and water the term water soluble polymer as used herein includes polymers which are freely permeable to water such as eudragit rl likewise the term water insoluble polymer as used herein includes polymers which are slightly permeable to water such as eudragit rs the polymeric material may consist solely of a water soluble polymer or alternatively it may include a minor proportion of a water insoluble polymer the ratio of water soluble to water insoluble polymer is determined by the particular combination of polymers selected the water soluble polymer is suitably hydroxypropylmethylcellulose polyvinylpyrrolidone or a polymer sold under the trade mark eudragit rl the water insoluble polymer is suitably a cellulose ether such as methyl ethyl or propylcellulose shellac or a polymer sold under the trade mark eudragit rs the core suitably has between 20 and 120 layers and is built up in a manner known per se further preferably the multi layer arrangement of methyldopa organic acid and polymeric material is built up on a central inert core suitably consisting of a non pareil seed of sugar or starch having an average diameter in the range 0 3 0 7 mm especially 0 4 0 5 mm in a conventional coating pan the core may also include other components such as a lubricant a dispersing agent or a surfactant a suitable lubricant is talc and a suitable surfactant is sodium lauryl sulphate in a preferred embodiment the core comprises a a powder mixture comprising methyldopa or a pharmaceutically acceptable salt thereof and at least one organic acid selected from the group consisting of adipic acid citric acid fumaric acid malic acid succinic acid and tartaric acid and b a polymeric material comprising a major proportion of a pharmaceutically acceptable water soluble polymer and a minor proportion of a.