In early 2020, the Food and Drug Administration responded to decades of escalating concerns about a commonly prescribed drug for asthma and allergies by deploying one of its most potent tools: a stark warning on the drug’s label that it could cause aggression, agitation and even suicidal thoughts. The agency’s label, which was primarily aimed at doctors, was supposed to sound an alert about the 25-year-old medication, Singulair, also known by its generic name, montelukast. But it barely dented u