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Hemlock Poisoning

  • Author: Daniel E Brooks, MD; Chief Editor: Asim Tarabar, MD  more...
Updated: Apr 21, 2015


Plant ingestions continue to be a very common exposure for humans (particularly children) and animals, accounting for almost 44,000 calls to national poison centers in 2012.[1] Pediatric patients comprise more than 80% of plant-related exposures. Only a few plants, poison hemlock and water hemlock included, are associated with potentially life-threatening toxicity, and less than 20% of plant exposures require medical management.

Hemlock poisoning may refer to poisoning by either poison hemlock (Conium maculatum) or water hemlock family (Cicuta species and Oenanthe crocata L.). Historically, poison hemlock was reportedly used to execute Socrates, and the Old Testament describes rhabdomyolysis in Israelites who consumed quail fed on hemlock.

Although related, poison hemlock and water hemlock toxicity have different pathophysiologies and clinical presentations. The root contains the greatest concentration of toxin in both species, although all plant parts are toxic. Poison hemlock causes "crooked calf disease," a congenital abnormality, among cattle formed via fetal exposure. No antidote is available for either toxin.

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). Poison hemlock is shown in the photo below.

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

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.[2]

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.

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.[3] 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.




United States

In 2012, 43,947 plant exposures were reported to the American Association of Poison Control Centers National Poison Data System.[1] No human deaths from hemlock ingestion have been reported to US Poison Control Centers during the past 10 years. Prevalence was low for US livestock.


Livestock exposures in New Zealand, South America, Europe, and southern Canada have been reported. Cattle appear to be most vulnerable to hemlock toxicity.


Poison hemlock poisoning is potentially lethal with large ingestions; water hemlock fatalities have occurred following a few bites of the root.

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.


Younger patients are theoretically at increased risk due to smaller body mass.

Contributor Information and Disclosures

Daniel E Brooks, MD Co-Medical Director, Banner Good Samaritan Poison and Drug Information Center, Department of Medical Toxicology, Banner Good Samaritan Medical Center

Daniel E Brooks, MD is a member of the following medical societies: American Academy of Clinical Toxicology, American College of Emergency Physicians, American College of Medical Toxicology

Disclosure: Nothing to disclose.

Specialty Editor Board

John T VanDeVoort, PharmD Regional Director of Pharmacy, Sacred Heart and St Joseph's Hospitals

John T VanDeVoort, PharmD is a member of the following medical societies: American Society of Health-System Pharmacists

Disclosure: Nothing to disclose.

Chief Editor

Asim Tarabar, MD Assistant Professor, Director, Medical Toxicology, Department of Emergency Medicine, Yale University School of Medicine; Consulting Staff, Department of Emergency Medicine, Yale-New Haven Hospital

Disclosure: Nothing to disclose.

Additional Contributors

David A Peak, MD Associate Residency Director of Harvard Affiliated Emergency Medicine Residency; Attending Physician, Massachusetts General Hospital; Assistant Professor, Harvard Medical School

David A Peak, MD is a member of the following medical societies: American College of Emergency Physicians, Society for Academic Emergency Medicine, Undersea and Hyperbaric Medical Society, American Medical Association

Disclosure: Partner received salary from Pfizer for employment.


Michael Hodgman, MD Assistant Clinical Professor of Medicine, Department of Emergency Medicine, Bassett Healthcare

Michael Hodgman, MD is a member of the following medical societies: American College of Medical Toxicology, American College of Physicians, Medical Society of the State of New York, and Wilderness Medical Society

Disclosure: Nothing to disclose.

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Hemlock. Photo by Cornell University Poisonous Plants Informational Database
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