Drug reaction testing uses a genetic test to predict how a particular person will respond to various prescription and non-prescription medications. It checks for genes that code for specific liver enzymes which activate, deactivate, or are influenced by various drugs. There are currently four genetic markers commonly tested for: 2D6, 2C9, 2C19, and 1A2. People who are poor metabolizers of a drug may overdose while taking less than the recommended dose.