Welcome to the club, Arena Pharmaceuticals (NASDAQ:ARNA). You've got a lot of company from drugmakers such as sanofi-aventis (NYSE:SNY), Pfizer (NYSE:PFE), and Merck (NYSE:MRK), whose obesity drugs haven't lived up to their hope.

Arena's lorcaserin wasn't a complete failure; it actually met all three of its primary endpoints, so the drug is certainly doing something. But it doesn't look like it's doing enough to be a marketing success -- and the results might not even be enough to get it past the Food and Drug Administration.

After a year on lorcaserin, patients lost an average of 5.8% of their body weight compared with a 2.2% loss by patients on placebo. While the difference is statistically significant, that difference of 3.6 percentage points in placebo-adjusted weight loss isn't going to cut it. The FDA usually likes to see the difference between the two at greater than five.

Even if lorcaserin does get past the FDA, the weight loss probably isn't enough to get the drug to blockbuster status. Abbott Labs' (NYSE:ABT) Meridia and GlaxoSmithKline's (NYSE:GSK) alli have a lackluster effect on the waistline, which has led to lackluster performance in the revenue department.

The only good news for Arena was that there weren't any major problems with side effects. The obesity treatment area is full of drugs that were pulled from the market or never made it past (or to) the FDA in the first place, like Wyeth's (NYSE:WYE) Fen-Phen and sanofi-aventis' Acomplia.

Arena has two more phase 3 trials in progress. Its best hope is that the drug's performance in those trials is great enough that the FDA and doctors can look past the uninspiring results from this trial. That's a lot to hope for, and at a $250 million market cap as I write, the risk-reward trade-off doesn't look as appetizing as it did a few days ago.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. GlaxoSmithKline is an Income Investor selection. Pfizer is a Inside Value pick. The Fool's disclosure policy is as thin as a piece of paper.