Many intrinsic and extrinsic factors can affect an individual patient's drug exposure and response. The US Food and Drug Administration (FDA) has published a number of guidances that recommend how and when to evaluate these factors during drug development. The most recent FDA draft guidance on drug interactions provides advice for in vitro and in vivo drug interaction studies, including suggestions for study design, dosing strategies and analysis, and interpretation of data for medical product labels. The draft guidance updated the FDA's recommendations on the evaluation of important cytochrome P450 (CYP) enzyme- and transporter-based drug interactions during drug development.