An ab initio study of the formation and segregation energies of B and P doped and BP codoped silicon nanowires oriented along the [110] direction is performed for fully relaxed H-passivated wires with a diameter of 1.2 and 1.6 nm. We found that the B and P dopants will migrate to the edge of the wire and that the formation energy for codoping is smaller than that for the single doped cases. In ultrathin wires it is possible to have a larger number of dangling bonds than dopant atoms per unit length; the effect of these defects on the formation and segregation energy is substantial. We found that P dopants are more easier trapped, and thus become electronically inactive, than B dopants.