Ive been using borax salt (sodium tetraborate)for about 18-24 months now regularly with no ill effects whatsoever and I know a couple of others who also use it frequently again with no ill effects.
I havent really noticed any improvement in health as a result of taking borax though - but certainly no ill effects with the benefit that it helps remove fluoride and works well with mg, ca, vit d, for bone health etc.