Lean—also known as purple drank, dirty Sprite, sizzurp or simply syrup—is a drink usually associated with American hip-hop culture. Images of brightly coloured liquid in polystyrene cups have helped turn it into a recognisable cultural symbol, particularly on social media. Behind the branding, however, lean is essentially a form of opioid use. What is lean? The traditional American version is made by mixing cough syrup containing codeine and promethazine with a fizzy drink. S

Research has really yet to catch up with the full implication of new psychoactive substances. However, a recent study by Olga...