Devices, systems and methods for delivering one or more drugs to one or more internal body locations (such as the cerebrospinal fluid) are disclosed. In various aspects, the systems and methods may involve catheters having infusion sections with permeable membranes that develop significant back pressure to enhance uniform delivery of the drug over an infusion section catheters that have two or more infusion sections spaced apart along the length of the same catheter, catheters that include two or more infusion sections serviced by independent lumens (such that, e.g., different drug solutions can be delivered to the different infusion sections) implantable drug delivery systems with pumps and multiple reservoirs from which drugs can be delivered systems that are capable of delivering drug solutions with selected densities etc. Methods for treating diseases, including pain and spasticity are also discussed, as well as methods for screening patients and optimizing therapies. In addition, methods for delivering a drug to a brain through a spinal canal are described.
