It has been said that we live in a new era of "individualized medicine" or "personalized medicine." One of the primary drivers for this idea is the emerging field of pharmacogenomics (PGx). PGx is the study of how individual variations in the human genome affect responses to medications.
The term "pharmacogenetics" is also used for this discipline (people in the field use both terms; however, the term 'pharmacogenomics' is more popular).
The primary reason that individuals metabolize and respond to drugs differently is the inter-individual differences in receptor proteins and enzymes that metabolize the drugs. Mutations in these receptor proteins and enzymes can give rise to very different responses to drugs. In PGx, these mutations are referred to as variants.