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What: Arena Pharmaceuticals (NASDAQ:ARNA) was down as much as 12% today after its marketing partner Eisai released financial results for the first half of its fiscal year.

So what: Sales of the duo's obesity drug, Belviq, were up 95% year over year for the six months that ended in September compared to the same period in 2014. Unfortunately that's only 2.6 billion yen, which amounts to about $22 million at today's exchange rates. Annualize that out, and it isn't pretty.

Eisai has taken a cost-effective marketing strategy for the obesity drug with its costs to sell the drug controlled to approximately 40% year over year. That doesn't help Arena Pharmaceuticals directly though since the biotech collects royalty and manufacturing costs for the drug rather than sharing earnings.

Beyond the sales number, Eisai mentioned that it's preparing to launch Belviq in Brazil and Mexico and plans to submit the once-daily formulation to the Food and Drug Administration this fiscal year, which ends in March. Both may result in some incremental gain, but they won't suddenly make Belviq a blockbuster.

The companies' large 12,000 patient cardiovascular outcomes study will complete enrollment this calendar year. Having data that shows Belviq doesn't harm people's hearts -- or, even better, that it decreases the likelihood of heart attacks and strokes -- would go a long way toward competing with Orexigen (NASDAQ:OREX) and Takeda Pharmaceuticals' Contrave, which already has data showing the drug doesn't increase heart attacks and strokes. Takeda also reported earnings today, letting Orexigen's investors know that it sold $29 million worth of Contrave in the first half of its fiscal year. That's only slightly better than Eisai managed with Belviq, but keep in mind that Orexigen and Takeda got their drug approved a few years after Belviq was approved.

Now what: Investors should be focused on Arena's pipeline rather than Belviq, which may never amount to anything other than a niche drug.

On Tuesday, Arena Pharmaceuticals said it'll reduce its head count by 35% to help save money that can be used to develop its pipeline drugs. Yesterday it moved ahead, pushing its pain drug APD371into a phase 1b trial to determine the best dose to take into a proof-of-concept study.

With its most advanced pipeline drug in phase 2, Arena Pharmaceuticals is a long-term story. If you're going to own the stock now, it doesn't really make sense to worry about daily movements on sales of an approved drug that's hardly putting a dent in its cash burn.

Brian Orelli has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not 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.