At the same time, understanding the allelic variants CYP2C19*2, CYP2C9*3, and human leukocyte antigens (HLA) as pharmacogenomic biomarkers can be crucial in deciding whether to discontinue treatment, guide treatment with other antiseizure medications, or choose a safer therapeutic alternative for patients with epilepsy. This evidence concerns the gene CYP2C9 and epilepsy.