The U.S. Food and Drug Administration announced late Thursday it has approved expanding the use of Eli Lilly's (LLY) blockbuster antidepressant Cymbalta to treat chronic musculoskeletal pain, such as osteoarthritis and lower back pain.

"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."

And according to Lilly, osteoarthritis affects some 27 million U.S. adults. Also, an estimated 70% to 85% of adults experience lower back pain at some time, with some reports estimating that 2% to 10% of these people go on to experience chronic low back pain.

The approval could mean higher sales for Lilly's second-biggest drug, but the question is by how much. Doctors have already been heavily prescribing the drug as a painkiller, according to the FDA. Doctors are allowed to prescribe drugs for "off-label" uses, although companies are not allowed to market them for these uses. This new approval allows Lilly to market Cymbalta for pain. Further, Cymbalta, which accounts for 14% of Lilly's revenue, is set to lose its patent protection in 2013.

According to Bloomberg, Seamus Fernandez, an analyst at Leerink Swann & Co. in Boston, estimates annual sales of Cymbalta, which were $3.07 billion last year, may increase by $500 million, or 16%, with the new FDA approval.

Like other pharmaceuticals, the Indianapolis-based drugmaker must confront the potential loss of sales from top drugs as patent protection expires and much cheaper generics enter the market. Top-selling antipsychotic drug Zyprexa faces generic competition in 2011. Lilly's third-best selling drug, insulin Humalog, is also set to lose patent protection in 2013.

In addition, Lilly is battling pipeline setbacks, such as the suspension of its Phase III trial for Type 1 diabetes drug teplizumab, which came days after the FDA rejected its diabetes drug Bydureon, which could delay it to mid-2012. Amylin (AMLN) and Alkermes (ALKS) are Lilly's Buydureon partners.