ATLANTA (AP) — Federal authorities say they are working with Tennessee officials to investigate why at least 12 patients in that state who injected the pain reliever Opana ER for non-medical reasons since February have a rare blood disorder.

The Centers for Disease Control and Prevention issued the health advisory Friday night regarding the pain reliever, an extended-release form of oxymorphone. The CDC is working with the Food and Drug Administration and the Tennessee Department of Health.

The investigation is focusing on a cluster of patients with Thrombotic Thrombocytopenic Purpura, described by officials as a blood disorder in which clots form in small blood vessels throughout the body. It is an uncommon but serious illness.

Authorities say Opana ER is only supposed to be taken orally. The cases were reported in Tennessee from April 16 to Oct. 19. Most of the women did not have a prescription and reported a history of chronic IV use of the opioid pain reliever for nonmedical reasons.

All of the women were hospitalized, but no deaths have occurred.