  We study the impact of the Sommerfeld enhancement on the thermal relic
density of the lightest neutralino in the case of large co-annihilation effects
with a scalar particle. The proper way of including the Sommerfeld effect in
this case is discussed, and the appropriate formulas for a general scenario
with a set of particles with arbitrary masses and (off-)diagonal interactions
are provided. We implement these results to compute the relic density in the
neutralino sfermion co-annihilation regions in the mSUGRA framework. We find
non-negligible effects in whole sfermion co-annihilation regimes. For stau
co-annihilations the correction to the relic density is of the order of several
per cent, while for stop co-annihilations is much larger, reaching a factor of
5 in some regions of the parameter space. A numerical package for computing the
neutralino relic density including the Sommerfeld effect in a general MSSM
setup is made public available.
