The essential oil includes the chemicals thujone and phenol, which give it antiseptic properties. Its high concentrations of thujone and chemicals that stimulate the central nervous system, including pinocamphone and cineole, can provoke epileptic reactions. The oil of hyssop can cause seizures and even low doses (2–3 drops) can cause convulsions in children.
Hyssop Oil (aromatherapy)
Hyssop Oil is a pale yellow or faintly greenish yellow to almost colorless oil of a powerful, somewhat sharp, but sweet-camphoraceous odor, and with a warm-aromatic, spicy undertone. Its flavor is warm and sweet, slightly burning or biting, but rich aromatic and herbaceous-spicy.
Arctander, Steffen . Perfume and Flavor Materials of Natural Origin (p. 355).