Hundreds of lawsuits in state and federal courts have been filed in recent years by families who claimed that their children were diagnosed with autism or A.D.H.D. after use of Tylenol during pregnancy.

In the largest group of cases, filed in federal court, a U.S. judge in New York dismissed the lawsuits, citing a lack of reliable scientific evidence. The plaintiffs are appealing the decision, with a hearing in front of an appellate panel scheduled for Nov. 17.

This is very different from, say cigarettes, where the causal relationship was readily apparent for decades while politicians and industry denied it and worked to cover it up.