  We consider the van der Waals interaction between two ground-state atoms
embedded in adjacent semi-infinite magnetodielectric media, with emphasis on
medium effects on it. We demonstrate that, in this case, at small atom-atom
distances the van der Waals interaction is screened by the surrounding media in
the same way as in an effective (single) medium. At larger atomic distances,
however, its dependence on the material parameters of the system and the
positions of the atoms is more complex. We also calculate the Casimir-Polder
potential of an atom A arising from a uniform distribution of atoms B in the
medium across the interface. Comparison of this potential with the
corresponding result deduced from the Casimir force on a thin composite slab in
front of a composite semi-infinite medium, both obeying the Clausius-Mossotti
relation, suggests a hint on how to improve a well-known formula for the van
der Waals potential with respect to the local-field effects.
