Royal Sussex County Hospital morphine diluted
Bottles of painkilling liquid morphine were found diluted on a surgical ward at a Sussex hospital.
They were found at Brighton's Royal Sussex County Hospital over the 3 and 4 March weekend, the Brighton and Sussex University Hospitals NHS Trust said.
The trust said no patients had been harmed and no arrests had been made by police, who are investigating the case.
Stricter controls on the storage and dispensing of the drug have since been introduced at the hospital.
Trust chief executive Duncan Selbie said there had been no further incidents.
"This is nevertheless a serious matter and until we fully understand the nature and cause, these stricter controls will remain in place across the hospital," he said.