More than 30 out of every 100 patients admitted to New Zealand hospitals are harmed by medications given to them, a new study has found.

The study, carried out by members of the Health Quality and Safety Commission, researchers at Auckland University and members of other institutions, was published in the New Zealand Medical Journal today and found for every 100 hospital admissions 34.7 people were harmed by the drugs prescribed.

While 61 per cent of incidents resulted in minor harm, 35 per cent prompted hospital admission or prolonged the patient's hospital stay.

In 1.6 per cent of cases the medication resulted in permanent harm or death. Harm that resulted in treatment to save the patients life accounted for 2.4 per cent of cases.

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The study was based on a random selection of 20 medical records provided by four major metropolitan hospitals and two provincial ones every month for two years.

Compared with similar international studies, New Zealand's rate of harm was in the high range, although the results were very similar to a previous New Zealand study on the issue.

The top five harms, which represented 70 per cent of the cases, were: constipation; hypotension; bleeding; nausea or vomiting; and delirium, confusion or over-sedation.

Of the instances of serious harm, bleeding; hypotension; delirium, confusion or over-sedation; constipation; and IV volume overload or electrolyte imbalance were the top five problems.

The medication which most commonly caused harm was morphine which accounted for 16 per cent of cases followed by other opioids like fentanyl, oxycodone, codeine and tramadol which accounted for 14 per cent. They caused constipation, nausea or vomiting and delirium, confusion or over-sedation in harmed patients.

Aspirin was the fourth most dangerous and predominately caused bleeding. Warfarin and enoxaparin also caused issues meaning anticoagulation and antiplatelet drugs together accounted for 7 per cent of harms and were the most common cause of serious harm.

Nearly 30 per cent of the identified harm originated in the community and was found because the harm led to a hospital admission.

One of the study authors, executive director of medical services at West Moreton Hospital and Health Services Mary Seddon, said medications were a common treatment and because they were used so frequently the risk of harm was higher.

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She said it was important to remember some of the harms were not preventable and most of them were minor.

"They still take a toll on the patients and many of them prolong the hospital stay and cost them the ability to look after other patients," she said.

Seddon said there was no magic bullet but the Health Quality and Safety Commission had been working with DHBs to identify the causes of harm from opioids and were now trialling ways of reducing the instances of harm.

Some DHBs were taking it upon themselves to reduce the harm caused by anticoagulants and antiplatelets although warfarin, one of the drugs which caused the most harm, was no longer as commonly used.