Pictured: Hemlock

Level of toxicity: Generally moderate to severe, life-threatening

Common signs to watch for:

  • Drooling
  • Dilated pupils
  • Weakness
  • Agitation
  • Nervousness
  • Twitching
  • Seizures
  • Cardiac abnormalities
  • Difficult breathing
  • Death from respiratory paralysis

Cowbane, which is more commonly known as water hemlock, poison parsnip, or poison parsley, is often referred to as the most violently toxic plant in the United States. Cowbane is considered a natural wildflower and prefers wet areas, such as irrigation ditches, marshes, damp areas in pastures, and riverbanks. Cowbane contains the toxins cicutoxin and cicutol, which affect the neurons in the brain and central nervous system. All parts of the plant are poisonous, with the roots containing the highest concentration of the toxin. It is highly poisonous to horses, and only takes about 0.2 to 2 lbs hemlock root per 1,000 lbs of a horse’s body weight to cause death. Clinical signs include drooling, dilated pupils, weakness, agitation, nervousness, twitching, seizures, cardiac abnormalities, difficult breathing, and death from respiratory paralysis.

Content written by: Dr. Lynn Hovda, DVM, RPH, MS, DACVIM, Pet Poison Helpline

Poison type: plants

Alternate names: Water hemlock, poison parsnip, poison parsley