Health officials are warning a shortage of anti-psychotic drug - olanzapine - could cause relapses for people with schizophrenia symptoms.

Supply of the injected medication is already low, and a shipment didn't arrive last month.

Royal College of Psychiatrists New Zealand chair, Hiran Thabrew, says it's critical for people to get their regular medication, or a substitute.

"What might happen is that people might experience a deterioration in their mental state, they might notice symptoms of psychosis start to occur and people around them might notice their behaviour changing."

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