is defined as being strong when it has a THC level of more than 10 per cent.
High-strength cannabis, such as skunk, has levels of the psychoactive substance tetrahydrocannabinol (THC) above 10%.
In those cities, strong pot is widely available. "This proportion is as high as 30 percent in South London and 50 percent in Amsterdam". "Di Forti said, "...twenty years ago, there wasn't much high potency cannabis available [in the market]".
Legalization of cannabis for medical and recreational use made it increasingly available worldwide. "Given the changing legal status of cannabis across the world, and the associated potential for an increase in use, the next priority is to identify which individuals are at risk from daily potent cannabis use, and to develop educational strategies and interventions to mitigate this".
"Maintaining cannabis prohibition, unfortunately, achieves neither result", he concluded. The finding should be considered by both drug users and lawmakers alike.