Commercial preparations of essences of hyssop has caused human intoxication in eight cases, from which tonico-clonic convulsions were the major symptom. The experimental study of the toxic properties of commercialized essential oils of sage and hyssop has revealed that their convulsant action was of central nervous system origin from pinocamphone in hyssop oil.
– Clin Toxicol 1981 Dec;18(12):1485-98 — Toxicity of some essential plant oils. Clinical and experimental study.

Survey of the literature shows essential oils of 11 plants to be powerful convulsants (eucalyptus, fennel, hyssop, pennyroyal, rosemary, sage, savin, tansy, thuja, turpentine, and wormwood) due to their content of highly reactive monoterpene ketones, such as camphor, pinocamphone, thujone, cineole, pulegone, sabinylacetate, and fenchone. Three cases strongly support the concept of plant-related toxic seizure. Nowadays the wide use of these compounds in certain unconventional medicines makes this severe complication again possible.
– J Neurol 1999 Aug;246(8):667-70 — Plant-induced seizures: reappearance of an old problem. — Burkhard PR, Burkhardt K, Haenggeli CA, Landis T.

The oil has been reported to cause seizures in laboratory animals as well as in humans. Do not use if you have epilepsy or are pregnant or nursing.

– Tisserand R, Balacs T. — Essential Oil Safety: A Guide for Health Care Professionals. — Churchill Livingstone.