What Is Hemlock Poisoning?

  • Finley Hansen BSc, Neuroscience, Cardiff University / Prifysgol Caerdydd, UK
  • Ellen Rogers MSc in Advanced Biological Sciences, University of Exeter, UK
  • Sophie Downton BSc, Biomedical Sciences, University of Reading, UK


“Hemlock" is commonly used to refer to a group of plants known for their toxic properties and is a term that has evolved from its historical connection to both accidental and deliberate poisonings. Notably, the term "hemlock" gained prominence as the substance used to execute the renowned philosopher Socrates. However, it was also used to execute criminals and political prisoners. It has also found its place in literature, featuring in works such as Shakespeare's 'Macbeth' and Keats 'Ode to a Nightingale', which both alluded to its potential as a means of death. Intentional cases of hemlock poisoning are now highly uncommon, but there are still occasional reports of humans and livestock accidentally ingesting these dangerous plants.

If you think you may have eaten this plant, call emergency services immediately or go to the nearest emergency room. Severe symptoms, such as seizures, may occur within 15 minutes of ingestion.

What is hemlock?


“Hemlock” refers to a group of plant species belonging to the Apiaceae or Umbelliferae family. This family is often referred to as the "parsley" family, and it encompasses various edible vegetables (like carrots, celery, and parsley) as well as toxic species of hemlock. These plants tend to thrive in grasslands and damp environments.

The family name 'Umbelliferae' derives from the plants' unique flower arrangement, which resembles open umbrellas with clusters of short stems. This distinctive petal pattern serves as the basis for categorising these plants within the family rather than their shared ancestry. As such, the plants in this family often differ significantly in terms of their habitat, height, colour, edibility, toxicity levels, and nutritional value.

Given their shared petal pattern, people often confuse the plants within this family - even though they often belong to entirely different genera or species. For instance, the (generally) harmless Giant hogweed (Heracleum mantegazzianum) closely resembles the highly poisonous 'water hemlock' (Cicuta maculata). Interestingly, giant hogweed merely causes skin rashes.1 Similarly, wild carrot (Daucus carota) bears a striking resemblance to poison hemlock (Conium maculatum), another extremely poisonous hemlock species.

Types of hemlock plants

While certain hemlock species are highly poisonous, the toxicity levels of different family members can vary significantly. Furthermore, some non-poisonous hemlock plants look very similar to their poisonous relatives - so it is extremely important to be certain of which plant you come into contact with before prolonged exposure and or ingestion.

Poisonous hemlock plants

There are three primary poisonous hemlock plants:

  1. Poison hemlock (Conium maculatum)
  2. Water hemlock (Cicuta species)
  3. Hemlock water dropwort (Oenanthe species)

These plants share a striking visual resemblance; all three species possess small white flowers situated at the tips of umbrella-like stalks and are highly toxic to both humans and livestock. Further, poison and water hemlock can both be found in Europe, North America, South America, Australia, and New Zealand, but their specific habitats are quite different.2 

Poison hemlock is found on roadsides, waste areas, field edges, and damp locations such as streams and irrigation ditches.3 When crushed, it emits a pungent odour.2 On the other hand, water hemlock and dropwort are exclusively found near water sources and lack poison hemlock’s finely divided, lacy leaves. They grow only a few feet tall; poison hemlock, with its fern-like leaves, can reach a mature height of 6-10 feet.4 

Plants that look like poison hemlock, water hemlock and hemlock water dropwort

  • Giant hogweed (Heracleum mantegazzianum)
  • Wild carrot (Daucus carota)
  • Cow Parsnip (Heracleum maximum)
  • Wild parsnip (Pastinaca sativa)
  • Wild Angelica (Angelica sylvestris)
  • Bishop's Weed (Aegopodium podagraria)
  • Hemlock Parsley (Conioselinum spp)
  • Wild Chervil (Anthriscus sylvestris)

Some of these plants are mildly toxic to humans and can be safely consumed in small amounts. However, some of these species, such as Wild Angelica, might still be classified as 'slightly toxic' due to their ability to cause mild skin irritation. Others, like wild carrots, could be harmful to humans and animals when ingested in large quantities. However, this level of mild toxicity significantly contrasts with the dangerous toxicity of poison hemlock, water hemlock, and hemlock water dropwort, which can prove fatal to humans and animals even in very small doses.

How does hemlock poisoning occur?

Method of exposure: ingestion

Hemlock poisoning is most commonly observed in children and young adults, with a mortality rate estimated between 30-70%.6 However, this mortality rate varies based on various factors, such as the amount of hemlock ingested and the age of the individual.

Casual skin contact with any part of poisonous hemlock plants is not hazardous, as the toxin resides within the plant's tissues rather than on its surface. Lethal poisoning only occurs when any plant part is ingested.  You can also get hemlock poisoning by consuming the produce from an animal that has ingested the plant, such as milk from a poisoned cow.7 

The toxins may also be absorbed through the skin. Additionally, inhaling crushed plant material may also cause lethal toxicity.

Each of the three poisonous hemlock plants (listed above) contains a distinct and highly toxic chemical. While the lethal compounds in these plants have been identified and their general effects understood, the precise biological mechanism leading to death after hemlock poisoning remains unknown. Further, although there is some overlap in the active chemicals of these three plants, the symptoms and mechanisms of their poisoning vary slightly. These differences, albeit subtle, are not yet comprehensively understood. We will now discuss the known active chemicals of each poison hemlock.

Poison hemlock - coniine

The most toxic chemical in poison hemlock is coniine. Coniine is present in all tissues of the plant, including the seeds. In humans and animals, coniine disrupts both the nervous system and muscular systems, disrupting their communication and eventually causing paralysis, suffocation, and death. As such, it is known as a neurotoxin. Coniine can also cause birth defects if ingested during pregnancy and is thus called a teratogen.2

Coniine affects your brain chemistry in a similar way to nicotine, as it works on the same type of biological receptor. These are known as nicotinic acetylcholine receptors. However, coniine is highly potent and significantly disrupts these receptors. Eventually, your muscles stop responding to the chemicals which normally activate them - causing paralysis, loss of vital breathing muscles, and death by suffocation. There is, however, an odd course of symptoms during death where respiration is first stimulated, then depressed, which causes a lack of oxygenated blood to reach vital organs and respiratory failure.2

Water hemlock and water dropwort – cicutoxin and oenanthotoxin

The most toxic chemical in water hemlock is cicutoxin, which is also a neurotoxin. Cicutoxin works more like a different poison called strychnine and acts directly on the nervous system.

It is thought to stop GABA receptors from working. GABA receptors normally slow down electrical activity between nerves, so when they are blocked, the nervous system goes into overdrive, causing seizures. These seizures are thought to cause cardiopulmonary arrest, which is the abrupt stopping of the heart's pumping and the lungs' ability to provide oxygen to the body.8

Dropwort contains oenanthotoxin, which is related to cicutoxin and is thought to have a similar neurochemical effect.8 

The ingestion of cicutoxin or oenanthotoxin has been reported to cause nausea, vomiting, excessive sweating, and repeated increasingly severe convulsions - with death occurring within 10 hours.9  However, death may occur in under 2 hours.

Just how lethal are these plants?

All parts of these poisonous hemlock plants can be fatally toxic if ingested. However, the amount of toxin in each plant part can vary. Determining the exact lethal dose of these toxins, or the plant containing them, is challenging due to variations in the poison within plant parts and an individual’s sensitivity to the poison. Additionally, environmental conditions, season, and plant location can affect toxin levels,  making it even harder to pinpoint a precise lethal amount for any hemlock plant.8

Animals' reactions to the coniine toxin vary. For instance, a daily intake of ~1kg of poison hemlock is considered deadly for cows, while pigs seem more tolerant. However, humans will experience poisoning symptoms after consuming as little as 3mg of coniine, tolerating up to 150-300mg. This is equivalent to around 6g or 6-8 leaves.2

Like coniine, the lethal doses of cicutoxin and oenanthotoxin can differ between individuals. There have been several documented cases of accidental poisoning by water hemlock in humans. For example, college students on a white water rafting trip mistakenly ate water hemlock roots. One individual consumed a single root, experiencing multiple short seizures over 3 hours before recovering. The other consumed 2 roots, enduring intermittent seizures and ultimately passing away 2 and a half hours later.8

Symptoms of hemlock poisoning

Signs and symptoms of coniine (poison hemlock) poisoning 

Ingestion of lethal amounts of poison hemlock induces respiratory failure. The following symptoms may also occur:2,10 

  • Mouth irritation
  • Muscle weakness
  • Lack of coordination
  • Trembling
  • Excessive salivation
  • Rapid and weak pulse
  • Loss of appetite
  • Dilated pupils
  • Convulsions
  • Coma
  • Paralysis
  • Seizures
  • Nausea and vomiting
  • Heavy legs
  • Numbness

After a lethal dose, symptoms may appear within 1 hour of consumption, followed by respiratory paralysis within 2-3 hours. Generally, symptoms improve after 10 hours of ingestion.

Signs of cicutoxin/oenanthotoxin (water hemlock) poisoning

The characteristic symptoms of water hemlock/dropwort poisoning are increasingly severe seizures and violent muscle activity. The order of symptom onset is well-documented: 

Stage 1 -  uneasiness and muscle twitching

Stage 2 - evident seizures, with jaw champing and frenzied activity

Stage 3 - bellowing and violent seizures

 Other signs and symptoms may include:

  • Nausea
  • Vomiting
  • Muscle weakness
  • Uneasiness
  • Dizziness
  • Excessive salivation
  • Sweating
  • Muscle twitching of shoulders, nose, lips, face, ears.
  • Loss of coordination
  • Pupil dilation
  • Chest spasms
  • Tonic-clonic seizures

Symptoms may begin within 30 minutes of ingestion, and neurological symptoms tend to resolve within 3-4 hours. However, severe symptoms (such as seizures) may persist for several days. Further, residual effects such as muscle soreness and coordination issues may persist for months. 

Treatment for suspected poisoning by water hemlock, hemlock water dropwort, and poison hemlock

Get urgent medical attention

Seek medical help immediately if any hemlock poisoning is suspected. It is crucial that you seek treatment within 24 hours, as prompt medical intervention can be life-saving. With adequate treatment, survival beyond 8 hours, without life-threatening symptoms or complications, indicates a positive prognosis.8 


Diagnosis involves assessing blood gases, electrolytes, and a plant sample.2  

Hospitalisation and first aid management 

Rapid admission to a medical facility is vital due to the swift onset of poisoning symptoms. Immediate steps for asymptomatic patients include gastric lavage followed by activated charcoal with a drug to induce vomiting to prevent toxin absorption. However, in symptomatic patients, such as those already violently seizing, artificial respiration techniques like intubation can be employed to save the patient's life. The focus is on ensuring proper respiratory function.2

Specific treatment

If patients are showing symptoms, managing seizures and supportive care are vital. Recommended treatments for cicutoxin and oenanthotoxin poisoning involve barbiturates and benzodiazepines, which can stop seizures by targeting the GABA receptors.2 While other treatments (like fluid replacement and dialysis to filter the blood) might be employed to manage symptoms, the most impactful interventions focus on controlling seizures and providing supportive care.9

Prevention and safety measures

To prevent hemlock exposure, especially outdoors, follow these guidelines:

Plant Identification: Learn to identify hemlock and distinguish it from similar plants. Be cautious around wild plants, especially those resembling hemlock. Avoid prolonged exposure to suspicious plants. 

Prompt but safe removal of hemlock plants: If hemlock is found on your property, you should take immediate action to eradicate it. Removal can be accomplished chemically or physically by digging. Regardless, you should take steps to ensure safety is maintained while doing this, such as covering skin and keeping any contaminated surfaces far from airways and skin.

Medical attention: Seek medical assistance if exposure or ingestion is suspected.


Hemlock poisoning, which is historically notorious for its role in historic executions (like that of Socrates), stems from toxic plants that pose serious health risks if ingested. This group includes poison hemlock, water hemlock, and hemlock water dropwort, which all resemble harmless vegetables but are highly toxic. Though intentional cases are rare today, accidental consumption remains a concern. 

Ingestion is the primary mode of exposure to hemlock, with symptoms varying among species. Poison hemlock's coniine disrupts the nervous and muscular systems, leading to paralysis and suffocation. Water hemlock and dropwort contain cicutoxin and oenanthotoxin, causing seizures and cardiopulmonary arrest.

Treatment calls for swift medical attention, seizure control, and supportive care. Preventive measures involve learning to identify these plants and seeking medical help if exposure is suspected, emphasising the importance of awareness and safety.


  1. Launer, J. (2021) ‘Hemlock and hemlock poisoning’, Postgraduate Medical Journal, 97(1152), pp. 678–680. doi:10.1136/postgradmedj-2021-140976.
  2. Hotti, H. and Rischer, H. (2017) ‘The killer of socrates: Coniine and related alkaloids in the plant kingdom’, Molecules, 22(11), p. 1962. doi:10.3390/molecules22111962.
  3. Agricultural Research Service (2018) Poison Hemlock (Conium maculatum), Poison Hemlock (conium maculatum) : USDA ARS. Available at: https://www.ars.usda.gov/pacific-west-area/logan-ut/poisonous-plant-research/docs/poison-hemlock-conium-maculatum/ (Accessed: 23 August 2023).
  4. Eric Anderson, I.B. and E.H. (2023) Poison Hemlock Identification and control, Agriculture. Available at: https://www.canr.msu.edu/news/poison-hemlock-identification-and-control (Accessed: 23 August 2023).
  5. Wild carrot (no date) Minnesota Department of Agriculture. Available at: https://www.mda.state.mn.us/plants/pestmanagement/weedcontrol/noxiouslist/wildcarrot (Accessed: 23 August 2023).
  6. Downs, C. (2002) ‘A hemlock water dropwort curry: A case of multiple poisoning’, Emergency Medicine Journal, 19(5), pp. 472–473. doi:10.1136/emj.19.5.472.
  7. López, T.A., Cid, M.S. and Bianchini, M.L. (1999) ‘Biochemistry of hemlock (conium maculatum L.) alkaloids and their acute and chronic toxicity in livestock. A Review’, Toxicon, 37(6), pp. 841–865. doi:10.1016/s0041-0101(98)00204-9.
  8. Burrows, G.E. and Tyrl, R.J. (2013) ‘Apiaceae Lindl’, in Toxic plants of North America. 2nd edn. John Wiley & Sons, pp. 63–66.
  9. Furbee, B. (2009) ‘Neurotoxic plants’, Clinical Neurotoxicology, pp. 523–542. doi:10.1016/b978-032305260-3.50053-8.
  10. Wendt, S. et al. (2022) ‘Poisoning by plants’, Deutsches Ärzteblatt international [Preprint]. doi:10.3238/arztebl.m2022.0124.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Finley Hansen

Bachelor of Science - BS, Neuroscience, Cardiff University / Prifysgol Caerdydd

Finley is a Neuroscience graduate with a culturally and practically diverse working background, with experiences in customer service, data analysis and research, healthcare, teaching and childcare. His roles have spanned the UK, India, Ghana and his current location in Thailand, where he works as a high school science and english teacher, online tutor and writer for Klarity Health.

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