Although we don't believe in timing the market or panicking over market movements, we do like to keep an eye on big changes -- just in case they're material to our investing thesis.

What: Shares of fat-busting drug company VIVUS charged higher by as much as 14% after two research reports indicated that sales of Qsymia improved in November.

So what: Now before you go clicking the buy button, keep in mind that the figures from the two research companies were far from similar. Health-care research firm Symphony Health Solutions pointed to a 44% increase in Qsymia prescriptions for the week ending Nov. 12. On the other hand, IMS Health noted just a 15% increase during the same period. Who's right? It's anyone's guess at the moment -- but both at least point to a double-digit increase in Qsymia sales.


Now what: The conflicting data doesn't exactly inspire confidence that these figures can be trusted, but I would have to assume that with Aetna now on board and covering Qsymia -- and with other insurers likely to join it -- that sales are going to jump by double digits for quite some time. Out-of-pocket costs to the patient are the prime deterrent for Qsymia, and eliminating those will go a long way to boosting sales. It's still going to be a long road, however, as Qsymia failed to gain approve in Europe, and the door is now wide open for Arena Pharmaceuticals , which has a licensing partnership with Eisai to sell its fat-busting pill, Belviq, to step in and fill the international void.

The potential market for obesity drugs is massive, but so are the risks. If you're looking for more information on the top two obesity drug players, grab copies of our premium research reports on Arena Pharmaceuticals and VIVUS today. In the reports, our senior biotech analyst, Brian Orelli, Ph.D., breaks down each company's strengths and weaknesses, and explains the critical issues you need to know about. News in this space moves fast, so both reports come with a full year of updates. Click now for exclusive information on Arena and VIVUS.

The article Why VIVUS' Shares Grew Fatter originally appeared on Fool.com.

Fool contributor Sean Williams has no material interest in any companies mentioned in this article. You can follow him on CAPS under the screen name TMFUltraLong, track every pick he makes under the screen name TrackUltraLong, and check him out on Twitter, where he goes by the handle @TMFUltraLong. Try any of our Foolish newsletter services free for 30 days. We Fools don't all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.

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