Hemlock Poisoning

Updated: Jan 29, 2021
  • Author: Daniel E Brooks, MD; Chief Editor: Michael A Miller, MD  more...
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Practice Essentials

Hemlock poisoning may refer to poisoning by either poison hemlock (Conium maculatum; see the image below) or plants in the water hemlock family (Cicuta species and Oenanthe crocata). Hemlocks are among the few plants that can cause life-threatening toxicity. In both species, the root contains the greatest concentration of toxin, although all plant parts are toxic.

Hemlock. Photo by Cornell University Poisonous Pla Hemlock. Photo by Cornell University Poisonous Plants Informational Database

Both water hemlock and poison hemlock have caused severe poisoning in both humans and livestock. Historically, poison hemlock was reportedly used to execute Socrates, and the Old Testament describes rhabdomyolysis in Israelites who consumed quail fed on hemlock. [1] Poison hemlock causes skeletal deformities in the offspring of livestock that eat the plants during gestation (eg, crooked calf disease in cattle). Fatal toxicity has occurred in children who played with whistles made from hollow stems of poison hemlock. [2, 3]

Although related, poison hemlock and water hemlock toxicity have different pathophysiologies and clinical presentations. With water hemlock toxicity, profuse salivation, perspiration, bronchial secretion, and respiratory distress leading to cyanosis develop soon after ingestion; severe toxicity tends to cause seizures, with death resulting from status epilepticus. [4] Poison hemlock toxicity is characerized by rapidly progressive muscle weakness and paralysis, which may eventuate in respiratory failure. [5]

No antidote is available for either toxin. Gastrointestinal decontamination, if appropriate, and aggressive supportive care are the mainstays of treatment.

For patient education resources, see Poisoning.



Poison hemlock, an exotic species introduced to the United States, is a ubiquitous plant with fernlike properties that may reach a height of 2 meters. Poison hemlock grows in diverse settings, including wooded areas, ditches, and waysides throughout the United States, and may be mistaken for other plants such as fool's parsley (Aethusa cynapium).

Water hemlock is typically found growing in moist habitats, such as drainage ditches, marshes, and near bodies of fresh water. Water hemlock has compound leaves; small white or green flowers; and tuberous, large, hollow roots. Water hemlock may reach a height of 2.5 meters and can also be confused with other plants such as wild carrot, also known as Queen Anne's lace (Daucus carota), poison hemlock (C maculata), pignut, sweet flag, watercress, wild parsnip, wild celery, wild ginseng, and kvanne. [6]

See 11 Common Plants That Can Cause Dangerous Poisonings, a Critical Images slideshow, to help identify plant reactions and poisonings.



Poison hemlock

Poison hemlock contains several piperidine alkaloid toxins (namely coniine) that are structurally similar to nicotine. Coniine has direct effects on nicotinic (cholinergic) receptors, both agonist and antagonist. Clinically, initial manifestations include gastritis and CNS stimulation (tremor, ataxia, and seizures). Nicotine activation at autonomic ganglia can cause tachycardia, salivation, mydriasis, and diaphoresis. In severe cases, acetylcholine (nicotinic) receptor antagonism develops. This leads to bradycardia, ascending paralysis, and CNS depression (coma). Death is typically from respiratory failure.

Symptoms are produced with ingestion of as little as 3 mg of conline but up to 150–300 mg coniine can be tolerated, which translates to 6–8 leaves (6 g). [7]

Water hemlock

Water hemlock contains cicutoxin, a potent, noncompetitive gamma-aminobutyric acid (GABA) receptor antagonist. Using a rat model, Uwai et al showed that cicutoxin is an antagonist of GABA-mediated chloride channels. [8] Cicutoxin rapidly produces GI symptoms (nausea, emesis, abdominal pain) typically within 60 minutes of ingestion. CNS excitation leads to tremor and seizures, often refractory to therapy. A single bite of the root, which contains the highest concentration of cicutoxin, has been reported to kill an adult. [6]



No exposures or human deaths from hemlock ingestion have been reported to US Poison Control Centers during the past 10 years. [9]  However, a case report of a death due to intentional intravenous poison hemlock injection was published in 2017. [10]  

A systematic review of 30 reported hemlock poisonings found the majoity of cases occurred in Italy (56.7%) and Greece (10%). Males (76.7%) and those over 38 years old (66.7%) were the most commonly affected. [11]

Prevalence was low for US livestock. [2, 3]  Livestock exposures in New Zealand, South America, Europe, and southern Canada have been reported. Cattle appear to be most vulnerable to hemlock toxicity.





The prognosis is good if the patient presents early and receives appropriate decontamination and supportive care. Complications of hemlock ingestion may include the following:

  • Death (secondary to respiratory failure or status epilepticus)
  • Seizures (status epilepticus)
  • Rhabdomyolysis (acute kidney injury) - Hyperkalemia
  • Coma
  • Aspiration pneumonitis
  • Permanent neurologic sequelae

Poison hemlock poisoning is potentially lethal with large ingestions. Water hemlock fatalities have occurred following a few bites of the root. [4] Poison hemlock's human median lethal dose (LD50) is not known. Mortality from poison hemlock ingestion is usually secondary to respiratory paralysis.

Water hemlock had a 30% mortality rate in one series of 86 patients. It is recognized as one of the most toxic plants in North America. Mortality from water hemlock is usually secondary to refractory status epilepticus.