Could cannabis make somebody kill? In the past week, a picture has been forming of Khairi Saadallah, 25, arrested after last weekend’s Reading knife attack in which David Wails, Joe Ritchie-Bennett and James Furlong died.

Saadallah had suffered mental health problems, receiving treatment for post-traumatic stress disorder and other difficulties. He also reportedly smoked cannabis — a fact to which some commentators drew attention. But scientists, too, are becoming increasingly interested in such details.

Sir Robin Murray, professor of psychiatric research at King’s College London, is reviewing research into the drug’s impact on crime and believes there is a relationship between heavy consumption and violence.

“I’m a psychiatrist: I see people who have become psychotic, smoke excessively and become paranoid,” he says. “[When you give]