Northland District Health Board is investigating the region's use of strong painkillers after a national commission found the prescription rate was much higher than some parts of the country.
Northland had the fourth highest rate of strong opioid use in the country after Wairarapa, Nelson-Marlborough and Bay of Plenty.
The figures came from the Health Quality and Safety Commission's Atlas of Healthcare Variation, a new database of health services and outcomes. The doses were dispensed from community pharmacies in 2013, and did not include drugs given in hospitals.
However, nearly half of those given the strong opioids had been treated at a public hospital in the week before.
Strong opioids include methadone, morphine, fentanyl and oxycodone. Commonly used weak opioids include codeine, tramadol and dihydrocodeine.
Northland DHB physician Dr Alan Davis said the DHB wasn't yet sure why its use of strong opioids was higher than the national average.
"While it seems likely that changing disease patterns and an ageing population has led to some increase in need, some parts of the country get by with much lower levels of use," he said. "We are seeking to understand why."
The DHB was focused on reducing the harms of such painkillers, he said.
"We know that opioids frequently cause unpleasant side effects such as nausea, vomiting and constipation. These can be so severe as to cause people to be admitted to hospital. We will be looking to reduce the use in this setting and use other treatment strategies for people at high risk of adverse events."
Dr Davis, also chairman of the commission's expert advisory group, said opioids were effective in managing pain but were also the class of medicine most often implicated in patient harm - including addiction or over-sedation. "The question is, do we need to use strong opioids as much as we do?"
The lowest rates of strong opioid use were at the Capital and Coast, Auckland and Canterbury district health boards.
The commission also found women were dispensed significantly more strong and weak opioid drugs. Use increased with age, and people of European ethnicity had two to four times higher use of strong opioids than Maori, Pacific or Asian people.