  We reanalyze the prompt muon neutrino flux from gamma-ray bursts (GRBs), at
the example of the often used reference Waxman-Bahcall GRB flux, in terms of
the particle physics involved. We first reproduce this reference flux treating
synchrotron energy losses of the secondary pions explicitly. Then we include
additional neutrino production modes, the neutrinos from muon decays, the
magnetic field effects on all secondary species, and flavor mixing with the
current parameter uncertainties. We demonstrate that the combination of these
effects modifies the shape of the original Waxman-Bahcall GRB flux
significantly, and changes the normalization by a factor of three to four. As a
consequence, the gamma-ray burst search strategy of neutrino telescopes may be
based on the wrong flux shape, and the constraints derived for the GRB neutrino
flux, such as the baryonic loading, may in fact be already much stronger than
anticipated.
