We're finally going to get to put some numbers behind the obesity drug competition, with Arena Pharmaceuticals (NASDAQ:ARNA) and VIVUS (NASDAQ:VVUS) set to release second quarter earnings in the coming weeks.

A long time coming
VIVUS launched its obesity drug, Qsymia, back in September, but Arena -- despite having been approved first -- had to wait for the Drug Enforcement Agency to make a ruling about the potential abusiveness of its obesity drug, Belviq, before it could hit the pharmacy shelves.

It's a good thing they didn't launch at the same time because it wouldn't have been a fair fight since Qsymia had to be sold through mail-order pharmacies when it was launched. The FDA has since signed off on selling the drug though retail pharmacies, facilitating an increase in sales.

What to look for
Arena is scheduled to release earnings first on Thursday. The launch, facilitated by Arena's marketing partner Eisai, has slightly exceeded where Qsymia was at the same timepoint, but that's based on prescription data. It's hard to guess exactly how much Belviq Eisai is giving away through free samples to get patients to take the drug.

VIVUS hasn't scheduled a date for its earnings report, understandably since the company has been distracted by a proxy fight that resulted in a new CEO. When it does finally release earnings, don't expect any major changes. Qsymia became available in retail pharmacies in July, so there won't be any sales spike in the second quarter from the increased availability.

Instead, investors should keep a keen ear on what management says about how July progressed. Weekly prescription data makes it look like the increased availability is having a solid effect on the increased sales, but we'll have to see how that translated into higher sales.

Expectations in check
While both drugs have blockbuster potential, neither Belviq nor Qsymia is anywhere close to hitting the magic $1 billion level. VIVUS sold $4.1 million worth of Qsymia in the first quarter. Second-quarter sales will be higher, but even if they triple, it wouldn't be anything to get excited about.

Since Belviq didn't launch until June, there will only be one month's worth of sales in the second-quarter statement. Arena will have higher revenue this quarter and could even be profitable, but that's because of milestone payments that Eisai had to pay when the drug launched.

If you're going to buy Arena, VIVUS, or even Orexigen (NASDAQ:OREX), which hasn't even gotten its obesity drug Contrave approved yet, you've got to be in it for the long term. With market caps around $1.5 billion, there's room to go a lot higher if they can reach their full potential. And Orexigen is even cheaper, although you're taking on additional risk because it still has to pass its safety trial and get FDA approval.

But it's going to take more than a quarter, or even a year, for any of the drugs to get to blockbuster status. Investors should focus on the companies' plans for growth rather than paltry second-quarter sales.

Fool contributor 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.