Consider what happens to a drug when it’s ingested. Through the gut wall it goes, into the hepatic portal vein, and directly into that vast shredder we know as the liver. Various enzymes go to work tearing your unrecognized drug structure apart, the better to sluice it out through the kidneys as quickly as possible. And there’s the opportunity: a great many of those enzymatic reactions involve breaking carbon-hydrogen bonds. What if they were deuteriums instead?