When two drugs launch for the same indication, it's easy for investors to expect one to dominate and the other to fall into the "also ran" category. Vertex Pharmaceuticals' (NASDAQ:VRTX) Incevek was approved just weeks after Merck's (NYSE:MRK) Victrelis, with the former taking the fast majority of the hepatitis C market.

But for VIVUS' (NASDAQ:VVUS) Qsymia and Arena Pharmaceuticals (NASDAQ:ARNA) and Eisai's Belviq, it looks to me like there's room enough for two obesity drugs, especially since the two drugs have wildly different risk-benefit profiles. Qsymia offers more weight loss with more side effects, while Belviq has fewer side effects with less weight loss.

Watch and learn
On the surface, it looks like VIVUS has the upper hand, being able to launch potentially four to six months before Arena and Eisai, which are waiting for the DEA to sign off on their drug. In reality, based on sales so far, it doesn't look like VIVUS will be able to tap that much of the market before Belviq is launched, so the advantage of launching first is minimal.

In fact, you could argue that Arena and Eisai have an advantage because they get to see how the market reacts to Qsymia. Of course, VIVUS has some challenges that Arena and Eisai won't face, such as the potential for birth defects if the drug is taken by a pregnant women and that Qsymia is only available through mail-order pharmacies at the moment.

Take two, they're (not) cheap
Given the humongous potential market, it's hard to see how two obesity drugs can't coexist. While it's true that one drug would have the market all to itself, that's essentially impossible since the drugs have stopping rules that recommend they be terminated if they haven't led to weight loss after a set period of time.

For Belviq, the Food and Drug Administration recommends that doctors discontinue use if the patient hasn't lost 5% of their body weight after 12 weeks. Only 42% of patients in the clinical trials met that goal; it was even lower in a separate trial that just enrolled diabetics.

Qsymia's rules are a little less stringent because there are two potential doses that patients can be on. If they haven't lost 3% of their body weight after 12 weeks, it's recommended that they discontinue use or go on the higher dose. If the patient hasn't lost 5% of his or her body weight at the higher dose after 12 weeks, the FDA recommends that the drug be stopped. In the clinical trials for Qsymia, the 5% cutoff criteria was met by 80% of patients taking the high dose, albeit with the caveat that the patients didn't start on the low dose in the clinical trial.

What's going to happen to all those patients that fail to reach the appropriate level of weight loss with Belviq and Qsymia? My guess is many of them will try the other drug. These patients are looking for a quick fix. If one drug isn't going to get the job done, maybe another one will?

Getting covered
The limiting factor in patients potentially trying multiple drugs is the cost. VIVUS said 30% of patients prescribed Qsymia abandoned their prescription because they didn't want to pay $160 per month, the average retail cost for patients not covered by insurance.

For both Qsymia and Belviq to reach blockbuster status, it's pretty clear that they need to get onto the formularies of a large number of insurance companies and pharmacy benefit managers. Gaining tier 2 status, which usually comes with a $5 to $20 copay would be nice, but even getting tier 3 status with a copay that usually hovers around $50 would go a long way toward reducing the prescription abandonment rate.

While Arena and Eisai haven't been able to sell Belviq yet, there's nothing that's stopping them from having discussions with insurance companies about the potential to add the drug to the formulary once it's available for sale.

The companies working on getting their drugs covered by insurers at the same time could be beneficial to each other. Most insurers haven't covered obesity drugs because the drugs were seen as treating a cosmetic issue, although the companies are now trying to spout the benefits of weight loss on other comorbidities such as heart problems and diabetes. It's not hard to imagine that having two new drugs available might cause an insurance company to give more attention to whether it wants to change its policy on obesity drugs than if there were a single drug asking for coverage.

The same goes for doctors. Educating them about the benefits of weight loss -- no matter which sales rep does it -- should increase the use of obesity drugs. When Novo Nordisk (NYSE:NVO) launched Victoza, it took some sales from Byetta, but it also expanded the market for GLP-1 drugs. Bristol-Myers Squibb (NYSE:BMY) and AstraZeneca (NASDAQ:AZN) should benefit from that doctor education with Bydureon, the once-weekly version of Byetta.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.