HOUSTON (AP) – Jurors in the nation’s first federal trial over Merck & Co.’s once-popular painkiller Vioxx ended a third day of deliberations Saturday without reaching a verdict on whether the drug contributed to a man’s fatal heart attack.

The nine-member jury is expected to continue deliberations Monday.

So far, the jurors have deliberated for more than 18 hours over three days. They told U.S. District Judge Eldon Fallon earlier Saturday that they could not reach a unanimous verdict, but he urged them to continue.

“It is your duty to agree on a verdict if you can do so without surrendering your own conscientious opinion,” Fallon told them.

He told the jurors that if they fail to reach a unanimous verdict, the case will be tried again, resulting in additional cost, time and effort by both sides. He added that another jury wouldn’t be any more qualified to consider the evidence.

The panel is considering whether the drug was defective, if the company failed to warn about its risks and was negligent in designing and marketing Vioxx.

If jurors answer yes, they then must decide whether any of those factors contributed to the 2001 death of Richard “Dicky” Irvin, 53, of St. Augustine, Fla., whose widow is suing Merck.

Merck pulled Vioxx from the market last year when a study showed the drug could double the risk of heart attack or stroke if taken for 18 months or longer. Irvin died of a heart attack after taking Vioxx for about a month to ease back pain.

Merck claimed Vioxx couldn’t be responsible for Irvin’s death because he took it for such a short time. The plaintiff’s lawyers said several studies among the 58 clinical trials involving 10,000 patients conducted before Vioxx was launched showed dangers after only a few weeks’ use.

If the jury finds Merck liable, the punitive damages will be determined in a later hearing.

About 20 million people took Vioxx after its launch in 1999.

Merck has won one state Vioxx trial so far and lost another, and has more state and federal lawsuits pending.