GlaxoSmithKline P.L.C., the world's second-biggest drugmaker, begins a trial in Philadelphia next week in what may be a test case for more than 600 lawsuits over claims that the company's antidepressant drug Paxil causes birth defects.

Patients and their parents say internal company documents show Glaxo failed to warn consumers about the risks of Paxil until forced to do so in 2005 by the Food and Drug Administration. In the trial set to start Monday, Michelle David blames the drug for causing life-threatening heart defects in her son, Lyam Kilker, now age 3.

The company, based in London and with major operations in Philadelphia and its suburbs, faces two more such trials each month from October through January in state court in Philadelphia.

"The early cases set the parameters for any global settlement negotiations," said David Logan, dean and professor of law at Roger Williams University in Bristol, R.I.

Paxil, approved by the FDA in 1992, generated about $942 million in sales last year, 2.1 percent of the total for the company.

Glaxo has settled other Paxil-related cases, including a suit brought by the New York Attorney General's Office accusing the company of withholding safety data about the antidepressant.

The drugmaker isn't liable for Lyam Kilker's heart defects, and it acted responsibly in testing Paxil and updating safety information, Kevin Colgan, a Glaxo spokesman, said in an e-mail.

"The scientific evidence simply does not establish that exposure to Paxil during pregnancy caused Lyam Kilker's condition," Colgan said. "Very unfortunately, birth defects occur in 3 to 5 percent of all live births, whether or not the mother was taking medication during pregnancy."

The FDA said in an alert to doctors on Dec. 8, 2005, that preliminary studies suggested Paxil might contribute to heart defects in infants when taken in the first three months of pregnancy. The government asked the company to update the label enclosed with the medicine, changing its birth-defect warning.

The FDA's action does not prove any connection between Paxil use and birth defects, Glaxo said in court filings in July.

"GlaxoSmithKline will show it acted properly and responsibly in conducting its clinical trial program for Paxil, in marketing the medicine, in monitoring its safety once it was approved for use and in updating pregnancy information in the medicine's label as new information became available," Glaxo's Colgan said.

Lawyers for patients say Glaxo documents show the company had known since 1980 that Paxil could raise the risk of birth defects.