  The effects of radiation drag force on the structure of relativistic
electron-positron and electron-proton outflows are considered within the
one-fluid approximation for quasi-monopole cold outflow. It is shown that for a
Poynting-dominated case the drag force does not change the particle energy
inside the fast magnetosonic surface. In this region the action of the drag
results in a diminishing of the Poynting flux, not the particle flux. Outside
the fast magnetosonic surface, for intermediate photon density the drag force
may result in additional acceleration of the plasma. This acceleration is a
result of the disturbance of magnetic surfaces under the action of the drag. At
even larger distances particles are not frozen into the magnetic field and the
drag force decelerates them efficiently.
  In the case of extreme photon densities, the disturbance of magnetic surfaces
becomes large and the drag force changes the total energy flux significantly,
the particles becoming nonrelativistic.
  We find that for Active Galactic Nuclei the photon density is too low to
disturb the parameters of an ideal MHD outflow. The drag action may result in
additional acceleration of outgoing plasma only for central engines with very
high luminosities. For cosmological gamma-ray bursts the drag force can
strongly affect the process of formation of a Poynting-dominated outflow.
