Methylthioninium chloride, commonly called methylene blue was first prepared in 1876 by German chemist Heinrich Caro and has been described as "the first fully synthetic drug used in medicine." It was originally developed as a dye to color clothing and was used by researchers as a stain for identifying bacteria and parasites. It was later discovered it could be used to kill them, it eventually became a successful treatment for malaria.

Today it has many uses including treating rare blood conditions.  

On November 22nd, 2005 artist Yann Marussich ingested 5 milliliters of the highly concentrated dye, he sat motionless in a temperature-controlled glass box stripped down to his underwear. Over the course of about an hour, the dye started making its way out of his mouth and nose, as the temperature in the box increased he began to sweat the dye out of his skin which really made for some stunning photographs.

Bonus picture: RFK allegedly adding methylene blue to his drink on a plane.