Common Tansy Tanacetum vulgare contains a volatile oil, tanacetin or "oil of tansy," which is extremely variable in its chemical composition, but usually contains large amounts of thujone, a monoterpene ketone present in many essential oils. It can cause convulsions and psychotic effects in people. Because of its strong taste and smell it is rarely touched by animals.

Several related plants also containing thujone have been used in herbal medicine, including Yarrow (Achillea millefolium), and various Sagebrush and Wormwood species (Artemisia spp.).1 Habitual abuse of the wormwood spirit absinthe was described in the 19th and 20th centuries as a cause for the mental disorder "absinthism" including the symptoms hallucinations, sleeplessness and convulsions.2 Plants containing thujone should be regarded with extreme caution.

Recent research studies have revealed that thujone works by blocking brain receptors for gamma-aminobutyric acid, or GABA. Without access to GABA, which is a natural inhibitor of nerve impulses, neurons fire too easily and their signaling goes out of control leading to epileptic convulsions.3