For example, cytochrome P4503A4 catalyzes the breakdown of some pharmaceutical sedatives, the antihistamine terfenadine, antihypertensives, and the immunosuppressant cyclosporin. CYP2D6 catalyzes such rapid breakdown of the pain reliever codeine that patients within the haplotype whose liver contains large amounts of CYP2D6 derive virtually no benefit from taking the standard dose of codeine.

Another example, is that consumption of the pharmaceuticals tolbutamide, warfarin, or phenytoin can be riskier for people who possess a mutation (i.e., an SNP which codes-for less or no expression of CYP2C9) within their liver tissue. That is because CYP2C9 enzyme causes rapid metabolism of tolbutamide, warfarin, phenytoin (and some other pharmaceuticals); so the "typical dose" could result in higher-than-expected bloodstream levels of those pharmaceuticals in people possessing that particular SNP.