It checks for genes that code for specific liver enzymes which activate, deactivate, or are influenced by various drugs.
Strattera is the first drug to mention the test in the official documentation, although it doesn't specifically recommend that patients get the test before taking the medication.
[1][2] People who cannot metabolize a drug will require a much lower dose than is recommended by the manufacturer, and those who metabolize it quickly may require a higher dose.
Some drugs, such as codeine, will not be effective in people without the requisite enzymes to activate them.
People who are poor metabolizers of a drug may overdose while taking less than the recommended dose.