Just as a sidenote to what Aunty said (which was stated superbly) I just want to add that I also have nickel allergy and that nickel is in dental appliances such as braces and retainers. I don't know if you have had braces, but I did for 6 years. Having nickel AND silver fillings or any two or more dental metals in your mouth creates a 'current', which adds to problems.