Is White Snakeroot Poisonous? – Toxic to Humans & Animals

Yes. White snakeroot (Ageratina altissima) is highly poisonous to humans, dogs, cats, and horses. All parts of the plant contain tremetone and related ketones (collectively called tremetol). The plant is dangerous fresh or dried. Humans are most commonly poisoned through milk from affected cows or goats, but direct ingestion of any plant part is a real medical concern. Deaths have occurred in both humans and livestock.

What Makes White Snakeroot Toxic?

Tremetone is the primary toxic agent, along with dehydrotremetone and related ketones. These compounds cause skeletal muscle necrosis and cardiotoxicity. The toxin accumulates in the body over time, which is why symptoms may not appear for days or even weeks after exposure.

Both green and dried plants are toxic. Tremetol passes through the milk of lactating animals, which is why humans who drink raw milk or eat dairy from exposed animals can develop “milk sickness.” The same mechanism poisons calves, foals, and other young livestock nursing from exposed mothers.

The toxin does not break down when the plant is dried or baled into hay, making contaminated forage a persistent risk for grazing animals.

Which Parts of White Snakeroot Are Toxic?

All parts - leaves, stems, flowers, roots, and seeds. The toxin is distributed throughout the plant. Even small quantities can cause symptoms in sensitive individuals or smaller animals.

Symptoms of White Snakeroot Poisoning

Symptoms vary by species and depend on the amount ingested versus body weight.

Humans (milk sickness): Weakness, loss of appetite, nausea, vomiting, abdominal pain, severe thirst, tremors, acetone-scented breath, delirium, coma, and death in severe cases. Symptoms develop over days to weeks as the toxin accumulates. Direct ingestion of plant material produces the same symptom profile faster.

Dogs and cats: Vomiting, diarrhea, lethargy, tremors, excessive salivation, labored breathing, weakness, and collapse. Onset ranges from days to weeks depending on dose and animal size.

Horses and livestock: The characteristic “trembles” - visible muscle tremors, weakness, stiffness, weight loss, constipation, acetone-scented breath, cardiac arrhythmias, and terminal collapse. Fatalities are common in untreated severe poisoning.

Children: Same symptom profile as adults. Smaller body weight means a proportionally greater risk from any ingestion. Children who put plant material in their mouth - a realistic scenario for toddlers - are at real risk.

What to Do If White Snakeroot Is Ingested

1. Do not induce vomiting unless a medical professional or veterinarian specifically directs you to do so.
2. Contact poison control (1-800-222-1222 in the US) or your veterinarian immediately.
3. If possible, photograph the plant to confirm white snakeroot identification. Bring a plant sample if you are instructed to visit a clinic.
4. If livestock are involved, isolate them from the plant immediately. Do not consume or sell milk or dairy from affected animals.
5. Monitor for symptoms for 24 to 48 hours. Keep in mind that symptoms can take days to weeks to appear.
6. Bring the plant sample to the vet or ER if instructed to visit in person.

Early intervention matters. The toxin accumulates, but symptoms caught early and managed supportively give a significantly better outcome.

Why “Milk Sickness” Matters - The Historical Context

White snakeroot caused “milk sickness” throughout the 18th and 19th centuries before the link was understood. Cows that ate the plant passed tremetol through their milk. Anyone who drank that milk - or ate butter, cheese, or other dairy from those cows - accumulated the toxin over time.

Nancy Hanks Lincoln, Abraham Lincoln’s mother, died from milk sickness in 1818 at a frontier settlement in Illinois after drinking raw milk from a family cow that had grazed on white snakeroot. The condition was common in rural communities where families relied on a single cow. The toxin was not identified until the early 20th century.

Commercial dairy operations today pool milk from many herds, which dilutes any tremetol to negligible levels. Raw milk drinkers, homesteaders with small dairy operations, and anyone in rural areas where white snakeroot grows adjacent to pastures or woodlots are still at genuine risk.

Safe Handling and Garden Precautions

White snakeroot grows in shaded woodlands and wooded edges across eastern North America. It may appear in yards near wooded areas, along fence rows, or in shaded garden corners.

If you weed around the plant:

• Wear gloves. Skin contact can contribute to toxin absorption, especially if you have cuts or abrasions.
• Wash your hands thoroughly after handling, even with gloves.
• Do not burn the plant. Smoke can carry the toxin and cause respiratory exposure.
• Remove the plant by the root before it sets seed. Pull when soil is moist for easier removal.
• Keep dogs and children away from any area where white snakeroot grows.

How to Identify White Snakeroot

White snakeroot (Ageratina altissima), formerly classified as Eupatorium rugosum:

• 1 to 5 feet tall, perennial
• Opposite, serrated leaves with long petioles (stalks)
• Clusters of small white flowers at the top of stems, typically July through September
• Found in shaded woods, woodland edges, and shady pastures across eastern North America

Most common look-alike: Late boneset (Eupatorium serotinum)

These two are frequently confused. White snakeroot has broader, shorter, toothed leaves and grows in shadier, drier spots up to 5 feet tall. Late boneset has longer, narrower leaves, prefers sunnier, wetter areas, and can reach 6.5 feet.

If unsure, treat any unknown plant as potentially toxic and remove it following the handling precautions above.

Frequently Asked Questions

Is white snakeroot poisonous to dogs and cats? Yes. Both dogs and cats can be poisoned by ingesting any part of white snakeroot. Symptoms include vomiting, diarrhea, lethargy, tremors, excessive salivation, and weakness. Contact your veterinarian immediately if you suspect ingestion.

Can white snakeroot kill a horse or cow? Yes. White snakeroot kills cattle, horses, goats, and other grazing animals. Deaths are common in untreated severe poisoning. The “trembles” in livestock are a hallmark sign.

How did Abraham Lincoln’s mother die from white snakeroot? Nancy Hanks Lincoln died of milk sickness in 1818. She drank raw milk from a family cow that had grazed on white snakeroot. The tremetol passed through the milk and accumulated in her body over weeks until it reached a fatal threshold.

Is it safe to drink raw milk if white snakeroot grows nearby? No. If white snakeroot grows in an area accessible to cows or goats whose milk you drink, there is real risk. The toxin passes directly through the milk. Commercial dairy is generally safe because pooled milk dilutes any tremetol to negligible levels. Small-scale raw milk operations do not have that buffer.