  In this work we are concerned with the understanding of the collisionless
drag or entrainment between two superfluids, also called Andreev-Bashkin
effect, in terms of current response functions. The drag density is shown to be
proportional to the cross transverse current-current response function, playing
the role of a normal component for the single species superfluid density. We
can in this way link the existence of finite entrainment with the exhaustion of
the energy-weighted sum rule in the spin channel. The formalism is then used to
reproduce some known results for a weakly interacting Bose-Bose mixture.
Finally we include the drag effect to determine the beyond mean-field
correction on the speed of sound and on the spin dipole excitations for a
homogeneous and trapped gas, respectively.
