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 use: The drug was still prescribed to 12 million people in the United States in 2022.

Children face the greatest risks of the drug’s ill effects, and while usage by minors did decline, it was still taken by 1.6 million of them — including Nicole Sims’s son. Ms. Sims had no idea why, at 6, her son started having nightmares and hallucinations of a woman in the window. When he told her that he wanted to die, Ms. Sims went online, desperate for answers.

Only then did she learn about the F.D.A. warning. She also found a Facebook support group with 20,000 members for people who had experienced side effects of the drug. Members of the group recounted a haunting toll that they linked to the drug with the help of peers, not their doctors.