The Swedish Accident Investigation Board (SHK) will now investigate the incident.
The patients continue to receive care with a heart-lung machine in the intensive care unit and need a heart transplant, SHK writes on its website.
Medication mix-ups are a known risk, but serious incidents continue to occur.
"This indicates that there are remaining risks and vulnerabilities in drug handling," writes SHK.
The Swedish Accident Investigation Board supplements investigations carried out by healthcare providers and oversight by other authorities.
