The active ingredient in HeadOn, a roll-on headache treatment, is potassium dicromate, which is used in developing photos and should not come into contact with skin, according to Larry Newman, MD, director of the Roosevelt Headache Institute in Manhattan.
HeadOn is an OTC product and is subject to much less regulation than prescription drugs. In fact, I’m not sure if it is regulated at all.
Update, 5/6/07: This site is not affiliated with HeadOn, I’m just reporting on what I’ve read. For more information, here are a couple follow-up posts: