NEW YORK (CNNMoney.com) - Merck's got three victories and only one loss under its belt in the ongoing Vioxx litigation battle, but the upcoming cases could get a lot tougher.

Merck (down $0.39 to $35.66, Research) faces nearly 10,000 lawsuits regarding Vioxx, the arthritis painkiller that the drug maker pulled off the market in 2004 after its study demonstrated an increased risk of heart attack and stroke in patients who took the drug for 18 months or more. The previous plaintiffs against Merck took Vioxx for short periods of time, but the plaintiffs in the upcoming cases could present a tougher challenge, because they took Vioxx for longer than the 18 months that Merck's own study cited as problematic.

In the fourth case to reach a verdict, a federal judge in New Orleans ruled in favor of Merck on February 17. Andy Birchfield, the lawyer who represented plaintiff Evelyn Irvin Plunkett, the widow of Richard Irvin who died of a heart attack in 2001 after taking Vioxx for about one month, blamed the loss on a ruling from Judge Eldon Fallon. Fallon shot down Birchfield's attempts to get a cardiologist and a pathologist to testify that Vioxx caused Irvin's heart attack.

"With all due respect, we think that [ruling] was in error," said Birchfield, in a statement provided by his firm.

A fifth case is ongoing in Texas state court in Rio Grande City, a town of 12,000 near the border with Mexico, though the trial has been suspended for the next few days.

Thousands of cases have been filed in superior court in Merck's home state of New Jersey, where the drug maker defeated one of its plaintiffs. Next week, Merck faces two more plaintiffs in a sixth case in New Jersey Superior Court in Atlantic City, and the company has two causes for concern.

First, both of the plaintiffs that Merck will face in New Jersey next week were patients who Vioxx for at least 18 months. The previous plaintiffs took Vioxx for less than 18 months, which was the time period cited in the Merck study that prompted the Vioxx withdrawal. Merck defended itself in earlier trials by saying the plaintiffs didn't take Vioxx long enough to develop heart risks from the drug. That defense won't work with the upcoming plaintiffs.

"The big hill for [Merck] is dealing with the cases where people have taken the drug for over 18 months and had a cardiac event," said Jeff Cooper, a partner with Simmons Cooper who represents about 700 plaintiffs who sued Merck in New Jersey, but is not connected to the sixth case.

Also problematic for Merck is the fact that one of these plaintiffs, Tom Cona, is represented by Merck's adversary Mark Lanier, the Texas plaintiff lawyer who won the verdict in state court in Angleton, Texas in November. Lanier is the only lawyer to win a victory against Merck so far.

"Merck has admitted that Vioxx causes heart attacks after 18 months," Lanier said in an email to CNNMoney.com. "The ethical thing would have been to warn [the public.] I expect [to see] a further lack of ethics as Merck denies that Vioxx is dangerous at all now. They will try to sell the jury on the idea that Vioxx is a vitamin."

The jurors awarded Lanier's first Vioxx plaintiff Carol Ernst $253 million, though the damages will be capped by Texas law and Lanier said the plaintiff will collect about one-tenth of that.

Lanier, a Baptist preacher with five children, has a reputation for winning over juries with his down-home delivery and colorful anecdotes. Lanier has compared Merck to Saddam Hussein, and to the iconic cartoon monkeys who see no evil, hear no evil, and speak no evil. He also successfully portrayed Carol Ernst, whose husband died after taking Vioxx, in a sympathetic light. This is something that Chris Seeger, the lawyer representing Frederick Humeston, failed to do in N.J., when Merck lawyers attacked his client's reputation and portrayed him as untrustworthy.

"From Merck's victories, I've leaned that you must tie their lawyers to the truth," said Lanier. "They specialize in mudslinging."

Cona, 59, of Cherry Hill, NJ, survived a non-fatal heart attack at age 56 after 22 months of taking Vioxx, said Lanier. The other plaintiff, John McDarby, 78, of Park Ridge, took Vioxx for four years before suffering a non-fatal heart attack in 2004, according to his lawyer Rob Gordon of the firm Weitz and Luxemberg. The two cases are being tried together, though last week Merck filed a motion to Judge Carol Higbee to sever the cases.

Plaintiff lawyers in the upcoming cases interpret the Vioxx withdrawal as an admission of fault to plaintiffs who took the drug for at least 18 months. But Merck denies those claims and has said all along that Vioxx did not kill anyone.

Plaintiff's attorney Rob Gordon told CNNMoney.com that "Merck is in denial of their own studies" that show Vioxx increases heart attack risks after 18 months.

However, if Merck can defeat plaintiffs who took Vioxx for more than 18 months, then the company could shore up one of its weakest links.

"If Merck can win on those, then they can really say they've got some momentum on their side," said David Stahl, a legal expert and partner with Eimer Stahl Klevorn & Solberg, who is not connected to the Vioxx cases.

But Merck will still have to present itself as a trustworthy company, said Stahl. During the Plunkett v. Merck trial, the company was accused in an editorial in the New England Journal of Medicine of deleting information about Vioxx-related heart attacks. The company has denied the accusation, but it may still have to prove this to a jury. Plaintiff lawyers see the journal editorial as an opportunity to depict Merck as a shady company, and will probably use it as a legal weapon in upcoming trials.

"Juries don't react very well when they think the defendant is hiding something," said Stahl.

To find out how much Merck stands to lose from Vioxx, click here.