Merck, one of the world's top five drug makers, pulled the arthritis and acute pain drug from the market worldwide Sept. 30, saying it was acting in patients' best interest. Vioxx has been taken by about 20 million Americans and had produced 11 percent of Merck's total revenues.

Also Monday, Prudential Equity Group analyst Tim Anderson downgraded the stock's rating to "neutral" from "overweight." Anderson wrote that he believes there is value in the company's drug pipeline, but said it is likely to get lost in the negative Vioxx coverage.

Additionally, Standard & Poor's placed Merck on CreditWatch with negative implications, which sends a strong signal that the company's debt could be downgraded within the next three months. When Merck withdrew Vioxx, S&P lowered its outlook to "negative" from "stable." The additional, more serious step taken Monday reflects the momentum in the Vioxx litigation and the delay in launching Arcoxia, Vioxx's successor drugs, S&P analyst Arthur Wong said.

Hundreds of lawsuits have been filed against Merck over Vioxx, and one analyst thinks that it could cost the company up to $12 billion. Friday, the FDA said it wouldn't approve Arcoxia without additional safety and efficacy information. Wong said it looks like the drug will be delayed more than a year.

Merck shares were down $3.03, or 9.7 percent, to close at $28.28 on the New York Stock Exchange after the Wall Street Journal reported that internal e-mails and marketing materials show that the company knew as far back as 2000 that Vioxx was linked to an increased risk of heart attack but tried to discredit such evidence.

The news led to Merck posting the largest percentage decline Monday among stocks in the S&P 500 Index.

Despite a March 9, 2000, e-mail from Merck research director Edward Scolnick to colleagues conceding an elevated risk of heart attack and stroke was "clearly there," according to the newspaper, Merck continued to try to discredit academic researchers critical of the drug.