As the desire to decriminalize cannabis continues to spread among lawmakers, it’s important to determine just how safe the drug actually is. Marijuana might have various medicinal properties, but a new study published in The Lancet Psychiatry reports that high-potency cannabis is “strongly linked” to psychosis. The finding should be considered by both drug users and lawmakers alike.

People suffering from psychosis struggle to differentiate between what is real and what is not, often experiencing hallucinations or delusions. Some people might just experience a single episode of psychosis, while others live with it on a daily basis. Psychosis is linked to a number of mental health conditions, such as schizophrenia, severe depression, and bipolar disorder.

Worryingly, the research found that a high proportion of new psychosis cases in Europe can be linked to the use of high-potency pot. For example, the team found that in Amsterdam, half of new cases are connected to the drug, while in London, just under a third are.

Strong cannabis is that which contains a high level of delta-9-tetrahydrocannabinol, aka THC. This is one of cannabis’ active ingredients, and is responsible for the psychoactive effects of the drug. Over the last two decades, THC levels in cannabis in both Europe and the US have risen four to five-fold.

"The use of cannabis with a high concentration of THC has more harmful effects on mental health than the use of weaker forms," lead author Marta Di Forti of King’s College London said in a statement.

"Our findings also indicate for the first time how cannabis use affects the incidence of psychotic disorder at a population level."

To reach their conclusions, the researchers turned to 11 sites across Europe and another in Brazil and examined new cases of psychosis between 2010 and 2015, finding a total of 901. These patients were then compared to 1,237 healthy individuals. The researchers then determined whether they used cannabis and what strength they used – high-potency cannabis was defined as having a THC concentration of above 10 percent.

The team found that almost 30 percent of people who had experienced psychosis used cannabis daily, while less than 7 percent of the controls did. When it came to strong pot, these figures were 37 and 19 percent respectively.

Overall, daily cannabis users were three times more likely to suffer psychosis than those who did not use the drug, and those that used high-potency cannabis daily were five times more likely to experience a psychotic episode.

The researchers say that if strong cannabis became unavailable, 12 percent of new psychosis cases across Europe would be prevented. In Amsterdam, where cannabis has been decriminalized for decades, the frequency of first-time psychosis diagnoses would decline from 38 people per 100,000 each year to 19 people per 100,000.

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. Still, the research suggests that cannabis users should think twice before using the strong stuff, especially if they use the drug on a regular basis. Meanwhile, those looking to decriminalize cannabis should consider the harmful effects that it can have.

“If you are going to legalise cannabis, unless you want to pay for 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.