We consider elastic scattering of electrons off a proton target. The parity-violating (PV) asymmetry arises at leading order in α due to interference of γ and Z exchange. The radiative corrections to this leading mechanism were calculated in the literature and included in experimental analyses, except for γZ-box and cross-box contributions. We present here a dispersion calculation of these corrections in forward kinematics. We demonstrate that at the GeV energies of current PV experiments, such corrections are not suppressed by the small vector weak charge of the electron, as occurs in the atomic parity violation. Our results suggest that the current theoretical uncertainty in the analysis of the QWEAK experiment might be substantially underestimated, and more accurate accounts of the dispersion corrections are needed in order to interpret the PV data.