In the last few decades, a number of probabilistic models for drug delivery have been developed. Of particular interest are those that model controlled release systems to provide targeted dose delivery. Controlled release is achieved by using polymers with dierent dissolution characteristics. We present here a model based on Monte Carlo and Cellular Automata approaches, for simulating drug release from coated microspheres in the gastro-intestinal tract. Controlled release is obtained using ethylcellulose as the coating polymer. Modelling features, such as the drug and coating dissolution are nontrivial, since material is non-homogenously dispersed and the dissolution exhibits complex behaviour. Important underlying mechanisms of the process, such as erosion, are described here.