A 75-year-old woman died after a Lakes District Health Board doctor gave her two drugs that should not be used together.
An investigation by Health and Disability Commissioner Anthony Hill slammed the doctor for the "medical error" and found the DHB in breach for failing to have a "safety-net" plan in place to check and challenge prescribing if needed.
The woman, who has not been named for privacy reasons, was rushed to hospital in 2016 with suspected pulmonary embolism (PE) - a condition in which one or more arteries in the lungs become blocked by a blood clot.
That day, she was given a drug called enoxaparin to prevent blood clots. Three days later, after she was confirmed to have PE, she was given another blood thinner drug called dabigatran, in addition to enoxaparin.
Either medicine can be used to treat PE, but they should not be administered together.