We calculate corrections to the BCS gap equation caused by the interaction of electrons with the collective phase and amplitude modes in the superconducting state. This feedback reduces the BCS gap parameter Δ and leaves the critical temperature Tc unchanged. The feedback effect is proportional to (Δ/F)2, where F is the Fermi energy. This is a negligible correction for type-I superconductors. However, in type-II superconductors the feedback effect is greatly enhanced due to smaller Fermi velocities vF, and may be responsible for effects seen in recent experimental data on organic superconductors.