The Health and Disability Commissioner is formally investigating the use of the controlled drug ketamine on mental health patients at Dunedin hospital.
Ketamine is licensed for use as a general anaesthetic; it is also a recreational drug and used by vets as a horse tranquilliser.
Drugs may be used for a purpose other than the one they are licensed for, but a Dunedin mental health consumer group is worried that about 10 vulnerable patients may have been part of a trial without having given proper consent.
A report has been completed by the Health Ministry's National Health Board, which is not public. It will say only that it has referred the findings to the commissioner Anthony Hill.
Mr Hill confirmed that he expanded his earlier inquiries into a formal investigation on Friday into the use of ketamine on ward 1A of Dunedin hospital during the past two years.
The investigation is expected to take months.