A Toxic Remedy - Important Information on Helenalin and the Side Effects of Arnica Montana

   I was taught originally that Arnica, or A. montana, is not particularly safe to eat. After my researching the most recent medical studies on PubMed, I now have a different opinion.

   Is Arnica toxic? The health benefits of A. montana are derived from sesquiterpene lactones, a class of chemical compounds commonly found in many plants throughout the world. Our research into the potential effects of Arnica montana introduced us to the chemical Helenalin. 

What Is Helenalin?
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   Helenalin is also a sesquiterpene (20). It is reported all over the internet as being a toxic chemical.

   The studies that first brought this to my attention were focused on Arnica consumption by livestock, not humans.

Here is an example of some confusing information:

   The U.S. National Library of Medicine (NIH), also known as TOXNET (Toxicology Data Network) has a detailed review on Helenalin. At first glance, it reports that the herb is toxic, with the first lines reading:

Human Health Effects - Human Toxicity Excerpts:

"INTENSELY POISONOUS, CAPABLE OF CAUSING PARALYSIS OF VOLUNTARY & CARDIAC MUSCULATURE & FATAL GASTROENTERITIS." - TOXNET (20)

   However, if we continue to read the next lines of their Summary Report under Clinical Effects, nearly every example they give of poisoning is directed to animals, specifically livestock. Only a few bullet-points mention allergic reaction to humans. 

  They also don't include how severe the allergic reactions could or can be. Quoted from Line B:

"Toxic ingestions in humans are exceedingly rare" - TOXNET (20)
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Only Toxic To Livestock?
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   As it turns out, humans have been ingesting Arnica for possibly thousands of years. Some cultures even smoked it (although there is a serious lack of research on the benefits of smoking Arnica).

    Okay, so It poisons humans, but only enough to cause allergic reactions. ​So why has it been reported for over three decades that the herb is poisonous if ingested? Is Arnica safe? 

   Could it be that the earlier reports from the 70's and 80's were misinterpreted? Or perhaps we just didn't know as much about the chemical then as we do now?

This report from 1976 by Witzel DA, Ivie W, and Dollahit JW, suggests that:

​"The oral median lethal dose of helenalin for 5 mammalian species was between 85 and 105 mg/kg." - pubmed.gov (21)

   Subsequently, after this report another from 1980 suggests that 7 specific sesquiterpene lactones were considered toxic depending on their number of alkylating centers: ​

 "The toxicity of a sesquiterpene lactone depends on the number of alkylating centers such as cyclopentenone, alpha-methylene-gamma-lactone, or hemiacetal moieties in the molecules." - pubmed.gov (22)

   By 1983 and 1987, Elissalde MH Jr and Ivie GW were convinced that helenalin should be considered a poisonous chemical to livestock specifically:

"This phenomenon may have a significant role in the toxicity of some sesquiterpene lactones of poisonous plants when ingested by livestock." - pubmed.gov 1983 (23)

"These data indicate that sesquiterpene lactones containing an alpha-methylene-gamma-lactone moiety are potent inhibitors of macrophage adenylate cyclase activity. This moiety may have a significant role in the toxicity of some sesquiterpene lactones in poisonous plants when ingested by livestock." - pubmed.gov 1987 (24)

   Around the same years, helenalin was being tested as an anti-tumor chemical and examined for its effects on cancer. However, it was still considered to demonstrate cellular toxicity when taken at the doses required to be effective:

"Thus, microlenin may be a more likely therapeutic agent than helenalin which has demonstrated cellular toxicity." - pubmed.gov 1983 (25)
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Helenalin And Liver Function
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   In 1988, a study conducted by the Department of Veterinary Physiology and Pharmacology at the College of Veterinary Medicine at Texas A&M University shows the clearest understanding of Helenalin toxicity on mammals, including humans. While their study involved immature male mice, they revealed something important:

"These studies suggest that the in vivo and in vitro toxicity of hymenoxon and helenalin is strongly dependent on hepatic glutathione levels, which hymenoxon and helenalin rapidly deplete at very low concentrations." - College of Veterinary Medicine, Texas A&M University 1988 (26)

   What is Glutathione and why is it important? Here is the understanding as described by the Yale Journal of Biology and Medicine:

"Glutathione plays a key role in the liver in detoxification reactions and in regulating the thiol-disulfide status of the cell.....  Under conditions of oxidative stress, the liver exports oxidized glutathione into bile in a concentrative fashion, whereas under basal conditions, mainly reduced glutathione is exported into bile and blood." - Yale Journal of Biology and Medicine 1981 (27)

   These two reports tells us that ingesting Helenalin depletes the body of glutathione, which is a key component in liver detoxification. Eventually, this may allow the function of the liver to fail. 

Conclusion: Toxic to Mammals - Ingest with Caution

   Arnica contains a sesquiterpene called Helenalin. Helenalin is a known toxic substance to all mammals. It is toxic to mammals because it interferes with liver function and may lead to liver failure. 

   While it is specifically poisonous to livestock and smaller mammals, and cases of Arnica poisoning are exceedingly rare, it is still considered poisonous to humans because it interferes with liver function. If our liver fails, we cannot process toxins from our body. Our bodies become toxic and unbalanced, leading to potentially serious health conditions. If consumed, do so only at low doses and not over long periods of time. Consult your health physician before ingesting Arnica.

What This Information Means For TOPICAL APPLICATIONS:

   The above information is specific to ingestion of arnica tablets, arnica tea, or other arnica supplements. The above information does not pertain to arnica gels, arnica creams, arnica oils, ointments, or any other arnica based topical applications. Topical applications of this supplement do not effect the liver in the same way.

Learn More About Arnica

Works Cited:

20) Helenalin. https://toxnet.nlm.nih.gov/cgi-bin/sis/search/a?dbs+hsdb:@term+@DOCNO+3490

21) Mammalian toxicity of helenalin, the toxic principle of Helenium microcephalum CD (smallhead sneezeweed). https://www.ncbi.nlm.nih.gov/pubmed/937811

22) Toxicity of sesquiterpene lactones. https://www.ncbi.nlm.nih.gov/pubmed/7394314

23) Considerations of the structure of sesquiterpene lactones on biological activity: influence of the alpha-methylene-gamma-lactone moiety on mast cell degranulation. https://www.ncbi.nlm.nih.gov/pubmed/6195945

24) Inhibition of macrophage adenylate cyclase by the alpha-methylene-gamma-lactone moiety of sesquiterpene lactones from forage plants. ​https://www.ncbi.nlm.nih.gov/pubmed/3826835
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25) Antitumor agents LXIII: the effects of microlenin on nucleic acid and protein syntheses of Ehrlich ascites cells. https://www.ncbi.nlm.nih.gov/pubmed/6631681

​26) Role of glutathione in the toxicity of the sesquiterpene lactones hymenoxon and helenalin. https://www.ncbi.nlm.nih.gov/pubmed/3343693

27) The importance and regulation of hepatic glutathione. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2596047/