The coupling constant between two nearby waveguides is usually predicted using formulas derived from the perturbative theory applied to the electromagnetic Maxwell equations. These formulas, however, fail to provide any reliable estimate when the index contrast between the core of the waveguide and its cladding becomes large. We demonstrate in this paper that a good accuracy can be retrieved if the local field effect is taken into account. Moreover, we show that in case of structured and inhomogeneous cladding, an effective background index must be taken into account so that the local field effect correction remains accurate. This theoretical study is the occasion for physics-oriented discussions regarding the impact of the substrate on the interwaveguide coupling constant.