10 Deadly Things People Used To Think Were Harmless
5. Radium

The human body is far from perfectly designed. You need only ask an appendicitis patient. A case in point is that, when we ingest radium, the body tends to misidentify it as calcium and helpfully place it inside your bones where it sits there doing little more than being radioactive and carcinogenic.
So, unfortunately for the people of the early 20th century, using it in toothpaste, drinking water, chocolate and suppositories proved to be a major misstep. Bone cancer for all!