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 nonmonotonous potential in full. We discuss different regimes for several cases and find an anomalous behavior of the retarded asymptote. It can change sign and lead to a trapping potential.