PHILADELPHIA — A U.S. jury ordered GlaxoSmithKline to pay US$2.5 million to a woman whose son was born with serious heart defects after she took the antidepressant Paxil during her pregnancy.

The closely watched verdict handed down Tuesday in Philadelphia was the first of about 600 similar cases pending across the country that blame Paxil for heart problems and other birth defects.

The jury found GlaxoSmithKline guilty of negligence but not outrageous conduct, and rejected punitive damages. The company vowed to appeal.

"The adverse events started to come in the late 1990s, early 2000. The evidence was overwhelming and alarming," said lawyer Jamie Sheller, who represented plaintiff Michelle David. "They could have known this way, way before they did, way before they changed the label in 2005."

Paxil was classified as a drug with no known link to increased birth defects from its introduction in 1992 through 2005. The Food and Drug Administration began warning in September 2005 that Paxil may be associated with birth defects and strengthened the warning four months later.

David, 28, of Bensalem delivered her full-term son, Lyam Kilker, in October 2005.

He was diagnosed with heart defects two months later and spent five months in a Philadelphia hospital, undergoing surgery to repair two holes in his heart, lawyer Jamie Sheller said Wednesday. He also has a third, separate heart defect and will need at least one more surgery as he grows, she said.

David, a dance teacher and former Philadelphia 76er cheerleader, has no history of heart defects in her family, her lawyer said.

GlaxoSmithKline argues that birth defects occur in 3 to 5 percent of all live births, whether or not the mother took medication during pregnancy.

"The scientific evidence does not establish that exposure to Paxil during pregnancy caused his condition," the drugmaker said in a statement. "Once approved for use, the company acted properly in marketing the medicine, including monitoring its safety, updating pregnancy information in the medicine's labeling as new information became available, and in communicating important safety information to regulatory agencies, the scientific community and the public.

Plaintiffs lawyers will continue to pursue punitive damages in the hundreds of remaining cases, the next of which is set for trial in Philadelphia in November.

"We're starting to chip away at this story, but even as we speak, we're still fighting them, documents are still being produced, depositions are still being taken," Sheller said.

Sales of Paxil totaled $849 million last year.

Paxil has competed fiercely in the marketplace at times with rival antidepressant blockbusters like Eli Lilly's Prozac and Pfizer's Zoloft. The drug no longer has patent protection and now competes against cheaper generic versions.