Smoking potent forms of cannabis greatly increases the risk of serious mental illness, a major study has found.
Taking any version of the drug daily was found to triple the chances of developing psychosis.
But using high-potency cannabis containing high levels of the active ingredient THC raised the risk five-fold.
Scientists carried out the study at 11 sites in five countries across Europe, as well as one in Brazil.
They found that the link with psychosis was strongest in London and Amsterdam, where high-potency cannabis is commonly available.
Psychotic conditions such as schizophrenia can have devastating effects, including paranoid delusions and hallucinations.
Researchers looked at the drug use history of 901 patients experiencing a first psychotic episode between 2010 and 2015, and 1,237 healthy individuals.
The study took place in England, France, the Netherlands, Italy, Spain and BraziI.
In total 29.5% of the patients had a daily cannabis habit compared with 6.8% of the “controls” untroubled by serious mental health problems.
Just over a fifth of the psychosis cases were thought to be linked to daily cannabis use across the 11 sites.
Lead researcher Dr Marta Di Forti, from King’s College London, said: “Our findings are consistent with previous studies showing that the use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms.
“They also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level.
“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 that we also consider the potential adverse effects that are associated with daily cannabis use, especially high potency varieties.”
Use of high-potency cannabis was a strong predictor of psychotic illness in London and Amsterdam, where powerful versions of the drug were widely available, said the authors.
Strong marijuana is commonly known as “skunk weed”.
The findings are published in the journal The Lancet Psychiatry.
Dr Adrian James, from the Royal College of Psychiatrists, said: “This is a good quality study and the results need to be taken seriously.
“Cannabis carries severe health risks and users have a higher chance of developing psychosis. The risks are increased when the drug is high in potency, used by children and young people and when taken frequently.
“Because of these risks, a good drugs strategy should focus on preventing and reducing harm, not on diverting people to the criminal justice system.”
Waht is Cannabis?
Cannabis is currently a controlled drug as classified by the Misuse of Drugs Act 1971.
A follow-up to this law, the Misuse of Drugs Regulations Act 2001, placed it under Schedule 1, which is the category for substances with no medicinal value. And this is the schedule being considered by the review.
Cannabis plants are made up of more than 100 different cannabinoids, which have different impacts on the body and are concentrated to different extents in certain parts of the plant.
The most well-known of these are THC and CBD.
THC is the psychoactive cannabinoid – the one that recreational users use to get “high”. CBD does not have this effect.
CBD – or cannabidiol as it is also known – is not controlled under the drugs act, and GPs in the UK are able to prescribe cannabis-derived products to patients.
CBD oil is becoming increasingly popular – read again about this Oxford artist’s new range of products.
An Oxford cafe became the first in the city to offer cannabis coffees last month – see how reporter Tom got on testing it out