Researchers have found that a common food preservative can treat schizophrenia patients whose symptoms do not respond to available antipsychotic drugs.

The results of the clinical trial, published in the journal Biological Psychiatry, showed that the food preservative, sodium benzoate, improves symptoms in clozapine-resistant schizophrenia. The study, led by Hsien-Yuan Lane of China Medical University in Taiwan, demonstrated that adding on sodium benzoate to the antipsychotic clozapine improved symptoms in patients who did not see results with any other medications, providing a new option for the hardest-to-treat patients.

"Clozapine is considered the last-line antipsychotic agent for patients with refractory schizophrenia," Lane said, referring to patients whose symptoms do not respond to available antipsychotics. But an estimated 40-70 per cent of patients with refractory schizophrenia fail to improve even with clozapine, referred to as "clozapine-resistant", exhausting all potential options for treatment. The new study demonstrated that sodium benzoate -- which has been shown to enhance other antipsychotic drugs -- works in clozapine-resistant patients.

"If the finding can be confirmed, this approach may bring hope for treating patients with the most refractory schizophrenia," said Lane. Sodium benzoate works by preventing the breakdown of D-serine, a brain chemical that plays an important role in signaling that is disrupted in the brains of people with schizophrenia, according to the researchers. The 60 patients with schizophrenia included in the study were all taking clozapine, and received a placebo or sodium benzoate as an add-on treatment for six weeks.

Compared with the placebo, sodium benzoate improved negative symptoms, such as lack of emotion and motivation, which have a greater influence on a patient's functional outcome than the more prominent psychotic symptoms, the study said. The researchers detected no side effects in the patients taking sodium benzoate.