We use essential cookies (authentication, your saved goals/stack) by default. With your permission we’ll also enable privacy-respecting analytics (Vercel Web Analytics, anonymous load-time metrics) and error-replay diagnostics (Sentry — DOM snapshots only when an error fires) so we can fix bugs faster. Learn more about cookies
Comfrey (Symphytum officinale)
A traditional poultice herb containing unsaturated pyrrolizidine alkaloids, which are hepatotoxic and carcinogenic. Comfrey is described here so that it can be SCREENED — it is never recommended, and internal use has caused fatal hepatic veno-occlusive disease.
Research compound — not a dietary supplement
Comfrey is a research compound, not a regulated dietary supplement. It is sold for research or off-label use. The evidence below is largely preclinical (animal and in-vitro) or early-stage, so no evidence score is assigned. This page is provided for transparency and education — it is not a recommendation to use. Consult a qualified healthcare provider, and be aware that purity, dosing, and legal status vary by jurisdiction.
What the evidence says
Most Comfrey studies are mechanism or observational rather than RCTs that measure a clinical effect — keep findings provisional.
Most evidence is from high-quality studies published 2000–2026.
Based on 4 studies
Confidence
Very low confidenceBy outcome
Scores at the floor for internal use. What controlled evidence exists concerns topical preparations; the oral literature is toxicological — hepatic veno-occlusive disease, cirrhosis and death from pyrrolizidine alkaloids.
No trials currently enrolling · 1 completed on ClinicalTrials.gov
Registered trials show research momentum for Comfrey, not proof of effect — a registration is a plan, and posted results are sponsor-reported, not peer-reviewed. They are never counted toward the evidence rating above.
Browse these trials on ClinicalTrials.govClinicalTrials.gov · as of Oct 2026
Comfrey (Symphytum officinale) has a long history as a topical poultice for sprains and bruises — the name 'knitbone' records that use.
Its hazard is chemical rather than traditional: the plant contains unsaturated pyrrolizidine alkaloids, which are bioactivated in the liver to pyrrolic esters that damage the sinusoidal endothelium. The result is hepatic veno-occlusive disease, and case reports describe cirrhosis and death after oral use.
The same alkaloid class caused endemic outbreaks in Jamaica, India and Afghanistan through contaminated cereals and bush teas. It is described so that it can be recognised and avoided, not as an option: it is linked to no goal and is left out of every recommendation.
A reader who scans a product or types the name is told what it is, instead of being told it could not be identified.
Hepatic CYP450 converts the unsaturated alkaloids to reactive pyrrolic esters, which bind protein and DNA and injure the hepatic sinusoidal endothelium.
Endothelial injury obstructs the small hepatic veins, producing the syndrome that has proved fatal after oral comfrey.
Avoid — pyrrolizidine alkaloids cross the placenta and are hepatotoxic and genotoxic.
Avoid — pyrrolizidine alkaloids pass into breast milk and an infant's liver is the target organ.
Avoid — the injury mechanism is hepatic and a damaged liver has less reserve.
Avoid — never give this herb to a child.
Injury is hepatic and additive with any other agent that damages the liver.
Induction increases conversion of the alkaloids to their reactive metabolites.
Tip: Seek medical assessment — these are features of veno-occlusive disease
Tip: Emergency medical care — this has been fatal
The current evidence for Comfrey is insufficient to assign an evidence score, based on 4 indexed studies. A traditional poultice herb containing unsaturated pyrrolizidine alkaloids, which are hepatotoxic and carcinogenic. Comfrey is described here so that it can be SCREENED — it is never recommended, and internal use has caused fatal hepatic veno-occlusive disease. Representative study: PMID 32105669.
Dosing for Comfrey: No appropriate oral dose. The herb is described here so it can be screened, not taken.
Timing is flexible for Comfrey — consistent daily use matters more than the time of day. No oral dose is appropriate; no timing guidance is given.
No consumer dose of Comfrey is endorsed, so there is no dose at which it can be called safe. Talk to a healthcare provider before taking it. Reported side effects are uncommon and include abdominal pain, nausea, ascites, progressive liver failure. Use caution if any of these apply to you: Any liver disease — do not take; the alkaloids are a known hepatotoxin; Pregnancy — do not take; the alkaloids cross the placenta; Breastfeeding — do not take; the alkaloids pass into breast milk.
Blue Cohosh
Too few graded studiesA herb traditionally used to induce labour, and a serious teratogen. Blue cohosh is described here so that it can be SCREENED — it is never recommended, and there is no dose at which it is appropriate in pregnancy or breastfeeding.
Pennyroyal
Mostly mechanism / observationalA mint-family herb used historically as an abortifacient, and a documented cause of fatal liver failure. Pennyroyal is described here so that it can be SCREENED — it is never recommended, and the oil in particular has no safe dose.
Psyllium Husk
Likely helpsGel-forming soluble fiber that lowers LDL cholesterol, regulates blood sugar, and promotes digestive regularity.
St. John's Wort
Likely helpsModulates serotonin, dopamine, and norepinephrine; studied for mood, with trials in mild-to-moderate depression comparing it to SSRIs — requires drug-interaction caution.
Reviewed by Dr. Baher Al Hakim · Last reviewed August 2026 · evidence from 4 studies · how we score · editorial policy
This information is for educational purposes only. It is not a substitute for professional medical advice. Always consult a qualified healthcare provider before starting, stopping, or changing any supplement or medication.