The Health and Disability Commissioner has found that a family doctor failed to look into a patient's history and wrongly advised him to stop taking heart medication.
A man was prescribed the blood thinning medication Warfarin following heart surgery.
Commissioner Anthony Hill found the GP did not investigate why the man was taking the drug, assuming it was for a rhythm disturbance.
Mr Hill said the GP twice told the patient to stop taking the drug, and the man suffered a series of strokes and died in hospital shortly afterwards.
"The problem was, the patient ended up having palpitations so the doctor again instructs him, very clearly, for the second time, stop taking Warfarin - but has never asked the question, why is this patient on Warfarin and what are the consequences of taking the patient off Warfarin," he said.
"In this case, sadly, the consequences were catastrophic."
The commissioner found the doctor breached patients' rights and ordered that he undertake an education programme.