In this work, drug release from matrices with an inert nucleus using Monte Carlo simulation was studied. Drug-excipient systems were simulated, where the drug is a soluble material while the excipient is a non-soluble material. In the center of these devices, an inert nucleus was placed. The release of the drug was unidirectional and the results were fitted to the square root of time law (Higuchi law), the power law and the Weibull equation. The percolation threshold of the drug was found to be near 0.35 close to the expected value for the cubic lattice, the difference is due to the finite and rather small size of the systems in study as well as to the fact that the lattice in use is not exactly cubic. Near the percolation threshold, the parameters of the different release models presented a drastic change; this was due to a phase transition of the system. On the other hand, it was found that the size of the matrix system modifies the transport properties of the release platform. In general, the release kinetics was adequately described by the Weibull equation.