(Reuters) - Pfizer Inc's pain drug Lyrica won expanded U.S. approval to treat nerve pain associated with spinal cord injuries, the world's largest drugmaker said on Thursday.

The additional approval by the Food and Drug Administration could boost sales of Lyrica, which is already one of Pfizer's biggest selling products.

Lyrica had previously been approved to treat fibromyalgia, diabetic nerve pain, post herpetic neuralgia -- or pain following a case of shingles -- and seizures in adults with epilepsy.

Pfizer reported worldwide first quarter Lyrica sales of $955 million, making it second only to its cholesterol fighter Lipitor, which is fast losing revenue in the face of competition from cheaper generic versions. U.S. Lyrica sales in the quarter were $395 million.

More than 100,000 patients with spinal cord injury in the United States suffer from the chronic, complex and often severely debilitating pain condition that Lyrica is now approved to treat, the company said.

"Given the clinical challenges of investigating neuropathic pain in this patient population, any advancements in treatment are welcome by physicians and patients alike," Dr. Diana Cardenas, who was involved in conducting the Lyrica spinal injury trials, said in a statement.

In clinical trials, the drug proved significantly better than a placebo in alleviating nerve pain from both traumatic and nontraumatic spinal cord injuries.

Pfizer shares were up 14 cents, or 0.6 percent, at $22.81 in late morning trade on the New York Stock Exchange.

(Reporting By Bill Berkrot; editing by Jim Marshall)