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In what looked like a return to the dark days of "Reefer Madness"—a 1936 film that portrayed cannabis as a drug that could drive people to madness and murder—a recent article in the Daily Telegraph claimed that cannabis users are "twice as likely to be violent."

The claim comes from a systematic review by King's College London that combined 63 studies covering more than 265,000 people. The researchers did find a link between cannabis use and violence, but the headline overstates what the numbers can tell us.

There are three reasons to be cautious: other factors that could explain the link (known as confounders), the way the studies were designed and potential biases in how the data were gathered.

Poverty, alcohol, other drugs and a difficult upbringing are all linked to violence in their own right. Cannabis is also illegal in many places, and buying it can bring people into contact with people and places where violence is more common. Some of these factors are more common among cannabis users, so these confounders could partly explain the link.

Ice cream sales rise and fall alongside drownings, yet nobody thinks ice cream causes drowning. Hot weather makes people eat more ice cream and swim more, linking the two. Cannabis and violence may be connected in a similarly roundabout way.

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