An effective control of blood levels is especially important for drugs with a narrow therapeutic index, such as nitroglycerin and fentanyl.5 After application of a TS and a short lag-time6 an increase in drug concentration occurs in the plasma. Ideally, plasma drug levels rapidly exceed the lower threshold of the therapeutic window and then continue to increase until a plateau is reached where drug levels are sustained for a prolonged time. Figure 1 shows a schematic concentration-time curve upon application of a TS.