 If a substrate of the enzyme is given to the patient as a medication, and if the patient has reduced CYP2D6 or CYP2C19 activity, the patient will have elevated drug concentration in their body, and therefore severe side effects may occur.On the other hand, for the UM patient, the drug concentration might be too low to have a therapeutic effect.So testing the phenotype of the patient is important to help determine the optimum dosage of the drug.