A simple mathematical model based on a system of reaction-diffusion equations makes it possible to study the influence of defect interactions on effective diffusion of implanted impurities and to determine the broadening of the impurity profiles during annealing. It is shown that special initial conditions may give rise to anomalous diffusion processes, e.g., the transient diffusion. The effective diffusion coefficient and its dependence on reaction constants is discussed under the assumption that the as-implanted profile has a δ-function or an LSS form.