Tansy ("Tanacetum vulgare") is a perennial, herbaceous flowering plant in the genus Tanacetum in the aster family, native to temperate Europe and Asia. It has been introduced to other parts of the world, including North America, and in some areas has become invasive. It is also known as common tansy, bitter buttons, cow bitter, or golden buttons. The Latin word "vulgare" means "common".

Tansy is a flowering herbaceous plant with finely divided compound leaves and yellow, button-like flowers. It has a stout, somewhat reddish, erect stem, usually smooth, 50–150 cm (20–59 in) tall, and branching near the top.

The leaves and flowers are toxic if consumed in large quantities; the volatile oil contains toxic compounds including thujone, which can cause convulsions and liver and brain damage. Some insects, notably the tansy beetle, have resistance to the toxins and subsist almost exclusively on the plant.

Tansy has a long history of use. It was first recorded as being cultivated by the ancient Greeks for medicinal purposes. In the 8th century AD it was grown in the herb gardens of Charlemagne and by Benedictine monks of the Swiss monastery of Saint Gall. Tansy was used to treat intestinal worms, rheumatism, digestive problems, fevers, sores, and to bring out measles.

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