Most biotech investor would be happy with 30% quarter-over-quarter prescription growth in a market that didn't grow much. But for Arena Pharmaceuticals' (NASDAQ:ARNA) Belviq, the growth is coming off such a small base that it's hard to get too excited.

Yet.

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Arena Pharmaceuticals' marketing partner Eisai recorded net sales of just $16.8 million in the third quarter. The sales garnered Arena Pharmaceuticals $5.7 million. That's not enough to pay the bills. Even with some other revenue, Arena Pharmaceuticals lost $10.7 million in the third quarter.

Monotherapy might not be the blockbuster answer
If Eisai could continue 30% quarter-over-quarter growth, Belviq would become a blockbuster sometime in 2017. But only time will tell if the salespeople that Eisai added and its direct-to-consumer advertisements can keep sales growth at this rate. Eisai doesn't break out costs for selling Belviq, but it's safe to say it's currently losing money selling the product for Arena Pharmaceuticals with plans to make up for it as sales increase.

The drug might get used more if it caused more weight loss than VIVUS's (NASDAQ:VVUS) Qsymia and Orexigen (NASDAQ:OREX) and Takeda's Contrave. The simple solution is to combine Belviq with phentermine, a generic drug that produces modest weight loss on its own.

Data from a 12-week phase 2 trial, which will be presented at this week's meeting of The Obesity Society and the American Society for Metabolic and Bariatric Surgery, showed that 84.2% of patients who completed the trial who took phentermine twice a day and Belviq lost at least 5% of their body weight. That compared to 68.2% of patients taking Belviq plus once-daily phentermine, and just 33.3% of patients who only took Belviq.

The results look promising, but keep in mind that these are only results from patients who completed the trial, so the overall results will be lower. The other issue with this trial is that there wasn't a phentermine-only group, so it's hard to know how much Belviq is helping to shed the pounds.

Even if the two drugs are only additive, doctors will probably use the drugs in combination, but only if they're proven safe, which can only happen after Eisai and Arena Pharmaceuticals run a longer trial with the combination.

It might not even be obesity
A potentially easier way for Eisai and Arena Pharmaceuticals to substantially increase sales of Belviq is to get it approved as a treatment to help people stop smoking. It reasons that if the drug can make people feel sedate so they don't need to gain pleasure through eating, it can also reduce the need to gain pleasure from smoking.

Data from a phase 2 clinical trial released today show that 15.3% of patients taking Belviq twice a day had stopped smoking during the last four weeks of the 12-week trial, compared to just 5.6% of patients taking placebo. The trial also included patients who took Belviq just once a day with an 8.7% quitting rate for those patients.

The twice-daily dose was statistically different than the placebo, and while the once-daily dose wasn't statically significant, the intermediate result gives you confidence that the drug is what's causing the increased quitting rate.

While the results were statistically significant, they're a little disappointing when compared to the currently available drugs. In clinical trials with Pfizer's (NYSE:PFE) Chantix, 40% to 51% of patients were able to quit smoking for the last four weeks of the 12 week clinical trials. Two of Pfizer's trials compared Chantix to GlaxoSmithKline's (NYSE:GSK) Zyban, which helped 30% of patients quit smoking in each trial.

It's not quite an apples-to-apples comparison because the placebo groups in Pfizer's trials also performed better, with quitting rates of 12% to 18%. More intense counseling might help increase the placebo rate and the treatment rates, while hopefully keeping the same ratio between the two groups.

The other possibility is that the lower placebo response is due to a decrease in the level of commitment of people who say they want to quit smoking -- that is, many of the people who really wanted to quit already have. Arena Pharmaceuticals and Eisai could solve that problem simply by comparing Belviq to Chantix and Zyban in their next clinical trial. If the hypothesis is true, the older drugs should produce lower quitting rates than they did a decade ago.

Even if Belviq can't get to the same quitting rates as Chantix and Zyban, it might be able to find a niche since both of the approved drugs have black box warnings about potential neuropsychiatric events, including suicidal thoughts and behaviors. If it's deemed safer, doctors might be willing to try Belviq first, even if the response rate was lower.

As an added benefit, the drug might also decrease the weight gain seen in patients who stop smoking. There was some evidence of that happening in this trial, although over just 12 weeks of treatment, the effect on weight was small.

Brian Orelli and The Motley Fool have 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.