Cannabis has a long history as a pain-reliever – it is mentioned in 3500-year-old medical texts from ancient Egypt. But there is still active debate over the extent of the drug’s analgesic properties.

On the one hand, there is some evidence that cannabis can relieve certain types of pain. For instance, in a small trial published about 15 years ago, researchers found the drug could reduce the pain associated with an injection of capsaicin, a compound in chilli peppers that induces a sensation of burning…