Helenalin

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.