We have studied the anisotropic three-dimensional nearest-neighbor Ising model with competitive interactions in an uniform longitudinal magnetic field H. The model consists of ferromagnetic interactions Jz= λ2Jx in the x(z) direction and antiferromagnetic interactions Jy=λ1Jx in the y direction (Ising superantiferromagnet). For the particular case λ1= λ2=1 we obtain the phase diagram in the H-T plane, using the framework of the differential operator technique in the effective-field theory with finite cluster of N=4 spins (EFT-4). It was observed first- and second-order transitions in the low and high temperature limits, respectively, with the presence of a tricritical point and a reentrant behavior is observed at low temperature. The critical curve in the classical approach is also obtained and the results are compared. Crown Copyright © 2013 Published by Elsevier B.V. All rights reserved.