If at first you don't succeed, try, try again. That's Depomed's (Nasdaq: DEPO) philosophy with its pain drug candidate Gabapentin GR, after it failed phase 3 testing last year.

In July, Depomed's extended-release version of Gabapentin GR failed as a treatment for post-herpetic neuralgia (PHN) -- post-shingles nerve pain. Endo Pharmaceuticals' (Nasdaq: ENDP) topical patch Lidoderm and the long-genericized immediate-release version of gabapentin (marketed by Pfizer (NYSE: PFE) as Neurontin) are also approved to treat this indication.

Shares of Depomed sank last year after it announced that Gabapentin GR failed to reach statistical significance on its primary goal -- pain reduction -- in the phase 3 PHN study. Considering that the immediate-release version of gabapentin has been around for various indications since 1994, proving its efficacy as a treatment for PHN, Depomed's failure here was somewhat surprising.

Depomed partially blamed the Gabapentin GR study failure on a higher-than-expected placebo effect in the control group (not uncommon in pain drug studies) and the size of the study's two drug-treated-patient groups. To prevent a larger-than-expected placebo effect from hampering results again, and to improve the trial's odds of success, Depomed is increasing the number of patients in this new phase 3 study, eliminating one of the drug treatment groups, and enrolling a more strictly defined patient population.

These changes will more than double the sample patient size for the Gabapentin GR once-daily treated patients (a good thing from a statistical point of view) in the study, and potentially help to reduce some of the placebo effect Depomed said it saw in the first study.

Last year was rough for Depomed, after partners King Pharmaceuticals (NYSE: KG) and Esprit Pharma -- now a part of Allergan (NYSE: AGN) -- both decided to stop marketing Depomed's two FDA-approved drugs because of poor sales. Depomed did find a new marketing partner for one of the drugs, but its shares haven't recovered yet. Data from this Gabapentin GR pain study won't be available until the first quarter of 2009, but the trial could be an important step in the company's rebuilding process.

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Fool contributor Brian Lawler does not own shares of any company mentioned in this article. Pfizer is an active pick of both the Income Investor and the Inside Value newsletters. The Fool has an A+ disclosure policy.