After a long wait, patients now have a choice between two new obesity drugs. They couldn't be more different.

Arena Pharmaceuticals' (NASDAQ:ARNA) Belviq, the newest offering, looks relatively safe. The top adverse reactions were headaches, followed by dizziness and fatigue -- nothing to be too worried about. The downside is that patients didn't lose all that much weight while taking Belviq compared to the placebo group. After one year, in two combined phase 3 studies, patients lost a placebo-adjusted 3.3% of their body weight. Belviq, which will be marketed by Arena's partner Eisai, should hit pharmacy shelves beginning on Tuesday.

Qsymia, sold by VIVUS (NASDAQ:VVUS), has been on the market since September. It produces substantially more weight loss: at the highest dose, 9.4% placebo adjusted in one study, and 8.6% placebo adjusted in another. Unfortunately, that weight loss comes with additional risks, including the potential to cause birth defects if taken by women who become pregnant.

Which one will end up winning?
I see a place for both drugs. We could see patients starting on Belviq, and then moving to Qsymia if they don't see satisfactory results. Morbidly obese patients might start directly on Qsymia, because they desperately need to lose a large amount of weight. Piddling around with 3% isn't going to cut it.

While the companies are clearly competing against each other, having another drug on the market may actually benefit VIVUS, because there will be more sales reps beating the "take drugs for weight loss" drum. Despite the large number of patients, few get drugs as their treatment. Increasing their use should expand the overall market.

Conversely, having VIVUS go first likely helped Arena and Eisai. Getting insurers to cover the drugs is the key to sales. VIVUS saw an abandonment rate of 30% when it first launched, likely because of sticker shock from those not covered by insurance. Qsymia, having already gone through the insurers' review process for coverage, likely made it easier for Eisai to gain reimbursement for Belviq.

The third player, Orexigen (NASDAQ:OREX), could end up finding a fit, too; its obesity drug Contrave appears to have a benefit and side effect profile somewhere in between Qsymia and Belviq. The company is still testing Contrave to confirm it doesn't cause heart problems, so it probably won't be on the market until next year at the earliest. With the extra safety data, Contrave might actually take the lead as the go-to drug.

And Orexigen will have Eisai, Arena, and VIVUS to thank for setting the stage.

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.