We have previously shown that a broad drug interaction screening can be performed using enzyme specific probes such as oral caffeine for CYP1A2, N-acetyltrasferase-2 (NAT-2), and xanthine oxidase (XO), warfarin plus vitamin K for CYP2C9, omeprazole for CYP 2C19, dextromethorphan for CYP2D6, and midazolam for CYP3A4/5. This combination of probes has been validated in the Cooperstown 5+1 Cocktail (5+1).1 The use of the 5+1 cocktail provides information on the drug metabolizing enzymes that metabolize 90% of hepatically eliminated drugs for a fraction of the costs of the individual studies. Using a cocktail of biomarkers reduces the overall cost of drug interaction screening. The FDA has indicated that the use of probe drugs can be done in place of specific drug interaction studies.2 This streamlines the detection of drug interactions and reduces costs. However, it is important to control for genetic polymorphism in drug interaction trials so that genetic makeup does not affect the outcome.