VIVUS' (Nasdaq: VVUS) obesity drug Qnexa is down, but certainly not out.

After a Food and Drug Administration advisory panel recommended against approving the drug in July, investors got a bit of good news today when VIVUS presented long-term data for patients taking the drug.

Patients taking the highest dose of Qnexa shed 11.4% of their body weight after two years. The lower dose caused a 10.4% decrease while those taking placebo lost only 2.5% of their body weight. There was a little bit of a rebound in weight after greater initial losses -- at the maximum, the higher dose caused almost a 14% decrease -- but the weight loss after two years is still rather impressive.

Of course, the fact that Qnexa works has never been much of an issue. Potential side effects were the main reason the panel of outside experts didn't recommend approving the drug. One of the components of Qnexa is the active ingredient in Johnson & Johnson's (NYSE: JNJ) migraine and epilepsy drug, Topomax, which has some nasty side effects.

Nothing new cropped up in the two-year study, but keep in mind that this is a continuation study using patients that were in a one-year study of Qnexa. The patients that are willing to stay in the study an additional year are likely to be the ones that didn't have any problems in the first year anyway. Whether it'll be enough to convince the FDA remains to be seen.

Theoretically, VIVUS could submit the data to the FDA now, which would likely trigger an extension of the review process. But, since the decision is expected in about a month, I expect VIVUS will wait for a complete response letter -- the FDA's euphemism for rejection -- and see exactly what the agency has decided VIVUS needs to do to get Qnexa approved.

Of course, there's also a remote chance that the FDA goes against the advisory panel and approves the drug based on the currently submitted data. Not likely, but if you're going to buy a lottery ticket, it might as well be for the next drawing.

In case you hadn't figured this out already, investing in the obesity space is tough work. I thought Qnexa had a better chance at approval than Arena Pharmaceuticals' (Nasdaq: ARNA) lorcaserin and Orexigen Therapeutics' (Nasdaq: OREX) Contrave because it produced substantial weight loss, but an approval was far from a guarantee.

It's difficult to know exactly how much companies need to do to satisfy the lingering fear of side effects. We may have a better idea once the FDA makes decisions about Qnexa and lorcaserin next month. Maybe.

Interested in keeping track of VIVUS as its FDA decision date approaches? Click here to add it to My Watchlist, which will help you keep track of all our Foolish analysis on VIVUS.

Motley Fool Options has recommended a diagonal call position on Johnson & Johnson, which is a Motley Fool Income Investor pick. Try any of our Foolish newsletter services free for 30 days.

True to its name, The Motley Fool is made up of a motley assortment of writers and analysts, each with a unique perspective; sometimes we agree, sometimes we disagree, but we all believe in the power of learning from each other through our Foolish community.

Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. The Fool owns shares of Johnson & Johnson and has a disclosure policy.