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LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet]. Bethesda (MD): National Institute of Diabetes and Digestive and Kidney Diseases; 2012-.

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LiverTox: Clinical and Research Information on Drug-Induced Liver Injury [Internet].

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Last Update: October 30, 2017.



Comfrey is an plant belonging to the Borganinaceae family, extracts of the leaves and roots of which has been used as an herbal to treat wounds and to decrease pain and inflammation associated with arthritis, sprains and bone fractures. Comfrey, however, also contains pyrrolizidine alkaloids and, when taken orally, can cause sinusoidal obstruction syndrome and severe liver injury.


Common comfrey (Symphytum officinale) is a perennial herb belonging to the family Borganinaceae which is native to Europe and Asia, but is now found worldwide. Leaf and root extracts have many constituents including allantoin, rosmarinic acid, triterpene saponins, silicic acid, and tannins, believed to be the basis for its antiinflammatory and wound healing activity. However, comfrey also contains several pyrrolizidine alkaloids (symphytine, echimidine, symglandine and lycopsamine) which are toxic and capable of causing sinusoidal obstruction syndrome (previously called veno-occlusive disease) and severe liver injury. Comfrey has been shown to cause similar liver injury in laboratory animals and has also been linked to liver cancer. Comfrey products are marketed as herbal teas, root powders and as capsules. Oral comfrey has been banned or restricted in most countries, but topical forms (ointments, creams and liniments) are available and advertised as useful for wound healing sprains and bone fractures. Human studies have shown that comfrey creams have mild analgesic effects and decreases muscle and joint pain.


Several cases of acute liver injury resembling sinusoidal obstruction syndrome (SOS) due to oral comfrey have been published. The injury usually arises within 1 to 2 months of starting the comfrey product (either extract in tablet form or large amounts of comfrey tea) with onset of right upper quadrant pain, nausea and weight gain (from fluid retention) followed by jaundice. Serum aminotransferase levels are usually only mildly elevated with a hepatocellular pattern of injury, although they may be markedly increased if tested during the early phases of the injury. Immunoallergic and autoimmune features are usually not present. The injury can be severe and rapidly lead to acute liver failure (acute sinusoidal obstruction syndrome), but more commonly presents insidiously with weight gain, ascites, weakness, and minimal serum aminotransferase elevations (subacute or chronic sinusoidal obstruction syndrome).

Mechanism of Injury

The pyrrolizidine alkaloids contained in comfrey include intermedine, lycopsamine, symphtine and echnimidine, which are metabolized by the cytochrome P450 enzymes into highly toxic pyrrole metabolites which have alkylating properties that can damage hepatic endothelial cells and can cause sinusoidal obstruction. The amount of pyrrolizidine alkaloids in comfrey varies by the part of the plant used, its age and time of harvesting. The toxicity of pyrrolizidine containing substances is increased by microsomal enzyme inducers such as phenobarbital. Infants appear to be particularly susceptible to pyrrolizidine alkaloid injury.

Outcome and Management

Hepatotoxicity from comfrey is now rare, as it is widely accepted as being toxic when taken internally and oral formulations are restricted or banned in most countries. Management should be directed at limiting further injury and specific treatment of complications (ascites, variceal bleeding). Anticoagulants have not been shown to be beneficial. Difibrotide (a complex mixture of single stranded DNA prepared from pig intestine) has recently been approved for therapy of severe SOS accompanied by renal or pulmonary failure occurring after myeloablation for hematopoietic cell transplantation. More extensive description of sinusoidal obstruction syndrome and its management are given in the introductory section and in discussion of the antineoplastic alkylating agents.

Other Names: Black root, common comfrey, knitbone

Drug Class: Herbal and Dietary Supplements



Comfrey – Generic


Herbal and Dietary Supplements


Comfrey84696-05-9Herbal mixtureNot applicable


References updated: 30 October 2017

Abbreviations used: SOS, sinusoidal obstruction syndrome; HDS, herbal and dietary supplements.

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    (Review of hepatotoxicity of herbal and dietary supplements [HDS] mentions that comfrey is one of more than 350 plant species that contain pyrrolizidine alkaloids and is capable of causing sinusoidal obstruction syndrome).
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    (26 year old woman developed abdominal pain and ascites, having used herbal tea for 2 years [bilirubin 0.8 mg/dL, ALT 28 U/L, Alk P 303 U/L], with splenomegaly, varices and intractable ascites, biopsy showing sinusoid obstruction syndrome; dying after failed portocaval shunt, analysis of tea showed pyrrolizidine alkaloids).
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    (6 month old developed distended abdomen and vomiting [bilirubin 0.5 mg/dL, AST 794 U/L] and was found to have ascites and biopsy showed sinusoidal obstruction syndrome; child had been fed large quantities of locally brewed tea which was made from Senecio longilobus).
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    (Plants have evolved poisons that are toxic to insects, but can also cause hepatic sinusoidal obstruction syndrome (veno-occlusive disease) in grazing animals and in humans who ingest these in teas and herbal medications: “a disastrous discontinuity of tradition”).
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    (49 year old woman developed edema and ascites after taking comfrey capsules and tea for 6 months [no liver test results provided], biopsy showing sinusoidal obstruction syndrome, ultimately requiring portocaval shunt with subsequent slow clinical improvement after stopping comfrey ingestion).
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    (Four young women brewed Indian herbal tea to treat psoriasis and 3 developed ascites from sinusoidal obstruction syndrome 19-45 days later [bilirubin 0.6, 0.6 and 3.3 mg/dL, ALT 63, 122 and 69 U/L], one dying of hepatic failure, analysis of tea revealed pyrrolizidine alkaloids).
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    (Presence of pyrrolizidine alkaloids in several common Australian weeds is a health hazard to livestock and humans; these include Heliotropium europaeum, Echium plantagineum, Senecio and Crotalaria species).
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    (Analysis of two commercial brands of comfrey-pepsin preparations sold as a digestive aid demonstrated pyrrolizidine alkaloids [symphytine and symglandine] in concentrations that could cause sinusoidal obstruction syndrome after a few months of regular intake).
  • Weston CFM, Cooper BT, Davies JD. Veno-occlusive disease of the liver secondary to ingestion of comfrey. Br Med J 1987; 295: 183. [PMC free article: PMC1247038] [PubMed: 3115370]
    (13 year old boy developed fever, abdominal pain and ascites after several years of treatment with an herbal tea containing comfrey for Crohn’s disease [bilirubin 1.5 mg/dL, AST 87 U/L, Alk P normal], biopsy showing sinusoidal obstruction syndrome).
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    (5 day old girl found to have jaundice, hepatomegaly and ascites [bilirubin 9.6 mg/dL, ALT 760 U/L, protime 13%], biopsy and autopsy showing severe sinusoidal obstruction syndrome and analysis of an herbal tea taken daily by the mother during pregnancy revealed pyrrolizidine alkaloids).
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    (47 year old woman developed liver test abnormalities and ascites having taken comfrey pills and tea daily for more than a year [bilirubin 1.2 mg/dL, ALT 24 U/L, Alk P 27 U/L], biopsy showing chronic sinusoidal obstruction syndrome).
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    (Letter in response to Bach [1989] reviewing the history of the association of comfrey and pyrrolizidine alkaloids and sinusoidal obstruction syndrome).
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    (Letter in response to Bach [1989] and Ridker [1989] calling for better documentation of components of implicated hepatotoxic herbs in view of the variable amounts of pyrrolizidine alkaloids found in different comfrey samples due to different species of Symphytum, problems of contamination, mislabeling, use of leaves vs roots and variability in alkaloid content by time of harvest and growing conditions).
  • Ridker PM, McDermott WV. Comfrey herb tea and hepatic veno-occlusive disease. Lancet 1989; 1: 657-8. (Concise review of the hepatotoxicity of comfrey herb tea and its association with sinusoidal obstruction syndrome, probably due to the presence of pyrrolizidine alkaloids which comprise more than 180 compounds and occur in at least 8 plant families; four genera –.
    Heliatropium, Crotalaria, Senecio and Symphytum – accounting for most toxic ingestions).
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    (23 year old man who had eaten young comfrey leaves for 1-2 weeks developed fatigue and abdominal pain followed in the next 2 months by swelling and edema [bilirubin 1.6 mg/dL, AST 365 U/L, Alk P 475 U/L, INR 1.4], with intractable ascites and hepatic failure; autopsy showed sinusoidal obstruction syndrome).
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    (Editorial on problems of herbal poisoning highlighting pyrrolizidine alkaloids, comfrey, germander, and suggesting guidelines on their use [never during pregnancy or while nursing, avoiding daily and high doses of a single agent, using products from reputable sources and never using comfrey]).
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    (77 year old woman developed fatigue followed by jaundice 6 months after starting an herbal product with comfrey and skullcap [bilirubin 3.5 mg/dL, AST 520 U/L, Alk P 390 U/L], resolving within 6 months of stopping).
  • Sperl W, Stuppner H, Gassner J, Judmaier W, Dietze O, Vogel W. Reversible hepatic veno-occlusive disease in an infant after consumption of pyrrolizidine-containing herbal tea. Eur J Pediatr 1995; 154: 112-6. [PubMed: 7720737]
    (18 month old boy treated with locally prepared herbal tea for over a year presented with ascites and hepatic encephalopathy [bilirubin 2.8 mg/dL, ALT 124 rising to 923 U/L], biopsy showing sinusoidal obstruction syndrome, but gradual improvement and resolution of clinical symptoms and signs after stopping the tea; the plant leaves were likely Adenostyles alliariae [Alpendost] which is known to contain pyrrolizidine alkaloids).
  • Stickel F, Seitz HK. The efficacy and safety of comfrey. Public Health Nutr 2000; 3(4A): 510-8. [PubMed: 11276298]
    (Review of comfrey which has a long tradition as an external treatment for inflammatory arthritis and trauma; for internal application it was claimed to be beneficial for gastritis, ulcers, diarrhea and various allergies including asthma; the pyrrolizidine content of comfrey products varies greatly, and hepatic toxicity largely associated with daily intake of tablets or multiple cups of tea).
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    (Review of hepatotoxicity of botanicals including comfrey, pyrrolizidine alkaloids, germander, celandine, chaparral, Chinese herbs and pennyroyal).
  • Stedman C. Herbal hepatotoxicity. Semin Liver Dis 2002; 22: 195-206. [PubMed: 12016550]
    (Review and description of patterns of liver injury, including discussion of potential risk factors, and herb-drug interactions; comfrey is listed as a health tonic that contains toxic pyrrolizidine alkaloids capable of causing sinusoidal obstruction syndrome).
  • Schiano TD. Hepatotoxicity and complementary and alternative medicines. Clin Liver Dis 2003; 7: 453-73. [PubMed: 12879994]
    (Comprehensive review of herbal associated hepatotoxicity; discusses comfrey as a cause of sinusoidal obstruction syndrome).
  • Pak E, Esrason KT, Wu VH. Hepatotoxicity of herbal remedies: an emerging dilemma. Prog Transplant 2004; 15: 91-6. [PubMed: 15264453]
    (Review of hepatotoxicity of herbal medications stressing the recent rise in numbers of cases, with literature review of comfrey).
  • Russo MW, Galanko JA, Shrestha R, Fried MW, Watkins P. Liver transplantation for acute liver failure from drug-induced liver injury in the United States. Liver Transpl 2004; 10: 1018-23. [PubMed: 15390328]
    (Among ~50,000 liver transplants reported to UNOS between 1990 and 2002, 270 [0.5%] were done for drug induced acute liver failure, including 7 [5%] due to herbal medications, but comfrey is not mentioned as a cause).
  • Seeff LB. Herbal hepatotoxicity. Clin Liver Dis 2007; 11: 577-96. [PubMed: 17723921]
    (Review of herbal induced hepatotoxicity, with detail of specific herbal compounds including review of the literature on comfrey hepatotoxicity).
  • García-Cortés M, Borraz Y, Lucena MI, Peláez G, Salmerón J, Diago M, Martínez-Sierra MC, et al. [Liver injury induced by “natural remedies”: an analysis of cases submitted to the Spanish Liver Toxicity Registry]. Rev Esp Enferm Dig 2008; 100: 688-95. Spanish. [PubMed: 19159172]
    (Among 521 cases of drug-induced liver injury submitted to Spanish registry, 13 [2%] were due to herbals but none were attributed to comfrey).
  • Chalasani N, Fontana RJ, Bonkovsky HL, Watkins PB, Davern T, Serrano J, Yang H, Rochon J; Drug Induced Liver Injury Network (DILIN). Causes, clinical features, and outcomes from a prospective study of drug-induced liver injury in the United States. Gastroenterology 2008; 135: 1924-34. [PMC free article: PMC3654244] [PubMed: 18955056]
    (Among 300 cases of drug induced liver disease in the US collected between 2004 and 2008, 9% of cases were attributed to herbal medications, but none were linked to comfrey).
  • Navarro VJ. Herbal and dietary supplement hepatotoxicity. Semin Liver Dis 2009; 29: 373-82. [PubMed: 19826971]
    (Overview of the regulatory environment, clinical patterns, and future directions in research with HDS; comfrey is discussed in the context of pyrrolizidine alkaloid hepatotoxicity).
  • Giannetti BM, Staiger C, Bulitta M, Predel HG. Efficacy and safety of a Comfrey root extract ointment in the treatment of acute upper or low back pain: results of a double-blind, randomized, placebo-controlled, multi-centre trial. Br J Sports Med 2009; 44: 637-41. [PubMed: 19460762]
    (Controlled trial of comfrey root extract ointment versus placebo in 120 patients with acute back pain found significant effect of comfrey ointment on pain intensity and movement; side effects were mild and transient and no more common than with placebo).
  • Mei N, Guo L, Fu PP, Fuscoe JC, Luan Y, Chen T. Metabolism, genotoxicity, and carcinogenicity of comfrey. J Toxicol Environ Health B Crit Rev 2010; 13: 509-26. [PMC free article: PMC5894094] [PubMed: 21170807]
    (Overview of the metabolism and toxicity of comfrey, which refers to several species in the genus Symphytum, most frequently "common comfrey" or S. officinale; the herbal is prepared from leaves and dried roots but is a potential health risk due to pyrrolizidine alkaloids which include retronecine mono- and diesters that are hepatotoxic and carcinogenic in laboratory animals).
  • Staiger C. Comfrey: a clinical overview. Phytother Res 2012; 26: 1441-8. [PMC free article: PMC3491633] [PubMed: 22359388]
    (Review of the efficacy and safety of topical comfrey in treatment of arthritis, myalgias, conusions and strains).
  • Teschke R, Wolff A, Frenzel C, Schulze J, Eickhoff A. Herbal hepatotoxicity: a tabular compilation of reported cases. Liver Int 2012; 32: 1543-56. [PubMed: 22928722]
    (A systematic compilation of all publications on the hepatotoxicity of specific herbals identified 185 publications on 60 different herbs, herbal drugs and supplements including five publications on comfrey).
  • Bunchorntavakul C, Reddy KR. Review article: herbal and dietary supplement hepatotoxicity. Aliment Pharmacol Ther 2013; 37: 3-17. [PubMed: 23121117]
    (Systematic review of literature on HDS associated liver injury discusses pyrrolizidine alkaloids including comfrey as causing sinusoidal obstruction syndrome).
  • Abdualmjid RJ, Sergi C. Hepatotoxic botanicals - an evidence-based systematic review. J Pharm Pharm Sci 2013; 16 (3): 376-404. [PubMed: 24021288]
    (A systematic review of hepatotoxic botanicals mentions that comfrey is a common garden plant and consists of several species which contain pyrrolizidine alkaloids that can lead to sinusoidal obstruction syndrome, when taken orally and brewed as teas).
  • Navarro VJ, Seeff LB. Liver injury induced by herbal complementary and alternative medicine. Clin Liver Dis 2013; 17: 715-35. [PubMed: 24099027]
    (Review of herbal hepatotoxicity mentions that comfrey is used topically for pain relief, but when taken orally or brewed as a tea it can be associated with sinusoidal obstruction syndrome).
  • Björnsson ES, Bergmann OM, Björnsson HK, Kvaran RB, Olafsson S. Incidence, presentation and outcomes in patients with drug-induced liver injury in the general population of Iceland. Gastroenterology 2013; 144: 1419-25. [PubMed: 23419359]
    (In a population based study of drug induced liver injury from Iceland, 96 cases were identified over a 2 year period, including 15 [16%] due to herbal and dietary supplements, but none were attributed to comfrey ).
  • Licata A, Macaluso FS, Craxì A. Herbal hepatotoxicity: a hidden epidemic. Intern Emerg Med 2013; 8: 13-22. [PubMed: 22477279]
    (Review and commentary on herbal hepatotoxicity discusses pyrrolizidine alkaloids [including comfrey] which are dose dependent hepatotoxins that cause vino-occlusive disease, probably as a result of biotransformation by the liver to pyrrole derivatives that act as alkylating agents).
  • Navarro VJ, Seeff LB. Liver injury induced by herbal complementary and alternative medicine. Clin Liver Dis 2013; 17: 715-35. [PubMed: 24099027]
    (Review of HDS induced liver injury including regulatory problems, difficulties in diagnosis and assessing causality; discusses comfrey which has been used for pain relief, but has been banned in most countries because of its association with severe SOS).
  • Navarro VJ, Barnhart H, Bonkovsky HL, Davern T, Fontana RJ, Grant L, Reddy KR, et al. Liver injury from herbals and dietary supplements in the U.S. Drug-Induced Liver Injury Network. Hepatology 2014; 60:1399-408. [PMC free article: PMC4293199] [PubMed: 25043597]
    (Among 85 cases of HDS associated liver injury [not due to anabolic steroids] enrolled in a US prospective study between 2004 and 2013, none were attributed specifically to a comfrey containing product).
  • Navarro VJ, Lucena MI. Hepatotoxicity induced by herbal and dietary supplements. Semin Liver Dis 2014; 34: 172-93. [PubMed: 24879982]
    (Review of HDS induced liver injury including regulatory problems, difficulties in diagnosis and assessing causality; mentions that the FDA issued a warning in 2001 about the hepatotoxicity of comfrey).
  • Seeff LB, Bonkovsky HL, Navarro VJ, Wang G. Herbal products and the liver: a review of adverse effects and mechanisms. Gastroenterology 2015; 148: 517-32. [PubMed: 25500423]
    (Extensive review of possible beneficial as well as harmful effects of herbal products on the liver mentions that comfrey contains pyrrolizidine alkaloids that can cause SOS).
  • Stickel F, Shouval D. Hepatotoxicity of herbal and dietary supplements: an update. Arch Toxicol. 2015; 89: 851-65. [PubMed: 25680499]
    (Extensive review of liver injury due to HDS, discusses the pyrrolizidine alkaloids and the clinical features, diagnosis and management of SOS).
  • Chalasani N, Bonkovsky HL, Fontana R, Lee W, Stolz A, Talwalkar J, Reddy KR, et al.; United States Drug Induced Liver Injury Network. Features and outcomes of 899 patients with drug-induced liver injury: The DILIN Prospective Study. Gastroenterology 2015; 148: 1340-52. [PMC free article: PMC4446235] [PubMed: 25754159]
    (Among 899 cases of drug induced liver injury enrolled in a prospective database between 2004 and 2012, HDS were implicated in 145 [16%], however, none of the HDS products listed included comfrey as a known component).
  • Frenzel C, Teschke R. Herbal hepatotoxicity: clinical characteristics and listing compilation. Int J Mol Sci 2016; 17. pii: E588. [PMC free article: PMC4881436] [PubMed: 27128912]
    (Extensive listing of herbal products that have been implicated in causing liver injury, including comfrey).
  • García-Cortés M, Robles-Díaz M, Ortega-Alonso A, Medina-Caliz I, Andrade RJ. Hepatotoxicity by dietary supplements: a tabular listing and clinical characteristics. Int J Mol Sci 2016; 17: 537. [PMC free article: PMC4848993] [PubMed: 27070596]
    (Review and tabulation of HDS causes of liver injury focusing upon illicit anabolic steroids, green tea, linoleic acid, usnic acid, Herbalife and Hydroxycut products, OxyELITE Pro, vitamin A, Ma Huang [ephedra] and Garcinia cambogia).
  • Avigan MI, Mozersky RP, Seeff LB. Scientific and regulatory perspectives in herbal and dietary supplement associated hepatotoxicity in the United States. Int J Mol Sc 2016; 17: 331. [PMC free article: PMC4813193] [PubMed: 26950122]
    (Summary of the regulatory issues surrounding HDS products and hepatotoxicity in the US, including difficulties of surveillance, causality assessment, detection of contaminants and FDA regulatory actions using examples of Lipokinetix [usnic acid], OxyELITE Pro [Aegeline], Hydroxycut [possibly green tea] and designer [anabolic] steroids; no specific discussion of comfrey).
  • Navarro VJ, Khan I, Björnsson E, Seeff LB, Serrano J, Hoofnagle JH. Liver injury from herbal and dietary supplements. Hepatology 2017; 65: 363-73. [PMC free article: PMC5502701] [PubMed: 27677775]
    (Summary of a workshop on the increase in frequency of HDS associated liver injury in the US, focusing upon green tea extracts, OxyELITE Pro, anabolic steroid jaundice and exploring issues of chemical analysis of HDS products, possible changes in regulatory requirements and needs for future research; comfrey is not specifically discussed).
  • de Boer YS, Sherker AH. Herbal and dietary supplement-induced liver injury. Clin Liver Dis 2017; 21: 135-49. [PMC free article: PMC5117680] [PubMed: 27842768]
    (Review of the role of HDS in causing liver injury, mentions comfrey as a source of pyrrolizidine alkaloid associated SOS occurring in the US).
  • Wong LL, Lacar L, Roytman M, Orloff SL. Urgent liver transplantation for dietary supplements: an under-recognized problem. Transplant Proc 2017; 49: 322-5. PubMed Citation (Among 2408 adult urgent liver transplants enrolled in the US Scientific Registry of Transplant Recipients between 2003 and 2015, 625 [30%] were listed as drug induced, mostly due to acetaminophen [n=300, 48%], and HDS products were the 4th most common cause [n=21, 3%], seemingly increasing in proportion during the 12 years of enrollment; comfrey was not among the agents listed).
  • Vega M, Verma M, Beswick D, Bey S, Hossack J, Merriman N, Shah A, et al; Drug Induced Liver Injury Network (DILIN). The incidence of drug- and herbal and dietary supplement-induced liver injury: preliminary findings from gastroenterologist-based surveillance in the population of the state of Delaware. Drug Saf 2017 May 29. [Epub ahead of print] [PMC free article: PMC5699929] [PubMed: 28555362]
    (In a statewide prospective registry of drug induced liver injury, 23 cases were identified during 2014 in Delaware, of which 6 [43%] were attributed to HDS products, but none listed comfrey as a component).


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