More than a quarter of patients at hospitals run by three district health boards were harmed by their medication in the year to February 2011. The 28.9% rate is higher than previously reported.
A review by Counties Manukau, Capital & Coast and Canterbury DHBs considered 1210 patient hospital charts.
A resulting report said 5% of these adverse drug events, causing permanent harm to four people and contributing to the deaths of five others.
Others caused temporary harm to 94.5% of the affected patients and medical intervention was needed.
Painkillers like morphine, tramadol, and warfarin, an anti-clotting drug, were most often cited.
The report said the rate of harm detected is much higher than that indicated by voluntary reporting in hospitals.
The report also said the rate of harm detected shows improvements to patient safety have been slow in the last decade.
One of the authors said that was due to the complexity of the health system. Dr Nigel Millar said patients can be taking several medications and moving between different health services that may not always communicate well with each other.
