All radioactive atoms will eventually decay, or change, in some way. When a carbon-14 atom decays, one of its eight neutrons turns into a proton, emitting an electron (with a charge of -1 ...
This is typically done by adding a radioactive atom (e.g., carbon-14) to the drug so that its movement through the body can be traced. A bit like a GPS can be used to track the movements of animals.