Approximate analytical closed energy formulas for semirelativistic Hamiltonians of the form σ p p 2 + m 2 + V (r) are obtained within the framework of the auxiliary field method. This method, which is equivalent to the envelope theory, has been recently proposed as a powerful tool to get approximate analytical solutions of the Schrödinger equation. Various shapes for the potential V (r) are investigated: power-law, funnel, square root, and Yukawa. A comparison with the exact results is discussed in detail.