After the man took the pills that were seven times more than his usual dose, the pharmacist realised the error and told the man but still allowed him to drive himself home.
His family then took the man to hospital as a precaution.
Today, the pharmacist has been found in breach of the Code of Health and Disability Services Consumers' Rights for dispensing the wrong dose of methadone to a consumer and failing to follow relevant professional guidelines.
"[The pharmacist's] actions in this case were potentially life-threatening," Health and Disability Commissioner Morag McDowell said in her report.
The man and the pharmacist have not been named in the Health and Disability Commission report.
In November 2019, the man was at the pharmacy waiting in line to receive his daily 11mg dose of methadone, a strong opioid for people whose pain is uncontrolled with morphine.
The pharmacist had just finished administering a dose for another client who had left the room.
The man, who had been standing behind the intended client but was unseen due to his size, stepped forward and was waiting at the counter with the dose the pharmacist had just measured.
As a result, the man was inadvertently given and consumed another patient's methadone dose of 75mg — almost seven times his usual dose.
The commissioner was critical of the pharmacist for allowing the man to drive home after the overdose, without advising him of the risks of doing so and the need to seek medical assistance and/or call an ambulance.
The pharmacist failed to adhere to the professional standards set by the Ministry of Health, the Pharmacy Council of New Zealand and the pharmacy's Standard Operating Procedures (SOPs), McDowell said.
She recommended that the pharmacist arrange for an assessment through the Pharmaceutical Society of New Zealand and provide the man's family with a written apology.
The commissioner also advised that the pharmacy review and update its SOPs to reflect the changes made since these events, arrange refresher staff training and conduct an audit on errors or near misses.
The pharmacist expressed regret at the error that occurred, and told HDC that he has reflected "at length" about how to change his practice to prevent a recurrence of this mistake, the report said.
