Eli Lilly & Co. has won approval to sell the antidepressant Cymbalta, its second-biggest drug, as a treatment for pain.

The Food and Drug Administration cleared Cymbalta for musculoskeletal pain such as arthritis and chronic lower back conditions, the agency said Thursday in a prepared statement. An FDA advisory panel voted 8-6 in favor of Indianapolis-based Lilly’s application to expand Cymbalta use on Aug. 19.

Lilly is under pressure to maintain sales as its top drugs prepare for generic competition and new products struggle to reach the market. Broader approval of Cymbalta may provide only a small reprieve because the medicine’s patent expires in three years and it is already being prescribed as a painkiller, said Seamus Fernandez, an analyst at Leerink Swann & Co. in Boston.

“Half of that market opportunity could already be potentially accounted for,” Fernandez said in a telephone interview. He estimates annual sales of Cymbalta may increase by $500 million, or 16 percent, with approval for pain.

Sales of Cymbalta were $3.07 billion last year, accounting for 14 percent of Indianapolis-based Lilly’s revenue. The drug is scheduled to lose U.S. patent protection in 2013, along with the insulin Humalog, which ranks third in revenue. Top-selling Zyprexa, an antipsychotic, faces generic competition in 2011.

“Up to three-quarters of the population experience chronic pain at some time in their lives,” Janet Woodcock, director of the FDA’s Center for Drug Evaluation and Research, said in the statement. “This approval means that many of those people now have another treatment option.”

About 30 million patients have used Cymbalta since its initial approval for major depressive disorder in 2004, the agency said.

About 6.5 percent of Cymbalta prescriptions through last year were for headaches and nerve pain, including “chronic pain syndrome,” and 7 percent were for musculoskeletal system diseases, including arthritis and back pain, FDA staff said in a report released before the advisory panel meeting. While companies can’t promote drugs for uses not cleared by the FDA, doctors are allowed to prescribe them as they see appropriate.

Lilly asked for broad approval in chronic pain in the second quarter of 2008 before withdrawing the application after the FDA raised concerns about study design and statistical methodology. The application was resubmitted in June 2009.

An advisory panel review initially slated for January was canceled so regulators could have more time to consider new information pertaining to the drug’s “benefit-risk balance” in pain. Most of the advisers supported approval of the drug in August over concerns that it can cause liver damage and may not work for all patients. While the FDA usually follows its panels’ recommendations, it isn’t required to do so.