I know Kryptonite is stated to be very rare on Earth but surely the very rich/powerful (presidents, monarchs) would have carried it everywhere to protect them from skin cancer?
Was it just the risk to Superman that prevented them from using it? (e.g. saving from natural disasters, automobile accidents and supervillains) or is there some inherent danger in constant exposure?
Is there a canonical use of kryptonite as some form of radiotherapy/cancer treatment?