If you develop a mild rash, then you are probably helenalin-sensitive and should stop using arnica.
Arnica is toxic if it gets inside the body.
Never apply arnica in any form on broken skin or on an open wound. Never take arnica internally unless it's in the form of homeopathic pills that contain too little arnica to cause harm. Rub arnica tincture freely onto injured area, so long as skin isn't broken. Massage it in. Mix 1 tablespoon of arnica tincture in 1 pint of spring water.