Mercury from Amalgam is documented to be the most common cause of oral lichen planus and lots of other oral health problems due to the high levels of mercury in the gums, jawbone, oral mucosa,etc.; and Amalgam replacement almost always cures such conditions. http://www.flcv.com/periodon.html
But mercury also causes oral cancers such as squamous cell cancer, and is a factor in other cancers throughout the body, due both to toxic/inflammatory effects as well as depressing the immune system. http://www.flcv.com/cancerhg.html