  Modified gravity theories predict in general a non standard equation for the
propagation of gravitational waves. Here we discuss the impact of modified
friction and speed of tensor modes on cosmic microwave polarization B modes. We
show that the non standard friction term, parametrized by $\alpha_{M}$, is
degenerate with the tensor-to-scalar ratio $r$, so that small values of $r$ can
be compensated by negative constant values of $\alpha_M$. We quantify this
degeneracy and its dependence on the epoch at which $\alpha_{M}$ is different
from the standard, zero, value and on the speed of gravitational waves $c_{T}$.
In the particular case of scalar-tensor theories, $\alpha_{M}$ is constant and
strongly constrained by background and scalar perturbations, $0\le \alpha_{M}<
0.01$ and the degeneracy with $r$ is removed. In more general cases however
such tight bounds are weakened and the B modes can provide useful constraints
on early-time modified gravity.
