Locally arnica is irritant, the preparations of the flowers being most powerful. Strong preparations should not be applied full strength, for in some instances of tender skin, or in accidents occurring after its use, an erysipelatous inflammation has followed. Even when used as a local dressing for wounds dangerous inflammation, with vesication, has occurred.
Internally, in large doses, arnica causes heat in the throat, nausea, vomiting, purging, spasmodic contractions of the limbs, difficulty of respiration, and sometimes inflammation of the alimentary canal, and coma. There is no known antidote to its poisonous influences; vegetable acids have been recommended. Two fluid ounces of the tincture has produced death.