Eli Lilly and Co. said Friday that it has received an extra six months of marketing exclusivity on the antidepressant Cymbalta, its biggest selling drug.

Lilly said marketing exclusivity on Cymbalta will now expire in December 2013, which means generic versions of the drug will not be approved until then.

The extension could mean more than a billion dollars in sales for the Indianapolis drug maker. 

The drugmaker said the Food and Drug Administration had determined that Cymbalta meets requirements for a pediatric exclusivity extension even though Cymbalta is not approved for use in children.

Cymbalta is approved for patients 18 and older, and Lilly said it does not plan to try to get the drug approved for use in children. But the FDA's finding still gives the company another six months to sell Cymbalta without generic competition.

U.S. sales of Cymbalta totaled $1.77 billion in the fourth quarter of 2011 and the first quarter of 2012. That was about three-quarters of all worldwide sales of the drug.

Shares of Eli Lilly fell 34 cents, to $42.51 each, in morning trading Friday. Its shares hit a 52-week high of $43.31 on Tuesday.