We study the effect of Bethe-Heitler (BeHe) pair production on a proton synchrotron model for the prompt emission in gamma-ray bursts (GRBs). The possible parameter space of the model is constrained by consideration of the synchrotron radiation from the secondary BeHe pairs. We find two regimes of interest. (1) At high bulk Lorentz factor, large radius, and low luminosity, proton synchrotron emission dominates and produces a spectrum in agreement with observations. For part of this parameter space, a subdominant (in the MeV band) power law is created by the synchrotron emission of the BeHe pairs. This power law extends up to few tens or hundreds of MeV. Such a signature is a natural expectation in a proton synchrotron model, and it is seen in some GRBs, including GRB 190114C recently observed by the MAGIC observatory. (2) At low bulk Lorentz factor, small radius, and high luminosity, BeHe cooling dominates. The spectrum achieves the shape of a single power law with spectral index alpha = -3/2 extending across the entire Gamma-ray Burst Monitor/Swift energy window, incompatible with observations. Our theoretical results can be used to further constrain the spectral analysis of GRBs in the guise of proton synchrotron models.