Tennessee authorities are investigating after a drug mix-up at a Nashville hospital caused patients to experience complications during what should have been routine surgeries. 

The error occurred at Ascension Saint Thomas Midtown on Friday, Aug. 14. CBS affiliate WTVF reported that four patients had received doses of potassium instead of standard pain drugs while undergoing joint replacement operations. All of the operations were supposed to be outpatient procedures, WTVF reported. 

It’s not clear how the medications became mixed up.

Dr. Shubhada Jagasia, the hospital president and CEO, said in a statement that the facility became aware of the “event” the day it occurred. Jagasia said the hospital self-reported to state regulators and launched an investigation. 

“We identified the cause and have implemented corrective safeguards,” Jagasia said. Jagasia did not specify what the cause of the error was or which safeguards were being put in place, but said clinical teams are “actively using these enhanced safety protocols to ensure the highest level of safety” at the facility. 

The Tennessee Bureau of Investigation told CBS News it is actively investigating the mix-up after the Tennessee Healthcare Facilities Commission alerted it to the incident. 

Jagasia said hospital staff have met with the families of the impacted patients. Jagasia said the hospital would not share “specific clinical details” about the affected patients. 

Potassium has a number of uses in hospitals. Potassium itself is an essential mineral and electrolyte that the body needs to maintain a certain level of to function. Potassium citrate can be used to treat kidney stones. Potassium chloride can support heart and nerve function. However, too much potassium can be dangerous: High doses of potassium chloride are used in executions.