MOUNTAIN VIEW ( TheStreet) - Vivus ( VVUS) announced the commercial launch of its weight-loss pill Qsymia Monday, beating rival Arena Pharmaceuticals ( ARNA) to first reach the bulging waistbands of obese Americans. Qsymia hits (mail order) pharmacy shelves at an opportune time, coming just as doctors who treat obesity are gathering in San Antonio later this week for the Obesity Society annual meeting. Both Vivus and Arena are expected to have a presence at the meeting but only Vivus will have a new weight-loss pill available for doctors to prescribe once they return home to their patients. Arena and its marketing partner Eisai are expected to launch Belviq early next year. Vivus previously announced pricing for Qsymia -- ranging from $4-6 per day depending on the dosage strength -- that was in line with investor expectations. Qsymia combines two drugs, phentermine and topiramate, that are available as generics for a combined price of around $1-3 per day. Will doctors choose to prescribe Qsymia to their obese patients or recommend the individual, generic components? Investors have been debating the question leading up to the drug's launch. "Feedback from doctors indicates that generic substitution is unlikely to be widespread given the associated liability of either prescribing or dispensing the generic components separately so long as the product Qsymia is not made too cost prohibitive," said J.P. Morgan biotech analyst Cory Kasimov in a recent research report. But in August, Jefferies analyst Thomas Wei downgraded Vivus, citing concerns that doctors will choose to prescribe the generic components of Qsymia instead of the branded (and higher priced) product. With Qsymia now available in the U.S., investors and analysts will be watching daily and weekly prescription sales tracked by independent services like IMS Health and Wolters Kluwer. Vivus shares were unchanged at $22.08 in early Monday trading. Arena shares were up 6 cents to $8.41. Both stocks have fallen since their respective weight-loss drugs were approved earlier this summer, reflecting investor uncertainty about the commercial potential of obesity drugs. --Written by Adam Feuerstein in Boston.