Inundated with calls about serious heart ailments linked to the prescription painkillers Darvon and Darvocet, the plaintiffs bar once predicted that thousands of claims eventually might be filed against the manufacturers of those drugs. A year later, those cases are on life support.

In March, U.S. District Judge Danny Reeves of Frankfort, Ky., who is overseeing the multidistrict litigation against the drugs’ manufacturers, dismissed the primary defendants from a large number of personal injury cases, which at one point numbered nearly 200.