A drug delivery system provides for mixing various drugs in an optimally controlled manner, for using flow controllers to guide multiple drugs into a single or into multiple catheters, for enabling a single lumen catheter to treat a specific region with several drugs, for allowing for dilution of a concentrated drug in order to both increase the time between refilling and also for providing any concentration of a drug that might be desired, for using a buffer fluid to deliver exact amounts of several drugs from the same catheter or to separate several drugs within a single catheter, for using external fluid present in the human body either as a diluent or buffer fluid, and for providing for a drug testing/filler apparatus to be used prior to implant to ensure proper function and easy means of filling multiple reservoirs with different fluids, and also after implant for refilling operations. The drug delivery system (DDS) can perform both bolus and continuous delivery of substances, and enable the measured delivery of any one of several drugs to one or more distal locations at independently programmable rates. New types of catheter systems and uses therefore are also described. Catheter hub assemblies allowing for easy replacement of drug delivery systems offer advantages when replacing drug delivery systems. New methods for using the DDS in the promotion of healthy pregnancy and treatment of a developing fetus are also possible.