In drug development, making a good drug is only half the battle; the drug also has to have side effects appropriate for the disease it's treating.

It's that last part -- the disease it's treating -- that's crucial. The side effects from Johnson & Johnson's (NYSE: JNJ) epilepsy and migraine drug Topamax are acceptable, but the same active ingredient looks like it might be enough to sink VIVUS' (Nasdaq: VVUS) obesity drug Qnexa.

Valeant Pharmaceuticals' (NYSE: VRX) and GlaxoSmithKline's (NYSE: GSK) epilepsy drug Potiga has a bad side effect, but I think it'll get past its Food and Drug Administration advisory panel tomorrow because the drug treats epilepsy. Specifically, it's being proposed as a treatment for patients with few other options because their epilepsy isn't being controlled with other medications such as Johnson & Johnson's Topamax, Pfizer's (NYSE: PFE) Lyrica, Abbott Labs' (NYSE: ABT) Depakene, and Glaxo's own Lamictal.

Given the strong efficacy, most of the only discussion is likely to revolve around whether the side effects swamp out the good the drug is doing. The biggest issue here is that the drug causes urinary retention, essentially the inability to urinate. For the most part, the issue was reversible, and given the symptoms of epilepsy that aren't under control in this patient population, I think it's likely that the panel of outside experts will mostly shrug off the side effects.

That's not to say Potiga will be a blockbuster; the side effects likely will keep Potiga from ever becoming a first-line treatment -- remember that "disease it's treating" part. But considering the lack of other options, Glaxo and Biovail (NYSE: BVF), which is in the process of buying Valeant, should be able to carve out a nice niche in the epilepsy market if the FDA gives Potiga the green light.

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