The Toxen relation has often been used in the past to extract, from critical-magnetic-field data, an approximate value for the ratio of the energy gap to the critical temperature. Here, we test the validity of this procedure against extensive theoretical results for real superconductors with known electron-phonon spectral densities. To get further insight we study scaled as well as delta-function model spectral densities and introduce functional-derivative techniques. Use of the Toxen relation is not recommended in the strong-coupling regime.