) -- The number of prescriptions for

Arena Pharmaceuticals'

(ARNA) - Get Report

weight-loss pill Belviq rose either 12% or 16% for the week ended July 12 -- the difference depends on which of the two major drug prescriptions data services (IMS Health or Symphony) you follow.

Either way, Belviq's launch is slow out of gate, says an institutional investor shorting shares of Arena. Here's his take:

While early, the prescription trends for Belviq -- launched without a restrictive risk management (REMS) plan -- are terrible.Qsymia had a significant REMS, was first to market, couldn't do any sampling, and as is obvious to all, wasn't competent in their launch even given the challenges.On the flip side, Arena and Eisai have had a year to sit back, watch, strategize, and so far, Belviq is a flop to say the least. Prescription data to date suggest Belviq is on the same initial slope as Qsymia when it launched (albeit left by a couple weeks and just slightly higher in total prescriptions.) But lost by many is the fact that relative to the apparent increase in prescriptions when compared to Qsymia, nearly 60 percent of Belviq's total prescriptions (TRx) for the last two weeks are for free drug (15-day trial.)Vivus (VVUS) - Get Report is now in the hands of a much more competent management team and new independent directors like Alex Denner. A less restrictive REMS is already boosting prescriptions. Orexigen Therapeutics (OREX) is likely coming in the near term. For all these reasons, Belviq is doomed here, as it should be, given its weak profile in comparison to the others weight-loss drugs.Ultimately, I believe Orexigen will take the case with ease.

-- Reported by Adam Feuerstein in Boston.

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Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback;

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