Drug metabolism is the chemical alteration of a drug by the body.
Some drugs are chemically altered by the body (metabolised). The substances that result from metabolism may be inactive, or they may be similar to or different from the original drug in therapeutic activity or toxicity.
Some drugs, called prodrugs, are administered in an inactive form, which is metabolised into an active form; the resulting metabolites produce the desired therapeutic effects.

Most drugs must pass through the liver, which is the main site of drug metabolism. Liver enzymes convert prodrugs to active metabolites or leave them as inactive drugs. The group of Cytochrome P-450 enzymes is the liver’s primary mechanism for chemically altering drugs. The levels of P-450 enzymes control the rate at which many drugs are metabolised. Genes 2D6 and 2C19 are coding the information for the production of these enzymes.

Peoples ability to metabolise drug by these enzymes can now be measured by AmpliChips and thus patients can be categorised into poor, intermediate, extensive, or ultra-rapid metabolisers.