alleles CYP2C19*2 and *3 may reduce the efficacy of clopidogrel (Plavix), an antiplatelet medication. The basis for this reduced effect of clopidogrel in patients who have a gene of reduced activity may seem somewhat paradoxical, but can be understood as follows. 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. In patients with a gene of reduced activity, clopidogrel may not be metabolized to its biologically active form and therefore not achieve pharmacological effect in the body. 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. A 2020 systematic review and meta-analysis also confirmed that the CYP2C19*2 variant has a strong association with clopidogrel resistance. In 2021 a higher risk of stroke at 90 days was found with clopidogrel than ticagrelor, which does not require metabolic conversion, among Han Chinese CYP2C19 loss-of-function carriers with minor ischemic stroke or TIA. Ligands The following is a table of selected substrates, inducers and inhibitors of CYP2C19. Where classes of agents are listed, there may be exceptions within the class. Inhibitors of CYP2C19 can be classified by their potency, such as: Strong being one that causes at least a 5-fold increase in the plasma AUC values, or more than 80% decrease in clearance of substrates. Moderate being one that causes at least a 2-fold increase in the plasma AUC values, or 50–80% decrease in clearance of substrates. Weak being one that causes at least a 1.25-fold but less than 2-fold increase in the plasma AUC values, or 20–50% decrease in clearance of substrates. See also Cytochrome P450 oxidase References External links PharmGKB: Annotated PGx Gene Information for CYP2C19 2