Background <p>30% of individuals with schizophrenia do not respond to first line dopamine-blocking antipsychotic medications and meet criteria for treatment resistant schizophrenia (TRS). Elevated glutamate levels in the anterior cingulate cortex (ACC) have been found to be associated with poor response to dopamine-blocking antipsychotics in early phase psychosis (EPP). Clozapine is the gold standard treatment for TRS with some evidence suggesting that clozapine may work in part through altering glutamate neurotransmission. However, the exact mechanisms through which clozapine is effective for TRS remain largely unclear and require further investigation.</p> Objective <p>To our knowledge, this is the first study to investigate glutamatergic changes after 6 months of clozapine treatment in patients with EPP. We hypothesized that there would be a decrease in glutamate concentration after 6-months of clozapine treatment.</p> Method <p>Treatment resistant EPP patients were recruited for a larger neuroimaging study; 7 individuals who met criteria for TRS underwent measurement of ACC glutamate prior to starting clozapine, and following clozapine treatment for 6 months. 1H-MRS acquisitions were performed using Point Resolved Spectroscopy in the bilateral dorsal ACC.</p> Results <p>One case was excluded from imaging analyses due to poor MRS quality (Signal to noise ratio within LCmodel &lt; 12). Paired sample t-test found a non-significant decrease in average glutamate concentration in the ACC 6 months after starting clozapine, (6) = 0.49, <i>p</i> = .647, 95% CI [− 2.72, 4.00]; however, a small effect size was detected (d = 0.34, 95% CI [-0.81, 1.47]).</p> Conclusions <p>While there was not a significant change in glutamate in the ACC 6 months after starting clozapine, declining glutamate with a low-moderate effect size is aligned with the hypotheses and may inform future studies. This work suggests that future studies are warranted using a larger sample size.</p> Clinical trial number <p>Not applicable.</p>