Some authorities consider Water Hemlock, or Cowbane (Cicuta spp.) to be the most violently poisonous plant of North America. Entire plant, especially roots and rootstock, is highly toxic, often fatal if ingested. Poisoning occurs rapidly. Water hemlock is sometimes mistaken for edible look-alike plants, with deadly results. Many edible plants, including Parsnip, Carrot, and Celery, are in the same family.

There are several species of Water Hemlock in North America, and all are very toxic. They are perennial herbs found in shallow ditches, wet meadows, lake and pond edges, marshes, and slow-running creeks.

The toxin is a complex alcohol known as cicutoxin, which acts on the central nervous system and causes violent convulsions, respiratory paralysis, and death.

A toxin, closely related to cicutoxin is found in another plant, Water Parsley (Oenanthe spp.) of the celery family.