CYP2C19, a member of the cytochrome P450 superfamily, is an important liver enzyme involved in the metabolism of xenobiotics in the body. Numerous drugs prescribed as platelet inhibitors (clopidogrel), anticonvulsants (diazepam, mephenytoin), antidepressants (citalopram, sertraline), proton pump inhibitors (omeprazole, pantoprazole), or for the treatment of malaria (proguanil) are substrates for CYP2C19. Patients with defective enzyme variants are at risk of developing severe adverse reactions due to drug accumulation and toxicity. Conversely, when formation of an active metabolite is essential for the action of a drug, these patients can exhibit diminished response to therapy compared to extensive metabolizers.