Limonene

d-Limonene (R-(+)-4-isopropenyl-1-methylcyclohexene; C₁₀H₁₆) is a cyclic monoterpene dominant in citrus peels and a principal terpene in many "sativa-leaning" cannabis cultivars, contributing a bright citrus aroma. Preclinical studies report anxiolytic and antidepressant-like activity in rodent models, partly attributed to 5-HT1A and adenosine A2A mechanisms (Komiya et al., 2006, Behavioural Brain Research 172:240; Costa et al., 2019). A small human inhalation study (d'Alessio et al., 2014) reported reductions in subjective anxiety. Limonene is also a potent skin-penetration enhancer used in topical formulations and has been studied as a chemopreventive agent in rodent cancer models. Evidence level: rodent with small human pilot data; not a proven therapeutic. → See also: Terpene, Entourage effect.