A polymeric formulation for use in treating or preventing a disease, wherein the formulation is used in the delivery of a therapeutic or prophylactic agent to a mucosal epithelial surface, wherein the formulation comprises: a therapeutic or prophylactic agent; a gel-forming polymer in an aqueous solution at a concentration which is less than the critical gel concentration of the polymer, and increases in concentration as water is absorbed from the formulation to or above the critical gel concentration of the polymer under isotonic conditions and a temperature between room temperature and body temperature, thereby forming a gel on the mucosal epithelial surface; and one or more excipients, wherein the critical gel concentration of the polymer is the concentration at which the polymeric formulation would gel in a test tube when warmed to 37°C, wherein the formulation is a sodium-containing formulation having a tonicity below 280 mOsm/L, or the formulation is a sodium-containing formulation having a tonicity below 450 mOsm/L for administration to a mucosal epithelial surface of the colorectum, wherein the formulation is hypotonic with respect to the mucosal epithelial surface to which it is applied, and wherein the gel-forming polymer is selected from the group consisting of polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymers, tetra-functional block copolymers based on ethylene oxide and propylene oxide, poly(N,N-diethylacrylamide), poly(N, N-dimethylacrylamide), poly(N-vinylcaprolactam), poly(N-alkylacrylamide), poly(N-vinylalkylamide), poly(N-isopropyl acrylamide), polyethylene oxide methacrylate polymers, poly(lactic-co-glycolic acid) (PLGA)-polyethylene glycol triblock copolymers (PLGA-PEG-PLGA and PEG-PLGA-PEG), polycaprolactone -polyethylene glycol triblock copolymers (PCL-PEG-PCL and PEG-PCL-PEG), and combinations thereof. a therapeutic or prophylactic agent; a gel-forming polymer in an aqueous solution at a concentration which is less than the critical gel concentration of the polymer, and increases in concentration as water is absorbed from the formulation to or above the critical gel concentration of the polymer under isotonic conditions and a temperature between room temperature and body temperature, thereby forming a gel on the mucosal epithelial surface; and one or more excipients, wherein the critical gel concentration of the polymer is the concentration at which the polymeric formulation would gel in a test tube when warmed to 37°C, wherein the formulation is a sodium-containing formulation having a tonicity below 280 mOsm/L, or the formulation is a sodium-containing formulation having a tonicity below 450 mOsm/L for administration to a mucosal epithelial surface of the colorectum, wherein the formulation is hypotonic with respect to the mucosal epithelial surface to which it is applied, and wherein the gel-forming polymer is selected from the group consisting of polyoxyethylene-polyoxypropylene-polyoxyethylene triblock copolymers, tetra-functional block copolymers based on ethylene oxide and propylene oxide, poly(N,N-diethylacrylamide), poly(N, N-dimethylacrylamide), poly(N-vinylcaprolactam), poly(N-alkylacrylamide), poly(N-vinylalkylamide), poly(N-isopropyl acrylamide), polyethylene oxide methacrylate polymers, poly(lactic-co-glycolic acid) (PLGA)-polyethylene glycol triblock copolymers (PLGA-PEG-PLGA and PEG-PLGA-PEG), polycaprolactone -polyethylene glycol triblock copolymers (PCL-PEG-PCL and PEG-PCL-PEG), and combinations thereof.