Neurotoxic. Avoid during pregnancy and with epilepsy. It is best not to use this oil with children. The ketone content of Hyssopus of ficinalis is cause for concern, as various case studies and animal studies show that the essential oil can cause convulsions because of its neurotoxic effects. Many aromatherapists substitute creeping hyssop (Hyssopus officinalis var.decumbens) because it does not have the toxic effects of hyssop.