A Philadelphia jury on Tuesday ordered Johnson & Johnson to pay $8 billion in damages to a Maryland man who said his use of J&J’s antipsychotic Risperdal as a child caused enlarged breasts and the company failed to properly warn of this risk.

It was the biggest award to date among more than 13,000 lawsuits against J&J alleging that Risperdal caused a condition called gynecomastia in boys, which involves enlargement of breast tissue. The lawsuits generally claim that J&J was aware of the risk of this side effect, but understated...