NEW YORK — Merck researchers sought to reformulate Vioxx in 2000 to reduce its cardiovascular side effects, even as the drug maker was publicly playing down a study that highlighted the pain reliever's potential risk for heart attack and stroke, an internal company document shows.

The study in March 2000 found that patients taking Vioxx were five times more likely to have heart attacks than individuals using the generic medicine naproxen. Merck insisted at the time that this was a result of naproxen's cardioprotective properties and not any defect in Vioxx.

Behind the scenes, company scientists were considering combining Vioxx with another agent to reduce the risk of heart attacks and strokes, according to a document mistakenly provided by Merck to plaintiff lawyers during evidence-gathering in one of the hundreds of Vioxx lawsuits.

That document, a communication between Merck researchers and the company's patent department, stated that the way Vioxx works to reduce pain might also increase cardiovascular problems.

The researchers suggested a patent be sought for a combination drug mixing Vioxx with another agent to lessen the risk.

Merck removed Vioxx from the market voluntarily in September after a later study showed it doubled patients' risk of heart attacks and strokes. Thousands of wrongful-death and injury lawsuits have been filed based on claims that Merck hid Vioxx's risks. Analysts estimate the company's liability could reach $18 billion.

Merck attorneys said the patent application proves only that the company sought to help Vioxx users who had switched from aspirin, which is known to help prevent heart attacks by acting as a blood thinner.

The document is potentially damaging because it calls into question the Merck defense that company officials were convinced of the drug's safety.

Material from The Philadelphia Inquirer is included in this report.