[7] These proteins are monooxygenases that catalyze many reactions involved in drug metabolism and synthesis of cholesterol, steroids and other lipids.

[8] CYP2C19 also possesses epoxygenase activity: it is one of the principal enzymes responsible for attacking various long-chain polyunsaturated fatty acids at their double (i.e. alkene) bonds to form epoxide products that act as signaling agents.

It metabolizes: Along with CYP2C19, CYP2C8, CYP2C9, CYP2J2, and possibly CYP2S1 are the main producers of EETs and, very likely EEQs, EDPs, and the epoxides of linoleic acid.

The Clinical Pharmacogenetics Implementation Consortium (CPIC) provides therapeutic guidelines that are widely utilized to suggest suitable treatment plans.

These recommendations are particularly relevant for patients requiring antiplatelet medication and are based on the results of genotype testing.

A key aspect of these CPIC guidelines is the translation of genotype to phenotype, a process that relies on the "star" nomenclature system[13] maintained by the Pharmacogene Variation Consortium[14] assigns labels to known polymorphisms that affect drug response.

In patients with an abnormal CYP2C19 variant certain benzodiazepines should be avoided, such as diazepam (Valium), lorazepam (Ativan), oxazepam (Serax), and temazepam (Restoril).

[21][26] In the case of proton pump inhibitors, PMs exhibit a drug exposure that is 3 to 13 times higher than that of EMs.

[26][29] Some studies have found that the *17 variant's effect on the metabolism of omeprazole, pantoprazole, escitalopram, sertraline, voriconazole, tamoxifen and clopidogrel[26][30] is modest, particularly compared to the impact of loss-of-function alleles (*2, *3), therefore, in case of these medications, the EM designation is sometimes applied instead of the UM one.

[26] For example, carriers of the *17 allele did not demonstrate different gastric pH comparing to *1 after taking the proton pump inhibitor omeprazole, a CYP2C19 substrate.

Clopidogrel is administered as a "prodrug", a drug that is inactive when taken, and then depends on the action of an enzyme in the body to be activated.

The relative risk of major cardiac events among patients treated with clopidogrel is 1.53 to 3.69 times higher for carriers of CYP2C19*2 and CYP2C19*3 compared with non-carriers.