Intermittent use of Vioxx (search) or even a day's use of the painkiller could be enough to cause a heart attack, a prominent heart and medication expert testified Monday on behalf of a man who is suing the maker of the drug, claiming it caused his heart attack.

Vioxx breaks down so slowly in the body that it takes about 85 hours to clear out of the blood, testified Dr. Benedict Lucchesi, a professor at the University of Michigan who helped develop the first pacemaker.

"Based on the science, there's every reason to believe that a single dose, multiple doses, whatever, can lead to an adverse event," such as a heart attack or stroke, Lucchesi said.

That could be a key point in 60-year-old Idaho postal worker Frederick "Mike" Humeston's case against Vioxx maker Merck & Co. (MRK ).

Vioxx, launched in May 1999, was pulled from the market in September 2004 by Whitehouse Station-based Merck after its own research showed the blockbuster arthritis drug doubled the risk of heart attack and stroke after 18 months' use.

Humeston had a heart attack in 2001 after taking Vioxx for only two months and skipping some doses.

The company contends that Humeston's condition and sedentary lifestyle led to his heart attack, and that Vioxx was not the cause.

Lucchesi also told jurors that repeated evidence and warning signs linked heart risks to the painkiller Vioxx before and shortly after its launch. Lucchesi said after reviewing correspondence among Merck executives and scientists that he was surprised the company didn't put more emphasis on those risks.

"This is bad news, obviously," he said, adding he didn't know how any scientists "could not know the potential for harm."

Lucchesi also testified that after a 2000 study showed significant heart risks to Vioxx users, Merck told doctors and the public that Vioxx was safe. Lucchesi said that instead there was "an explosion" of reports by Merck executives and consultants saying that the other drug in that study, Naproxen, prevented heart attacks, not that Vioxx caused them.

Plaintiff attorneys and some doctors have argued Merck knew the risks of Vioxx at least a few years before it took the drug off the market, yet downplayed its dangers and kept aggressively promoting the drug. Attorneys for Merck say the company acted responsibly, putting patients first and pulling the drug once the risks surfaced.

Humeston is the second person to have a lawsuit over Vioxx reach court.

Merck faces more than 5,000 lawsuits filed in state and federal courts — about half of them in New Jersey state courts — by former Vioxx users alleging the medicine harmed them.

The first trial ended last month when the jury in the Texas case stunned Merck with a $253 million liability verdict although that will be slashed to about $26 million because Texas caps punitive damages. Merck plans to appeal. Lucchesi also testified against Merck in that trial.

Lucchesi said Monday that he felt funny testifying against Merck after having a positive relationship with its scientists for years; the company even helped pay for training of some of his graduate students.

"Merck has been extremely good to me over the years," Lucchesi said.

On cross-examination, Merck attorney Stephen Rayber repeatedly tried to undermine Lucchesi's credibility, pointing out inconsistencies between his testimony Monday and prior statements. Lucchesi agreed when Rayber asked if ibuprofen and Tylenol also carried risk. He also conceded that Vioxx is safe for most people — but not those who have heart or kidney problems or are pregnant.

But Lucchesi refused to give ground otherwise, at one point correcting Rayber when he referred to a study as lasting three months.

"Twelve weeks," Lucchesi responded.

Later, he told Rayber: "You're not going to trap me."

At times, the jury appeared to struggle to follow brain-numbing details of the workings of Vioxx and the cardiovascular system, with expressions from puzzlement to weariness playing across their faces late in the day. But plaintiffs' lawyers packed the back rows to pick up pointers for future Vioxx trials.

On the New York Stock Exchange on Monday, Merck shares closed at $28.65, down 25 cents from Friday, but still more than $2 above where the stock bottomed out last November.