Both variants of the CYP2C9 enzyme, *2 (c.430C>T) and *3 (c.1075A>C), exhibit impaired function leading to poor metabolism (PM) phenotypes for various drugs. Individuals with PM phenotype (i.e. genotypes with homozygous or compound heterozygous *2 or *3 alleles) are at greater risk of severe bleeding during coumarin-based anticoagulation therapy with warfarin, acenocoumarol and phenprocoumon.