An agent formulator (30, 60) for an intravenous administration set (14) is provided. The intravenous administration set (14) includes a container (12) of an IV fluid (13), a drip chamber (22), an agent formulator (30, 60) and an adapter-needle assembly (28). The agent formulator (30, 60) has a fluid inlet (23) and a fluid outlet (25) for maintaining a flow of IV fluid (13 ) therethrough. A portion (36) of the formulator wall (32) is comprised of a window material which allows the agent to diffuse therethrough but which prevents convective loss of the IV fluid (13). A flow distributor (38, 68) is provided within the chamber for distributing the flow of IV fluid (13) along the interior surface of the window (36). A transdermal-type drug delivery device (40) is adhered to the exterior surface of the window (36). Drug is delivered by the delivery device (40) through the window (36) and into the flowing IV fluid (13). The device (40) delivers drug into the IV fluid (13) at a rate that is independent of the flow rate of IV fluid (13) through the formulator (30, 60). The rate of drug delivery from the device (40) into the IV fluid (13) is controlled by either the window (36) itself or by a membrance layer (46) in the drug delivery device (40). A plurality of drug delivery devices (40) may be adhered to the window (36) to deliver a plurality of drugs or to deliver a single drug at a higher dosage rate. A similar window (11) may be placed in a IV bag to deliver a drug into the IV fluid therein.