Clinical research on aromatherapy is strongest for modest, short-term, subjective outcomes: several controlled trials find inhaled lavender oil reduces self-reported anxiety before medical procedures, and peppermint oil has reasonable evidence for easing nausea, results broadly consistent with aromatherapy's traditional claims for these particular oils. Evidence for more specific or serious claims, treating infection, chronic disease, or replacing conventional care, is weak to absent, and aromatherapy has not been shown to alter the course of any diagnosed illness. Because trials vary widely in the oil, concentration, and delivery method used, its evidence base remains thinner and more mixed than its long history of use might suggest.
The clearer and better documented finding concerns safety rather than efficacy. Essential oils are established causes of allergic and irritant skin reactions, and concentrated oil ingestion, including of common oils such as tea tree, wintergreen, and eucalyptus, is a recognized cause of poisoning, particularly in young children who may swallow a bottle meant for topical or diffuser use. A 2007 case series published in the New England Journal of Medicine linked repeated topical use of lavender and tea tree oil to prepubertal gynecomastia in three boys, along with laboratory evidence that the oils have estrogenic and antiandrogenic activity in human cell lines; later industry-funded reanalyses have disputed the strength of that causal link, but the underlying finding, that these oils are hormonally active and not inert, has shaped current safety guidance for their use in children.