A woman suffered blurry vision, insomnia, drowsiness and dizziness after a pharmacist dispensed the wrong drug.
The case - one of two involving errors by pharmacists - was detailed in reports released today.
Health and Disability Commissioner Anthony Hill said a doctor prescribed the painkiller tramadol for the woman in November after a tooth extraction.
The unnamed pharmacist selected an antidepressant, fluoxetine, instead, however, and a colleague failed to detect the error.
The woman took up to 20 capsules of the antidepressant, including six times the recommended dose at least once.
Mr Hill said both pharmacists breached patient rights.
In a separate case, a third pharmacist also breached patient rights by prescribing the wrong drug to a woman who had serious blood clots.
The woman took a treatment for severe anaemia rather than an anticoagulant before a flight and had to be taken to hospital.