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NEW YORK (Dow Jones)--Shares of Acura Pharmaceuticals Inc. (ACUR) fell Thursday after the company and its partner, King Pharmaceuticals Inc. (KG), received a letter from the U.S. Food and Drug Administration that raised concerns about the abuse-deterrent ability of the pain killer Acurox.

Shares recently were down 1 cent at $5.90 but earlier were off as much as 15%. Shares of King Pharmaceuticals were off 2.6% at $9.57 in recent trading.

The companies said the complete response letter, which was regarding their new drug application for Acurox, raises issues about the benefits of abuse deterrents built into the drug. But the companies said at this time they don't believe additional studies will be needed. The companies had disclosed last month they were not expecting to get approval at this stage in the process. They intend to meet with the FDA after submitting a response to the agency's most recent concerns. [Full Story]