The nurse who administered the medication in July last year said that when the man was brought to the holding cell outside the health unit, she was told his brother was outside.
Believing she recognised the man as his brother, she asked a colleague to help her cross-check the brother's medication, including checking his photograph and medical chart.
Both nurses believed they were looking at the man's brother, but these checks were not carried out in the same room as the man who was about to receive the medication.
When the man was brought into the health unit the nurse called out his brother's name and said he answered in the affirmative.
She believed she heard the brother's name when she asked him to state his full name.
The man was not asked for another form of identification, such as his date of birth.
Without telling the man the specific medication he was about to receive, the nurse then injected the man with his brother's anti-psychotic medication.
Deputy Health and Disability Commissioner Kevin Allan had the nurse informed the man about the medication he was about to receive, she may have realised he was there to receive his hepatitis vaccination.
The patient would've also had the opportunity to refuse the medication.
"Without information about the medication to be administered, the man was not in a position to make an informed choice and give his informed consent to taking the medication," Allan said.
The nurse realised "something was not right" when the man asked her what the injection was for after she administered it.
She immediately apologised and arranged the man to be observed and monitored for potential side effects.
After consulting other specialists for advice she arranged for medication to counter any adverse effects to be on standby.
While the man reported feeling fine every time he was assessed, he was kept for observation overnight. He displayed no side effects from the anti-psychotic.
The report recommended the nurse provide a written apology to the inmate and undergo further training.
Allan also recommended that the Department of Corrections review its policy in light of the issues raised in this case.
The Department of Corrections was not found to have breached the code.