In August, 23 guards, four nurses and an inmate were taken to the hospital from the incident.
An Ohio State Highway Patrol report says a corrections officer was training new guards on cell searches when she noticed a cell with a closed door. She looked inside and found an inmate apparently unconscious and making gurgling noises.
Substances found in the cell and on the inmate tested positive as fentanyl and heroin.
The prosecutor says there's not enough evidence for criminal charges.