  We derive viscous forces for vortices in a thin-film ferromagnet. The viscous
force acting on vortex $i$ is a linear superposition $\mathbf F_i = - \sum_{j}
\hat{D}_{ij} \mathbf V_j$, where $\mathbf V_j$ is the velocity of vortex $j$.
Thanks to the long-range nature of vortices, the mutual drag tensor
$\hat{D}_{ij}$ is comparable in magnitude to the coefficient of self-drag
$D_{ii}$.
