This triple-blind, randomized, controlled crossover design study investigated the effect of a single dose of exogenous ketones (delta G Ketones) on prepulse inhibition (PPI) in individuals with Schizophrenia Spectrum Disorder (SSD) and Bipolar Disorders (BD). PPI is considered a neurophysiological biomarker for sensory gating deficits in SSD/BD. Previous preclinical research suggests that ketones may offer an alternative energy source for the brain, leading to a potential improvement in PPI. However, in the current study the hypothesis that a single dose of dGK would increase PPI was largely unsupported. This interim analysis, for 14 participants and two dropouts (n =10 SSD and n =4 BD), found a significant decrease in PPI for one prepulse stimulus type, 120ms ISI, 76 dB (p = .029). Other stimulus types showed no significant changes. Beside this, qualitative feedback was collected and revealed varying participant experiences, which gained insight into the subjective experience of participants. The unexpected findings suggest a more complex interaction between exogenous ketones and PPI in SSD/BD than expected, highlighting the need for further research.