GlaxoSmithKline (NYSE:GSK) and XenoPort (NASDAQ:XNPT) announced clinical trial data yesterday on their drug to treat nerve pain following shingles, and the news was anything but painful. XenoPort's shares jumped by a full 25%.

GSK1838262 -- also called XP13512 and gabapentin enacarbil, but for the sake of my typing fingers, let's just call it "the drug" -- is a modified version of Pfizer's (NYSE:PFE) Neurontin, which is available as a generic. The phase 2b trial showed statistical significance over placebo at all three doses tested and minimal side effects at the lowest dose.

If approved, it'll compete with Pfizer's Lyrica and Endo Pharmaceuticals' (NASDAQ:ENDP) Lidoderm patch, which are both approved to treat pain after shingles. But Glaxo and XenoPort's biggest nemesis could be Merck (NYSE:MRK), which has a vaccine for shingles. If people aren't getting shingles, they're certainly not going to need a medication for shingles-related pain.

The drug, however, has more uses than just pain after shingles. It's up for review as a treatment for restless leg syndrome, and there's potential for it to be used as a treatment for a related nerve pain that some diabetics get -- and that market is larger than the shingles-pain market.

Investors welcomed the 25% boost in XenoPort's shares yesterday, especially after a drug candidate crashed and burned near the end of last year and pulled the company's share price down with it. Right now, XenoPort's near-term future is in the hands of the Food and Drug Administration, which may decide whether the drug gets approved to treat restless leg syndrome by Nov. 9.

Until then, investors are likely to be a little restless -- but they can smile for now.

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Fool contributor Brian Orelli, Ph.D., doesn't own shares of any company mentioned in this article. Pfizer is a recommendation of the Inside Value newsletter. The Fool has a disclosure policy.