Pterospartum tridentatum Willk. (prickled broom) is an autochthonous plant, common in Portuguese territory. The yellow flowers are used in traditional medicine, as a potential cure for all body illnesses, mainly to treat throat irritations or for diabetes, hypertension and hypercholesterolemia therapy. Despite its traditional use, no toxicological assessment has been performed to our knowledge. A high antioxidant activity of P. tridentatum flower water extract was found, in good agreement with its electrospray ionisation-mass spectroscopy (ESI-MS) spectrum which revealed the presence of several flavonoids, such as luteolin-O-(O-acetyl)-glucuronide, luteolin-O-glucuronide or isorhamnetin-O-hexoside. Mitocondrial respiratory rates (state 4, state 3 and FCCP-stimulated respiration) and respiratory indexes (respiratory control and P/O ratios) showed no consistent decrease of respiratory and phosphorylative efficiencies for the concentrations tested (up to 500 μg.mL). Cytotoxicity evaluation, using MTT assay, was in agreement with the previous results. In conclusion, for the concentration range commonly used, P. tridentatum flower usage can be regarded as harmless and credible.