Essential oils are complex multicomponent mixtures of volatile plant-derived secondary metabolites that have attracted increasing scientific interest due to their broad spectrum of biological activities. Owing to their chemical diversity and multitarget mechanisms of action, essential oils represent promising bioactive systems with antimicrobial, antioxidant, and antitumor potential. The present review provides a comprehensive analysis of current literature data on the chemical composition of essential oils, with particular emphasis on GC–MS profiling as a prerequisite for reliable interpretation of biological activity. Variations in chemotype, botanical origin, geographical factors, and extraction methods are discussed as key determinants of bioactivity. The review summarizes experimental evidence demonstrating strong antimicrobial effects of essential oils, especially those rich in phenolic monoterpenes such as carvacrol, thymol, and eugenol, including activity against multidrug-resistant pathogens and biofilm-forming microorganisms. Particular attention is given to synergistic interactions between essential oils and conventional antibiotics, which result in reduced minimum inhibitory concentrations and enhanced antimicrobial efficacy. In addition, antioxidant properties of essential oils are critically evaluated using common in vitro assays (DPPH, ABTS, FRAP), highlighting the contribution of synergistic interactions among major and minor constituents. Furthermore, accumulating in vitro data on the antitumor activity of essential oils are reviewed, including mechanisms such as apoptosis induction, cell cycle arrest, and oxidative stress modulation. Overall, essential oils emerge as multifunctional bioactive systems with significant potential as natural antimicrobial adjuvants, antioxidants, and complementary agents in anticancer research. However, their clinical translation is currently limited by compositional variability, insufficient standardization, and the predominance of in vitro data. The review emphasizes the need for standardized analytical protocols, comprehensive toxicological evaluation, and well-designed in vivo and clinical studies to fully assess the therapeutic applicability and safety of essential oils.
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