(Arena Pharmaceuticals article updated with FDA advisory panel decision and news that the stock was down more than 40% after hours.)
NEW YORK (TheStreet) -- Arena Pharmaceuticals (ARNA - Get Report) shares plunged in after-hours trading after an FDA panel rejected its weight-loss drug lorcaserin.

Shares fell 41%, or $1.53 to $2.31 after they reopened for trading at 5:05 p.m. EDT. The stock had been halted during the panel hearing.

Arena appeared to handle its presentation of data on weight-loss drug lorcaserin to an advisory committee of the U.S. Food and Drug Administration with ease -- but the panel voted no on the drug.

"Seems like the 'no' votes won out because of unanswered concerns about the risk and middling efficacy -- in many ways, today's lorc vote mirrored the vivus qnexa vote," the TheStreet's biotech reporter Adam Feuerstein said in his live blog of the event.
Arena

"These panels are setting a very high bar for obesity drugs," Feuerstein said.

When the session began, company representatives simply went over the data, without exhibiting any defensive behavior, according to Feuerstein. "ARNA is doing a very good job at presenting the ARNA analysis of the data," Feuerstein said this morning, during an ongoing live blog of the event. But rival Vivus ( VVUS ) also did a good job. What matters is how the panel reacts," he said at the time.

On Thursday, Arena Pharmaceuticals brought its weight-loss drug lorcaserin to an FDA advisory committee to attempt what nemesis Vivus wasn't able to accomplish in July: persuade a panel of outside experts that it has a weight-loss drug that is both safe to use and works.

The outside experts have expressed doubt about the drug, citing possible cancer risk in animals and only modest weight loss for patients.

During the live blog, Feuerstein asked TheStreet users what their current position was on Arena stock and options. Roughly 65% said they were net long, while 23% said they were neutral or not involved and 12% said they were net short. On Wednesday, Arena stock settled at $3.74, down 45% over the past two days.

During the company's presentation to the FDA panel Thursday, Arena representative Dr. Gary Williams assured outside experts that lorcaserin doesn't pose a cancer risk to humans at recommended doses. The company also assured the panel that the drug has no negative effect on heart valve safety and did not increase depression severity scores.

-- Written by Andrea Tse in New York.

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