Phytochemical composition, antioxidant potential, and toxicity of infusions and decoctions of Greek mountain tea (Sideritis clandestina subsp. clandestina) from the Peloponnese

• First comprehensive study of Sideritis clandestina subsp. clandestina extracts. • LC/QTOF analysis revealed 18 bioactive secondary metabolites. • Decoctions yielded higher phenolic and antioxidant content than infusions. • Geographic origin influenced phytochemical composition and bioactivity. • Extracts showed measurable in vitro activity without toxicity concerns. Sideritis clandestina subsp. clandestina , endemic to the Peloponnese and traditionally used as a mountain tea, belongs to a group of herbal infusions widely consumed across the Mediterranean and Balkan regions. This study investigates the phytochemical composition and antioxidant potential of infusions and decoctions prepared from aerial parts collected in three Peloponnesian regions. LC/QTOF analysis identified 18 secondary metabolites, including acetylated and methylated flavonoid glycosides, phenylethanoid glycosides, phenolic acids, and diterpenes. Significant compositional variation was observed across geographic origins, with Mt. Mainalon samples exhibiting the highest total phenolic content and antioxidant activity. Decoctions consistently yielded higher concentrations of bioactive compounds than infusions. In vitro assessment using the Microtox assay showed consistent EC₅₀ values across samples, indicating measurable bioactivity without raising safety concerns. These findings highlight S. clandestina subsp. clandestina as a promising natural source of antioxidants for nutraceutical applications, with bioactivity influenced by both extraction method and geographic provenance.