Mental health patients in a secure hospital unit are getting regularly stoned on synthetic cannabis - forcing health bosses to enlist specially trained drug dogs to search out the legal high.

The problem has become so bad at Auckland City Hospital's acute mental health unit that it was shut down and patients banned from day-time release normally considered part of their treatment process.

The legal highs had previously been identified as a severe risk to the mental health of people receiving psychiatric treatment. One of the only formal studies done on the effect of synthetic cannabis focused on the harm done to mental health patients in Wellington.

The problems had emerged at Te Whetu Tawera about six months ago and escalated to the point where the hospital needed to make major changes to deal with the issue, said psychiatrist Dr Greg Finucane.

The increased use meant access to and from the unit was stopped two weeks ago. "We lost a bit of patience. We cancelled all sorts of leave. We got the drug dog in to do a search. We stopped it for 24 hours."

About 20 per cent of the unit's 60 patients were regularly getting stoned, he said. Synthetic cannabis products, including the Kronic brands, were available in dairies around the hospital.

The use of the legal drugs also defied the strict smoking ban instituted on Auckland City Hospital grounds.

Finucane said: "We'd rather they smoked cigarettes than smoked Kronic."

It was possible to detect the use of the products because of the patient's increased heart rate and puffy red face. He said combining their use with some health conditions could also make people more aggressive.

Staff were still deciding how to deal with the problem on a permanent basis. Options included approaching the dairies to ask them to stop selling the products.

Ministry of Health psychiatrist Susanna Every-Palmer studied the impact of synthetic cannabis products on those with mental health problems. Her study, published in the Drug and Alcohol Dependence journal, found the chemical in many of the products could bring on "psychosis in vulnerable individuals".