The U.S. Food and Drug Administration is expected to rule on whether to approve the drug by Wednesday.

Public Citizen, a nonprofit consumer advocacy group, said physicians at last month's FDA advisory panel meeting on lorcaserin raised serious concerns about evidence of increased valve disease risks in patients who used the drug in clinical trials. The panel of independent experts voted 18 to 4 to recommend approval of the pill despite the concerns.

If approved, lorcaserin would become the first new diet drug on the U.S. market in more than a decade.

Heart valve problems were among the issues that led FDA to pull the anti-obesity drug known as fen-phen from the market in 1997, Public Citizen said.

The group also noted that FDA's panelists were concerned about the modest weight loss associated with lorcaserin.

A spokesman for Arena declined to comment on Public Citizen's concerns and said the company is focused on preparing for the FDA decision.

An FDA spokeswoman declined to comment and said the agency would respond directly to Public Citizen.

Arena's pill is one of three new potential obesity treatments vying to gain U.S. approval. Vivus Inc and Orexigen Therapeutics Inc are also trying to get pills approved.

Vivus in February won backing for its pill Qnexa from the same advisory panel that voted in favor of lorcaserin. The FDA pushed back its final decision on Qnexa to July 17 to review the company's risk evaluation plan for the drug.

The FDA first rejected lorcaserin, which is being developed in partnership with Japanese drugmaker Eisai Co Ltd, in October 2010, citing potential cancer risks. Arena resubmitted its application with more data to show that the previous findings of tumors in rats did not apply to people.

At the May advisory meeting, most panel members said side effects from lorcaserin could be addressed in post-approval studies and suggested patients who take the drug should get regular echocardiograms.

The panelists said overweight and obese patients needed more options.

Arena's lorcaserin was designed to block appetite signals in the brain in a similar way to fen-phen. Another diet pill, Meridia, was withdrawn in 2010 after also being linked to heart problems.

Arena's stock plunged 15 percent last Friday in what some traders attributed to profit-taking ahead of the FDA's decision. At one point during the day it had shed a third of its value. The stock has appreciated more than 400 percent this year in anticipation of a favorable ruling.

Arena shares fell 3.8 percent on Tuesday to close at $8.85 on Nasdaq.

(Reporting by Susan Kelly in Chicago; Editing by Matthew Lewis, Richard Chang and Tim Dobbyn)

• A Permanent Economic Emergency