  Recent observations show that the temporal decay of the R-band afterglow from
GRB 990123 steepened about 2.5 days after the burst. We here propose a possible
explanation for such a steepening: a shock expanding in a dense medium has
undergone the transition from a relativistic phase to a nonrelativistic phase.
We find that this model is consistent with the observations if the medium
density is about $3\times 10^6 {\rm cm}^{-3}$. By fitting our model to the
observed optical and X-ray afterglow quantitatively, we further infer the
electron and magnetic energy fractions of the shocked medium and find these two
parameters are about 0.1 and $2\times 10^{-8}$ respectively. The former
parameter is near the equipartition value while the latter is about six orders
of magnitude smaller than inferred from the GRB 970508 afterglow. We also
discuss possibilities that the dense medium can be produced.
