Shares in US drug maker Merck tumbled nearly 10% on Monday after a press report suggesting the firm may have ignored problems with one of its drugs.
Merck withdrew Vioxx, a top-selling painkiller, on 30 September after evidence linking it to heart problems.
The Wall Street Journal has now published e-mails which, if genuine, suggest Merck may have known about the concerns for years, the paper says.
Merck shares closed down 9.7% on fears of patient lawsuits.
Vioxx is thought to have been prescribed to 20 million patients in the US since its launch in 1999. It is used to help patients cope with arthritis pain.
About two million people worldwide were using it when it was recalled in September. Vioxx sales were worth £2.5bn (£1.3bn) to Merck in 2003.
Merck already faces patient lawsuits, filed after its decision to recall the drug in September. Investors now fear that the picture could be grimmer and the potential costs higher.
The Wall Street Journal quoted internal company e-mails, including one from Merck's head of research Edward Scolnick in March 2000 to colleagues saying that cardiovascular problems with Vioxx "are clearly there".
"If the e-mails actually exist and say what they are purported to say, they appear at least superficially to be a smoking gun that lawyers could pull up as evidence against Merck," said drug industry analyst Trevor Polischuk of Orbimed Advisors.
The result is likely to be more litigation, though he believes it may be difficult for Vioxx users to prove the drug harmed them given that many will have had pre-existing heart problems.
Credit rating agency Standard and Poor's said it may downgrade Merck's credit rating and is seeking a meeting with the firm's management to discuss Merck's "long-term ability to retain its credit strength".
The agency said its actions "reflect Standard and Poor's increasing concern about the magnitude of possible litigation" over Vioxx.