Venlafaxine (VEN) is a type of serotonin-norepinephrine reuptake inhibitor. It is known that the pharmacogenetics of VEN is mainly affected by metabolizer status of the Cytochrome P450 (CYP) 2D6 and CYP2C19 enzymes. We investigate a fatal overdose case of VEN in relation to CYP2D6 and CYP2C19 genetic polymorphisms. Qualitative and quantitative analyses for VEN were performed using gas chromatography-mass spectrometry and it is subsequently confirmed using ultra-high-performance liquid chromatography-tandem mass spectrometry. Genetic analysis for CYP2D6 and CYP2C19 polymorphism was performed using polymerase chain reaction (PCR) followed by DNA sequencing. The result was determined to be a fatal overdose of VEN in a Korean man with CYP2D6 heterozygous genotype *1/*2 that did not affect enzyme activity and wild allele of CYP2C19. The use of genotyping of CYP2D6 and CYP2C19 may assist interpretation for a considerable fatal dose of VEN in individuals with different CYP2D6 genotypes.