Compositions comprised of a delivery vehicle or delivery system and an active agent dispersed within the delivery vehicle or system, wherein the delivery vehicle or system contains a polyorthoester polymer and a polar aprotic solvent. Also disclosed are low viscosity delivery systems for administration of active agents. The low viscosity delivery systems have a polyorthoester polymer, a polar aprotic solvent and a solvent containing a triglyceride viscosity reducing agent. Compositions described include an amide- or anilide-type local anesthetic of the “caine” classification, and a non-steroidal anti-inflammatory drug (NSAID), along with related methods, e.g., for treatment of post-operative pain or for prophylactic treatment of pain. The compositions are suitable for delivery via, e.g., direct application and instillation, intradermal injection, subcutaneous injection, and nerve block (perineural).