Merck will start with a potential bonus on Monday in its second trial over personal injury claims related to its withdrawn drug Vioxx, as the New Jersey state court could limit evidence the jury hears to determine negligence.

And the US drugmaker has beefed up its defences for the critical federal court trials starting in November. It has hired attorney Phil Beck, credited with saving German drugmaker Bayer billions of dollars in liability when he led the defence of its withdrawn cholesterol drug Baycol in 2003.

Federal court cases make up about half of the 5,000 lawsuits claiming injury from Merck's pain drug Vioxx. Because of tighter evidence rules in federal court, these cases are expected to play the key role in determining its ultimate liability. Merck withdrew Vioxx in September after finding it increased risk of heart attack and strokes after 18 months' use and could face billions of dollars of liability.

But the group has vowed to contest each case and a state hearing in Atlantic City, New Jersey, begins today. Merck requested and received a split in how the jury determines damages in the trial over claims by Frederick Humeston, a 60-year-old from Idaho, that Vioxx caused his heart attack.

The jury is expected to determine whether Merck is at fault and liable for compensatory damages separately from any determination of punitive damages.

This so-called bifurcation makes the upcoming courtroom battle different from the first trial in Angleton, Texas, where Merck lost. The result is that evidence can be limited in the basic determination of fault particularly evidence used to attempt to show a company behaved badly, therefore requiring punitive damages.

The Texas jury was shown punitive damages evidence along with evidence attempting to show Vioxx caused a man's death. It awarded widow Carol Ernst $253m in total damages.

Merck expects Texas law to reduce that to $26m, and plans to file an appeal when the presiding judge formally enters the verdict.

But Merck said the Ernst verdict had in no way cracked its confidence in its defence. Kenneth Frazier, Merck's general counsel, said: “We are in this for the long haul. We have both the resources and resolve to address the cases one by one over many years.”

Jury selection begins today in the case of Mr Humeston, a postal worker and Vietnam war veteran, who began taking Vioxx for pain in a knee injured by shrapnel. Christopher Seeger, his lawyer, argues Mr Humeston suffered a heart attack in September 2001 from taking Vioxx for two months and survived but with damaged health. The case claims heart damage, shortness of breath and fatigue limit his activities.

Merck counters that Mr Humeston's medical condition contained factors that were ideal for a heart attack, including cholesterol problems, borderline hypertension and obesity. It says Mr Humeston only “intermittently” took his prescribed Vioxx, for less than two months, and no link shows it was responsible for his heart attack.

One setback for Merck, however, came on Friday when New Jersey Judge Carol Higbee ruled the Humeston trial could be televised. This denied Merck's request arguing it could prejudice juries in 2,400 other New Jersey cases and damage its ability to defend future cases.

According to court documents, Court TV asked to televise the trial. Other networks could also ask to have cameras there.