A Philadelphia jury on Tuesday hit Johnson & Johnson with an $8 billion verdict over its marketing of the anti-psychotic drug Risperdal, siding with a Maryland man who argued that the health care giant downplayed risks that the drug could lead to breast growth in boys.

The verdict in the Philadelphia Court of Common Pleas was the first to award punitive damages against Janssen Pharmaceuticals, a Johnson & Johnson subsidiary, said Thomas R. Kline, a lawyer who is part of a legal team representing the man and more than 10,000 people in similar lawsuits.

Compensatory damages of $680,000 were already awarded in the case in March 2016.

The plaintiff, Nicholas Murray, sued the company in 2013. He said that he grew breasts — a condition known as gynecomastia — after he began using Risperdal in 2003, at age 9, to treat symptoms of autism. The lawsuit accused Janssen of failing to warn doctors about Risperdal’s risks while improperly marketing it as a treatment for certain mental health disorders in children.

“This jury resoundingly told Johnson & Johnson that its actions were deliberate and malicious,” Mr. Murray’s lawyers, Mr. Kline and Jason Itkin, said in a statement Tuesday. “The conduct that the jury saw in the courtroom was clear and convincing that J&J disregarded the safety of the most vulnerable of children. This is an important moment, not only for this litigation, but for J&J, which is a company that has lost its way.”