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Helenalin is a sesquiterpene lactone found in Arnica montana and Arnica chamissonis foliosa. Helenalin is a highly toxic compound, with hepatic and lymphatic tissues particularly vulnerable to its effects. It is believed to be responsible for the toxicity and skin irritation associated with Arnica. If enough of the plant is ingested, the helenalin produces severe gastroenteritis and internal bleeding of the digestive tract.

Pharmacology
Helenalin has a variety of observed effects in vitro including anti-inflammatory and antitumor activities. Helenalin has been shown to selectively inhibit the transcription factor NF-κB, which plays a key role in regulating immune response, through a unique mechanism. In vitro, it is also a potent, selective inhibitor of human telomerase —which may partially account for its antitumor effects—has anti-trypanosomal activity, and is toxic to Plasmodium falciparum.

Animal and in vitro studies have also suggested that helenalin can reduce the growth of Staphylococcus aureus and reduce the severity of S. aureus infection.