The biggest concern expressed by the experts on Thursday's FDA panel was the risk that long-term use of lorcaserin could cause heart-valve damage, or valvulopathy. Arena presented data Thursday showing, mechanistically, lorcaserin doesn't hit receptors that cause heart valve problems; and FDA concurred. Experts on the panel were less convinced, particularly since the clinical data collected to date on lorcaserin could not rule out an increased risk of valvulopathy. Importantly, none of the panel experts, including the cardiologists, demanded that Arena be required to rule out a lorcaserin-valvulopathy link prior to the drug being approved. Dr. Bill Hiatt, a cardiologist from the University of Colorado School of Medicine, was notable during panel deliberations advocating strongly for the conduct of a post approval cardiovascular outcomes trial of lorcaserin. Hiatt's argument seemed to blunt a more cautious stance taken by Dr. Sanjay Kaul of Cedar-Sinai Heart Institute, who voted against lorcaserin although eventually conceding, too, that a post-approval heart safety study was appropriate. Setting parameters for a lorcaserin post-approval safety study could be another item that delays the drug's approval. Vivus will be running a similar study. So too, might negotiations over whether or not patients treated with lorcaserin should receive echocardiograms in order to more closely monitor any risk of heart valve damage. Many of the panel's experts supported echocardiograms for patients, some more frequently than others. Echocardiograms, if required, do have the potential to be an impediment to lorcaserin's commercial uptake. Arena did an excellent job Thursday presenting data showing that tumors in rats, particularly mammary tumors, caused by lorcaserin were not relevant to humans and therefore caused no potential safety concern once the drug is approved. The FDA largely agreed with Arena's conclusions. Panel members were somewhat less convinced, although again, not enough to warrant a negative vote that would keep lorcaserin from being approved. This was the biggest reversal from the September 2010 FDA advisory panel in which the lorcaserin-rat tumor concerns were chiefly responsible for the vote against approval. It is interesting to note, however, that FDA invited four panel members with toxicology and animal pathology expertise to review lorcaserin this time around. Two of these experts voted against lorcaserin on Thursday and a third, Dr. Ernest McConnell, the veterinary pathologist, was the lone abstention. Arena shares have performed very well since Thursday's positive panel vote, but keep in mind that with expectations for lorcaserin's approval baked in and investors thinking about the drug's peak sales potential, the company's market valuation starts to matter. In this regard, note that Arena has approximately 220 million shares outstanding when you include warrants from completed financings that are currently in the money. At Friday's close of $6.36, Arena's market cap was north of $1.4 billion already. Remember, too, that Arena shares lorcaserin sales and profits with Eisai. Vivus owns Qnexa outright and as of Friday, sported a market value of $2.5 billion. Assuming FDA approves both lorcaserin and Qnexa within weeks or months of each other, it will be fun to watch which drug doctors turn to first and which garners higher sales. Each drug has its advantages and disadvantages -- lorcaserin's efficacy is marginal but may be more tolerable and easier for doctors to prescribe. Qnexa helps patients lose more weight but side effects and restricted access to drug could be an impediment. Two things we sort of know: Obese patients have a hard time remaining compliant with medications and weight lost is easily regained. That likely means whichever drug is tried first -- lorcaserin or Qnexa -- the other is likely to be used in quick succession once the first stops working. You've heard of yo-yo diets, well, the approval of two weight-loss drugs will likely lead to yo-yo prescriptions. One more thing we know: No obesity drug has ever generated more than $1 billion in annual sales. It remains to be seen if Arena or Vivus can break that mark. --Written by Adam Feuerstein in Boston. >To contact the writer of this article, click here: Adam Feuerstein. >To follow the writer on Twitter, go to http://twitter.com/adamfeuerstein. >To submit a news tip, send an email to: firstname.lastname@example.org. Follow TheStreet on Twitter and become a fan on Facebook.