Japanese drugmaker Eisai Inc. said it will double its U.S. sales force to 400 representatives to market Belviq, the anti-obesity medication developed by San Diego-based Arena Pharmaceuticals Inc.

The company began selling Arena’s medication this June, almost a year after it received approval from the U.S. Food and Drug Administration.

Shares rose 5 percent for Arena following the news, closing day trading at $4.57. The company’s stock has taken a dive this year, having been at an $8.77 share price on June 11, when it began marketing the drug.

— Meghana Keshavan