Synthetic cannabis and other "legal highs" have caused a recent spike in incidents of aggression at Wanganui Hospital.
There have been 23 staff injuries over the past two years with physical aggression, verbal abuse, destruction of property and disruptive behaviour all giving cause for alarm.
Whanganui District Health Board director of nursing Sandy Blake said most incidents of patient aggression at the hospital's acute mental health unit were caused by patients under the influence of synthetic cannabis.
Such incidents had reduced significantly over recent years, but the sudden rise in synthetic cannabis use was now a problem.
"The number of people being admitted to the mental health unit under the influence of synthetic cannabis is causing concern," Mrs Blake said.
"People under its influence tend to exhibit aggressive behaviour. Almost all incidents of aggressive behaviour in mental health this year have been related to this issue."
There have been three assaults by patients on staff in the unit this year, though none was directly related to synthetic cannabis.
The attacks included hot liquid thrown in the staff member's face; a punch to the stomach; and an incident involving a heavy fall and cuts to the face.
One person has been convicted of assault and another has been charged and is awaiting a court appearance.
Throughout the hospital as a whole, there have been 50 incidents this year of patients being aggressive toward staff or other patients, and a total of 246 incidents over the past two years, including the 23 staff injuries.
Mrs Blake said the health board considered any assault against staff to be "major".
Staff often didn't report cases of verbal abuse because "they know the patient or their family is sick or stressed", she said.
"However, we encourage staff to report all abuse - verbal or otherwise."
The health board had policies to manage incidents, she said, and staff were trained to calm potentially violent situations. Staff at greatest risk receive self-defence training.
Mental health services and the emergency department had close working relationships with Whanganui police, Mrs Blake added.
There have been 2500 assaults on health workers in the upper North Island alone in the past three years and last month a Wairarapa nurse was seriously injured when attacked by a male patient in an ambulance.
In the Bay of Plenty, cases included a patient who tried to strangle an employee and another in which a boiling cup of tea was thrown over a worker.