A pharmaceutical composition for controlled release of an active substance, the composition being a matrix composition of: (a) a substantially water soluble or crystalline polymer, (b) an active substance, and optionally, (c) one or more pharmaceutically acceptable excipients having a water solubility of at least 1 mg/ml at ambient temperature. The matrix composition does not contain a water dispersible or water soluble surface active agent that has at least one domain, which is compatible with the polymer in the matrix composition, and which substantially eliminates water diffusion between the interface between the polymer crystals.