Is St. John's Wort Poisonous? – Safety Guide

Yes, St. John’s wort (Hypericum perforatum) is poisonous. It is toxic to humans, dogs, cats, and horses. The danger comes from hypericin, a compound that causes photosensitivity: once ingested, the skin becomes abnormally reactive to sunlight, leading to severe sunburn-like reactions on exposed skin.

The flowers and berries contain the highest concentration of hypericin. The plant is more toxic when fresh than dried.

What Makes St. John’s Wort Toxic

Hypericin is the primary culprit. When you swallow the plant, hypericin enters your bloodstream and travels to the skin. There it reacts with UV light, causing damage at the cellular level - essentially a chemical sunburn from ordinary sunlight exposure.

Hyperforin, another compound in the plant, may also contribute to toxicity, though it is less studied. The combination makes every part of the plant a risk, though the flowers and seed pods carry the most concentrated dose.

Fresh plants are significantly more toxic than dried material. Livestock poisoned through pasture grazing face a much higher dose than animals exposed to hay.

Is St. John’s Wort Poisonous to Humans

Yes, but the severity depends on how much is eaten.

Children are at the highest risk - they are most likely to eat the bright yellow flowers or berries out of curiosity. In humans, ingestion typically causes digestive upset followed by a painful skin reaction on any sun-exposed areas: face, arms, neck. Fever and lethargy can follow.

Adults who take St. John’s wort as an herbal supplement face a different but serious risk. The same plant sold as a mood booster carries dangerous drug interactions. It reduces the effectiveness of oral contraceptives, interacts dangerously with SSRIs (risking serotonin syndrome), and interferes with blood thinners and anti-seizure medications. This is separate from accidental plant poisoning - but if you are searching this page, you may not have known either risk existed.

If someone has ingested St. John’s wort - whether supplement or garden plant - call poison control immediately: 1-800-222-1222. Do not induce vomiting unless a medical professional directs you to.

Is St. John’s Wort Poisonous to Dogs and Cats

The ASPCA confirms St. John’s wort is toxic to both dogs and cats. Any ingestion warrants a call to your veterinarian or the pet poison hotline at (855) 764-7661.

Symptoms in pets include photosensitivity with visible skin damage on sparsely furred areas - ears, nose, belly, and around the eyes are common targets. Blisters, redness, and lesions develop where UV light hits. You will also likely see vomiting, diarrhea, loss of appetite, fever, and restlessness.

Light-colored and white-furred cats are especially vulnerable. Contact dermatitis can also occur if a pet brushes against the flowers and then gets sun exposure on their fur.

There is no well-established threshold for how much will poison a dog or cat. Treat any ingestion as an emergency.

Is St. John’s Wort Poisonous to Horses and Livestock

Horses and cattle are the most sensitive livestock. As little as 1% of an animal’s body weight in fresh St. John’s wort can cause serious poisoning. Sheep are more tolerant, tolerating up to roughly 4% of body weight before showing symptoms.

Pale-skinned animals suffer the most visible damage - reddening, swelling, and skin sloughing on the ears, mouth, nose, and udder. In severe cases, the damage extends to internal tissues.

If livestock are affected, move them to shade immediately and call a large animal vet. Remove them from the pasture entirely. Dried hay is significantly less dangerous than fresh plant material, but do not assume a recovered animal is safe to re-expose.

What to Do If Someone Ingests St. John’s Wort

For humans: Call poison control at 1-800-222-1222. Bring a photo of the plant if possible - accurate identification matters for treatment. Do not induce vomiting unless directed.

For dogs or cats: Contact your vet or the pet poison hotline at (855) 764-7661. Bring a plant sample or photo. Keep your pet calm and in shade while you arrange help.

For livestock: Move the animal to a dark, covered area immediately. Contact a large animal vet. Note which pasture the animal came from so the source can be identified and managed.

Taking a photo of the plant for identification helps professionals decide on treatment faster. Accurate species identification is critical - St. John’s wort is identifiable by its opposite leaves with translucent oil glands (hold a leaf up to light - you will see tiny light dots) and its characteristic five-petaled yellow flowers.

Preventing St. John’s Wort Poisoning in Pets and Children

If you have the plant in your garden, remove it from areas accessible to pets and young children. Container gardening is the safest option if you want to keep it. Clean up all pruning debris immediately - fallen flowers and seed pods are especially attractive to curious children and animals.

Do not compost plant material that you suspect has been consumed by a poisoned animal. Handle dead plant material with gloves and wash hands thoroughly after working with it.