Poison Hemlock Poisoning

Medically Reviewed by Dany Paul Baby, MD on November 04, 2022
5 min read

The name “poison hemlock” tells you what you need to know about this plant. While the plant itself looks like a typical wild weed, it has an awful smell to warn you of the poison within. 

Poison hemlock (Conium maculatum) is a weedy plant that can typically be found in or near bike paths, ditches, farms, fields, rivers, and railroads. It’s part of the Apiaceae family, which includes plants like parsnip, carrots, parsley, and dill. Despite being in the same family as some common herbs, poison hemlock has a musty, unpleasant smell.

The plant is native to Europe and North Africa but is an invasive species in North America. It was brought over to the U.S. from Europe in the 1800s. Called “winter fern,” it was advertised as a good garden plant thanks to its pretty white flowers.

Currently, poison hemlock can be found in all states within the U.S. except Florida, Mississippi, Alaska, and Hawaii. In Canada, it can be found in British Columbia, Alberta, Saskatchewan, Ontario, and Quebec. Some states are actively working to get rid of this invasive plant and prohibit or restrict the transportation or sale of poison hemlock. If you discover poison hemlock in the wild, let your state’s department of agriculture know.

So, what does poison hemlock look like? When poison hemlock is young, it starts as a rosette that grows low to the ground. As it matures, it becomes a tall, flowering stalk that can reach up to 8 feet tall.

Poison hemlock plants produce flowers from May through August. The flowers are tiny and white with five petals and grow in umbrella-shaped clumps. These clumps are usually 2 or 3 inches across.

The leaves of poison hemlock are dark green and can be up to 18 inches long. They have a fern-like look to them and alternate on the stem. Poison hemlock stems are hollow and light green with purple spots, especially near the base of the stem. Though the stems are smooth, the veins of the stem may make it look ridged.

The entire poison hemlock has a terrible but unique smell similar to mouse urine.

The name “poison hemlock” is no joke. All parts of the plant are highly toxic to humans and animals. Poison hemlock is most dangerous when eaten, but the plant's toxins can also be absorbed through the skin or breathed in.

The primary toxin in poison hemlock is coniine. Coniine stops the nervous system from working properly, which can lead to suffocation. Hemlock was used in ancient Greece to kill prisoners, and while there's no way to say for certain, evidence indicates that hemlock may have been the poison used to kill Socrates in 399 BCE.

Poison hemlock poisoning in humans typically happens when someone mistakes part of a poison hemlock plant for a wild herb like parsnip, parsley, or anise. There have been cases of children using the hollow stems as whistles, leading to death. Dead stems can contain poison for up to three years.

While the amount of poison can vary between plants, plants grown in high-sun areas and in the southern U.S. tend to have higher levels of poison. Drought conditions can also increase the amount of poison in a poison hemlock plant. All poison hemlock plants should be avoided, as even a small amount can cause poisoning. Symptoms can start after ingesting just 3 milligrams of coniine, and over 150 to 300 milligrams, about the amount in six to eight poison hemlock leaves, can be fatal.

Symptoms of poison hemlock poisoning can start within minutes of ingesting the plant. Common symptoms include:

In severe hemlock poisoning, some symptoms may be delayed, including:

Poison hemlock is incredibly toxic, but poisoning is preventable. Never eat unfamiliar plants. You should remove any poison hemlock on your property for safety. To safely remove poison hemlock:

  • Dig the plant out by the roots, working in small patches.
  • You can use herbicides in late fall or early spring but not while the flowers are blooming.
  • Place the uprooted plants in a plastic bag and put them in the trash.
  • Don't burn, cut, or mow poison hemlock, as the fumes can be toxic.
  • When removing the plant, wear protective gear that includes thick gloves, long sleeves, a face mask, and protective eyewear.
  • Shower after removing the plant to remove any toxic sap from your skin.
  • Wash the clothes you were wearing to remove any plant residue.

Poison hemlock poisoning can quickly become very serious. If you or a loved one has been exposed to poison hemlock, you should go to the nearest emergency room immediately. If you’re unsure, you can contact poison control through their website or their toll-free hotline at 1-800-222-1222.

Most of the time, poison hemlock poisoning can be treated in the hospital. There is no cure or antidote for poison hemlock. Instead, treatment focuses on cleansing the toxins from your system and supportive measures to keep your body stable. Treatments may include:

  • Activated charcoal and other methods of ridding the toxin from your body
  • Antiseizure medication to control seizing
  • Hemodialysis for kidney failure
  • IV fluids to prevent dehydration and supplement nutrients
  • Mechanical ventilation to help with breathing

The severity of your symptoms and your recovery time will depend on factors like how much exposure you had and how healthy you were before exposure.

There are several safe plants that look similar to poison hemlock. These include:

  • Angelica (Angelica atropurpurea): Has a completely purple stem and the leaves are oblong, not fern-like
  • Common elderberry (Sambucus canadensis): The stems are woody and don’t have purple spots
  • Tall meadow rue (Thalictrum dasycarpum): Sometimes has purple spots on the stem, but the leaves are not fernlike
  • Yarrow (Achillea millefolium): Leaves are fernlike but longer and less triangle-shaped