Disposition of a drug depends on absorption, distribution, metabolism, and elimination (ADME). Quantifying the ADME capacity of a patient may help to individualize target exposure by choosing the corresponding dose required for this individual. The large group of drugs eliminated by members of the CYP3A isozyme subfamily accounts for about 50% of all marketed drugs. Co-medication can modulate CYP3A activity 400-fold either by inducing isozyme expression or inhibiting expressed enzyme. Therefore, due to the variability of CYP3A activity, dose requirements of CYP3A substrates vary considerably between individual patients and may even rapidly vary within a patient. There is clinical interest in using CYP3A activity as a biomarker to predict optimal CYP3A drug dosing, improve therapeutic efficacy, and minimize adverse drug effects.