Controlled release pellet formulation

pharmaceutical composition comprising a coating heterogeneous mixture comprising: (a) at least 75% by weight of a polymer which is insoluble in water at both acidic pH and a basic pH; and (b) 1-25% by weight of an enteric polymer which is substantially insoluble in water at a pH below 4, 5 and which is substantially soluble in water at a pH greater than 6, 0, (c) 0 1-8% by weight of a water soluble polymer; wherein the weight ratio [(a) / (b)] between the insoluble polymer (a) and the enteric polymer (b) is in the range 3-20.