A composition for 
controlled delivery of at least one active substance into an 
aqueous medium by 
erosion at a preprogrammed rate of at least one surface of the composition, comprising a matrix comprising the active substance, the matrix being erodible in the 
aqueous medium in which the composition is to be used, and a 
coating having at least one opening exposing at least one surface of said matrix, the 
coating comprising a first 
cellulose derivative which has 
thermoplastic properties and which is substantially insoluble in the 
aqueous medium in which the composition is to be used, and at least one of a second 
cellulose derivative which is soluble or dispersible in water, a 
plasticizer, and a filler. The 
coating is a coating which crumbles and / or erodes upon 
exposure to the aqueous medium such as a 
body fluid. The first 
cellulose derivative may be, e.g., ethylcellulose, 
cellulose acetate, cellulose 
propionate or cellulose 
nitrate, and the second cellulose derivative may be, e.g. methylcellulose, carboxymethylcellulose or salts thereof, 
cellulose acetate phthalate, 
microcrystalline cellulose, ethylhydroxyethylcellulose, ethylmethylcellulose, hydroxyethylcellulose, hydroxyethylmethylcellulose, hydroxypropylcellulose, hydroxypropylmethylcellulose, hydroxymethylcellulose or hydroxymethylpropylcellulose.