Updated from 1:27 p.m. ET

A

Food and Drug Administration

review of

Merck's

(MRK) - Get Report

blockbuster arthritis drug recommends warning Vioxx users about possible cardiovascular side-effects.

A briefing document posted Wednesday on the agency's

Web site recommends warning Vioxx users about "cardiovascular thrombotic events," or blood clots. Notably, the document doesn't recommend removing from Vioxx labels a warning that Vioxx could cause gut problems, and says additional studies may be necessary to clarify outstanding questions. The findings appear to set back the company's efforts to make safety claims that could markedly expand the drug's already significant market.

The document will be considered Wednesday and Thursday, along with remarks from company officials, by a panel of outside experts who will recommend to the FDA whether to change the label for the bestselling drug. Advisory panel recommendations aren't binding, though the agency tends to follow their advice.

Merck said it was confident that the data, which will be presented tomorrow, support "the excellent GI (gastrointestinal) and overall safety profile of Vioxx." The company also said that "the real issue is how and where that information is going to be included in the labeling."

"Once the advisory panel is done, there will be many discussions between us and the FDA discussing how the labeling will be modified," a Merck spokeswoman said.

Merck and

Pharmacia

(PHA)

, maker of the competing Celebrex arthritis treatment (and co-marketer of Celebrex with

Pfizer

(PFE) - Get Report

), contend that Vioxx and Celebrex are safer than over-the-counter painkillers such as Aleve and Advil. The companies have been arguing that these prescription drugs, which belong to a class known as Cox-2 inhibitors, don't cause the ulcerlike side effects common with the nonprescription painkillers, which are also known as nonsteroidal anti-inflammatory drugs. If the agency accepts their argument, it could allow them to advertise Vioxx and Celebrex as safer than the so-called NSAIDs.

If the agency permits the companies to make the expanded safety claims, the market for Cox-2 drugs could expand significantly beyond the $5 billion or so a year that analysts already project Vioxx and Celebrex together will bring in. With many other blockbuster drugs on the verge of losing patent protection, increasing the sales of current blockbuster drugs has been among the drug companies' top priorities.

"Whatever change (in the Vioxx label) will not create market upside unless the FDA gives them a clear label indicating a lower risk of ulcer," said

Sanford Bernstein

analyst Richard Evans. "And I don't think they're going to get a license to put that message out."

An FDA advisory panel also indicated today that studies don't show competing drug Celebrex to be any safer than traditional painkillers. The panel found Celebrex had "no clinically meaningful safety advantage." Yesterday, the panel warned Pharmacia that its promotional activities for Celebrex didn't explain the "serious risk" of bleeding when the product is used with Coumadin, a clot-busting drug.

In a report issued this afternoon,

Deutsche Banc Alex. Brown

analyst Barbara Ryan said a favorable label change for Vioxx was unlikely and indicated that the additional cardiovascular warning is a "risk" for the company. Ryan maintained her buy ratings on both Merck and Pharmacia, and said the Cox-2s "will expand their share of the arthritis market, with or without a label change in the near term."

However, Ryan added that "the market will likely be skeptical of the companies abilities to meet their growth targets for these drugs" if they fail to win FDA approval to change the labels.

Merck closed down $2.51, or 3%, at $81.85, while Pharmacia lost $1.52, or 2.6%, to $56.13. Pfizer fell $1.35, or 2.9%, to $44.50 during the trading session.