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As noted above, comfrey contains substances called pyrrolizidine alkaloids that are both toxic to the liver and carcinogenic. 1 2 3 4 5 6 7 The main form of liver disease seen with comfrey is a blockage of small veins that can lead to liver cirrhosis and eventually liver failure (hepato-occlusive disease). Liver transplantation may be required. Oral use of comfrey for as brief a time as 5 to 7 days in a child and 19 to 45 days in adults has resulted in severe liver disease and death. Long-term use of very low dosages may also cause harm.
In general, the root of the plant contains more pyrrolizidine alkaloids than the leaves. Related species of comfrey such as Symphytum uplandicumand Symphytum asperumcontain even higher levels of these toxins, and may be mistakenly sold as ordinary comfrey.
Pyrrolizidine alkaloids in comfrey can be absorbed through the skin. For this reason, it has been recommended that when using comfrey preparations, the daily amount of pyrrolizidine alkaloids should not exceed 100 mcg. Unfortunately, few products are labeled to indicate their pyrrolizidine alkaloid content. Furthermore, the common analytic methods used for testing pyrrolizidine alkaloid content may fail to measure a certain chemical form of these toxins (the N-oxide form), leading to results that are too low by a factor of ten or more. For all these reasons, it may be prudent to avoid topical comfrey products entirely. If you nonetheless wish to use comfrey as a topical treatment, we recommend the following general guidelines:
- Do not apply comfrey for more than 4-6 weeks per year.
- Do not use for more than 10 days in a row.
- Do not apply to broken skin.
In addition, comfrey should not be used by children, pregnant or nursing women, or people with liver disease.
- Williams L, Chou MW, Yan J, Young JF, Chan PC, Doerge DR. Toxicokinetics of riddelliine, a carcinogenic pyrrolizidine alkaloid, and metabolites in rats and mice. Toxicol Appl Pharmacol. 182(2):98-104.
- Mei N, Guo L, Fu PP, et al. Mutagenicity of comfrey ( Symphytum officinale ) in rat liver. Br J Cancer. 2005;92:873–5.
- Stickel F, Seitz HK. The efficacy and safety of comfrey. Public Health Nutr. 3(4A):501-8.
- Couet CE, Crews C, Hanley AB. Analysis, separation, and bioassay of pyrrolizidine alkaloids from comfrey ( Symphytum officinale ). Nat Toxins. 1996;4:163–7.
- Winship KA. Toxicity of comfrey. Adverse Drug React Toxicol Rev. 10(1):47-59.
- Betz JM, Eppley RM, Taylor WC, et al. Determination of pyrrolizidine alkaloids in commercial comfrey products ( Symphytum sp.). J Pharm Sci. 1994;83:649–53.
- Yeong ML, Wakefield SJ, Ford HC. Hepatocyte membrane injury and bleb formation following low dose comfrey toxicity in rats. Int J Exp Pathol. 74(2):211-7.