Eli Lilly & Co said on Friday that it gained six more months of U.S. exclusivity on its Cymbalta antidepressant for conducting pediatric studies, extending Lilly's hold on the $4 billion-a-year drug until December 2013.

Drugmakers are frequently awarded six months of additional patent life on their medicines for conducting studies in children, an incentive U.S. lawmakers created because of a perceived lack of pediatric studies of medicines.

However, Lilly said that, based on the study results, it would not seek approval to expand use of Cymbalta for children. Cymbalta is approved in the United States only for use in individuals 18 years and older, the company noted.

Use of antidepressants for children has a controversial history. The Food and Drug Administration in 2004 warned of an increased risk of suicidal thoughts or behavior in children and adolescents treated with some antidepressants.

About 11 percent of adolescents have a depressive disorder by age 18, according to a survey cited by the National Institute of Mental Health.

The extension of exclusivity on Cymbalta is a boost to Lilly as it grapples with the loss last October of its U.S. patent for Zyprexa, an antipsychotic that was its previous top-selling product.

Shares of Lilly were off 0.5 percent at $42.64 in morning trade on the New York Stock Exchange, a slightly better performance than for the weak overall U.S. markets.