Drug metabolism is the formation of metabolite when a drug is introduced into the body. These drugs are organic molecule, when they are inside the body, they can break into their constituent metabolite. It is important to know the metabolite formed, as some metabolite formed can be toxic to the body, or may have some sort of side effect.
Usually isotopes of certain elements are added to the drug, as it helps in easy tracking. Hydrogen deuterium, carbon 14 is some isotopes that are added to certain position in the drug. It is easier to track these isotopes by various methods like nuclear magnetic resonance (NMR) and mass spectrometry.
The addition of these isotopes for example, hydrogen can be done by shaking the drug in a water solution having deuterium hydrogen. Apart from these frequently used isotopes methyl groups are also added, but they are less common as demethylation is a common process happening in the cell. The atropine is labeled using deuterium hydrogen and a methyl group.