“Are you making good health decisions?” reads one Robert F. Kennedy Jr. meme on social media, a slogan printed against an image of a smiling US health secretary. Such social media posts invariably invite lively comments beneath them, but the situation is deadly serious. On 22 September, Donald Trump and RFK Jr. publicly proposed a link between paracetamol – commonly referred to in the US by the brand name Tylenol – and autism.