  In the study of dispersion forces, nonretarded, retarded and thermal
asymptotes with their distinct scaling laws are regarded as cornerstone results
governing interactions at different separations. Here, we show that when
particles interact in a medium, the influence of retardation is qualitatively
different, making it necessary to consider the non-monotonous potential in
full. We discuss different regimes for several cases and find an anomalous
behaviour of the retarded asymptote. It can change sign, and lead to a trapping
potential.
