Merck’s best-selling asthma medicine, Singulair, has been linked for years to suicides and psychiatric problems, often in children. But lawsuits over the drug are stymied by one of Corporate America’s most effective liability shields: the doctrine of federal preemption.

Nicholas England, a healthy 22-year-old from Virginia, shot himself in the head in 2017, less than two weeks after he started taking an allergy medicine that had been linked for years to episodes of depression and suicidal thinking.

His parents soon started exploring a lawsuit against Merck, the developer of the blockbuster asthma and allergy drug, Singulair, along with the manufacturer of the generic version their son took. Nicholas had no history of mental-health problems, they said.

The Englands were shocked to learn from legal advisers that they had no case. Like countless other potential plaintiffs, they had run into one of Corporate America’s most effective liability shields: the legal doctrine of preemption, the principle that federal law supersedes state law.