Aspiration pneumonia is a serious health concern in older patients with schizophrenia. In this study, we examined clinical and demographic variables that could impact the plasma substance P level, which is a useful predictive biomarker of aspiration. Thirty-four patients were included (mean age ± SD: 70.9 ± 10.8 years). A greater number of cigarettes/day and a higher antipsychotic dosage were found to be associated with a lower plasma substance P level, while age showed a trend-level effect. This finding suggests the need for intensive observation for prevention of aspiration pneumonia in heavy smokers who are receiving a higher antipsychotic dose in this senior population.