The CYP2C9 gene is located on human chromosome 10. The CYP2C9 enzyme plays an important role in the metabolization of many drugs. Therefore, people with a lower enzyme activity increase their susceptibility to adverse drug reactions or present a lack of response to treatment. Indeed, the carriers of CYP2C9 mutations are not able to normally metabolize different drugs. The principal cause of a deficiency of CYP2C9 enzyme is genetic. The CYP2C9 enzyme is involved in the metabolism of phenytoin (anticonvulsant), tolbutamide (hypoglycemic drug) losartan (anti-hypertensive), ibuprofen (anti-inflammatory) and warfarin (anticoagulant), among others.

CYP2C9*2 (rs1799853, 430C>T or R144C)

CYP2C9*3 (rs1057910, 1075A>C or I359L)

These two common SNPs in the Caucasian population are responsible for a reduction in the metabolism since they are related with a reduction in CYP2C9 enzyme activity to approximately 12 to 70% (CYP2C9*2) and 5% (CYP2C9*3) compared to the wild-type CYP2C9 activity.