St. John’s Wort Toxic?

Updated 2026-03-06| 4 min read| 1,762 found this helpful

All plant parts (especially flowers and leaves) are toxic, with ~20% of toxicity retained even in dried feed

Primary toxic effect is photodermatitis: ulcerative, exudative skin lesions on sun-exposed unpigmented areas

Horses are uniquely sensitive to this toxin, with risk of severe neurological symptoms including coma

FAQ

Is St. John’s Wort toxic to Humans?

No specific human toxicity data provided in the source dataset; general clinical records indicate ingestion may cause mild to moderate photosensitivity reactions (Low)

Is St. John’s Wort toxic to Cats?

Photosensitization leading to ulcerative/exudative dermatitis on sun-exposed unpigmented skin, anorexia, restlessness, and possible convulsions (Moderate)

Is St. John’s Wort toxic to Dogs?

Photosensitization leading to ulcerative/exudative dermatitis on sun-exposed unpigmented skin, anorexia, restlessness, and possible convulsions (Moderate)

Is St. John’s Wort toxic to Horses?

Extreme species sensitivity: photosensitive dermatitis with high risk of secondary infection, inappetence, uncoordinated movement, convulsions, and potentially fatal coma (Moderate)

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Toxicity Details

Humans

Low

No specific human toxicity data provided in the source dataset; general clinical records indicate ingestion may cause mild to moderate photosensitivity reactions

Cats

Moderate

Photosensitization leading to ulcerative/exudative dermatitis on sun-exposed unpigmented skin, anorexia, restlessness, and possible convulsions

Dogs

Moderate

Photosensitization leading to ulcerative/exudative dermatitis on sun-exposed unpigmented skin, anorexia, restlessness, and possible convulsions

Horses

Moderate

Extreme species sensitivity: photosensitive dermatitis with high risk of secondary infection, inappetence, uncoordinated movement, convulsions, and potentially fatal coma

Toxicity Snapshot

Toxic PartsWhole plant, with highest toxic compound concentration in flowers and leaves; toxin levels vary significantly between individual plants

Exposure RouteIngestion of fresh or dried plant material, contaminated feed, or unregulated herbal supplements containing St. John’s Wort

Toxic CompoundsHypericin; Pseudohypericin; Flavonoids (including Hyperoside); Essential oils; Tannins; Antibiotic active compounds; Phenolic acids

First Aid & Prevention

First 30 Minutes

• Move the exposed animal immediately to a fully shaded indoor space to stop further phototoxic damage
• Remove all St. John’s Wort plant material, contaminated feed, or supplements from the animal’s access
• Gently rinse exposed unpigmented skin areas with cool clean water to remove residual plant sap

Contact Vet Immediately

• Any visible skin redness, edema, scabbing, or discharge on sun-exposed skin after confirmed or suspected ingestion
• Presence of neurological symptoms including restlessness, convulsions, uncoordinated movement, or lethargy
• Confirmed ingestion in horses, or if the animal refuses to eat after exposure

Prevention

• Eradicate all St. John’s Wort growth from pastures, pet yards, and areas accessible to companion animals and livestock
• Inspect hay and dried feed batches for contamination, as ~20% of the plant’s toxicity is retained when dried
• Do not administer unregulated human St. John’s Wort supplements to pets or livestock

Treatment & Prognosis

• Perform decontamination per standard emergency toxicology protocols (e.g. activated charcoal for recent ingestion)
• Administer symptomatic care: topical or parenteral antibiotics with glucocorticoids for skin lesions, zinc cod liver oil paste, and niacin supplementation
• Monitor closely for secondary skin infection and neurological deterioration, especially in exposed horses
• Antihistamines are not recommended as they have very low efficacy for this specific toxicity

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Professional Toxicology

Scientific NameHypericum perforatum

AliasesKlamath Weed

Toxicity Grade

HumansLow

CatsModerate

DogsModerate

HorsesModerate

Target Organs

SkinCentral nervous systemGastrointestinal tract

Mechanism of Action

• Hypericin, the main toxic principle, accumulates in skin cells after ingestion

• UV radiation from sunlight activates hypericin, triggering inflammatory phototoxic reactions that damage skin tissue

• Systemic absorption can cause central nervous system impairment, especially in highly sensitive species like horses

Other Species Risk (CliniTox)

Information from CliniTox veterinary text.

Cattle: Confirmed poisoning cases reported, presenting with photosensitive dermatitis and systemic symptoms

Sheep: Confirmed poisoning cases reported, with photodermatitis affecting sun-exposed unpigmented skin

Pig: Confirmed poisoning cases reported linked to ingestion of fresh or dried contaminated plant material

Confusion Risk

The following plants look similar and may be confused:

Other St. John’s Wort species Hypericum spp.

Visually similar to common St. John’s Wort; most other Hypericum species also contain hypericin and carry equivalent toxicity risks

Groundsel species Senecio spp.

Visually similar wild-growing plants; Senecio species are far more toxic, containing pyrrolizidine alkaloids that cause irreversible fatal liver damage

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Related Toxic Plants

Tutsan

Hypericum androsaemum

Moderate toxicity, associated with photosensitivity reactions in animals

Gamboge

Garcinia hanburyi

Severe toxicity, causes severe gastrointestinal irritation and multi-organ damage

Autograph Tree

Clusia rosea

Moderate toxicity, causes contact dermatitis and gastrointestinal upset on ingestion

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Data Sources

1. ASPCA Animal Poison Control

2. CliniTox (Universität Zürich)

This information is for reference only and does not constitute medical advice. If ingestion or poisoning is suspected, seek medical or veterinary attention immediately.