Relaxation effects of Shikuwasa (Citrus depressa Hayata) essential oil inhalation: A randomized controlled trial in young adult females

Background: Hayata) essential oil, rich in limonene and γ-terpinene, produces calming effects, but its psychological and physiological relaxation effects in humans remain unclear. This study examined the short-term relaxation effects of inhaling Shikuwasa essential oil compared to odor-free air among young adult females. Methods: In this two-arm, parallel-group, open-label, randomized controlled superiority trial, healthy females aged 18-39 were randomly assigned to either the Shikuwasa essential oil group (n = 20) or the odor-free air group (n = 20). The primary outcome was subjective tense-arousal, measured using the Japanese version of the University of Wales Institute of Science and Technology Mood Adjective Checklist (JUMACL). Secondary outcomes included miosis rate, peripheral skin temperature, and cerebral blood flow. Due to measurement constraints, the experiment was divided into three 2-min inhalation phases. Results: No significant between-group differences were observed in tense-arousal, miosis rate, or cerebral blood flow. Additional analyses of covariance, performed for each outcome variable using the Visual Analog Scale score and its interaction with the group as covariates, revealed a trend toward an interaction effect for the JUMACL tense-arousal subscale score, although the overall effects were not statistically significant. Conclusions: Short-term inhalation of Shikuwasa essential oil did not significantly reduce tense-arousal or affect physiological indices in healthy, normal-to-low stress young adult females. However, aroma preference may influence psychological and neural responses. Future studies should consider pre-assessed aroma preference, longer inhalation durations, single-phase protocols, extended rest periods, and participants' baseline mood. Trial registration: This study was prospectively registered with the UMIN-CTR, UMIN000053914.