Phenotyping involves measuring the metabolism of a probe drug. For example, with CYP2D6, dextromethorphan or debrisoquine can be given to a patient to see how well the drug is metabolized. Both these drugs are safe and extensively metabolized by CYP2D6.

By measuring the parent drug and the metabolite in urine, the metabolic capacity of a CYP450 enzyme can be estimated.

Such testing is complex and tedious, however, and has not become routine in clinical laboratories. Therefore, genotyping is likely to be the main tool that is used for assessing the PGx of a patient.