  Most of the massive states in superstring theory are expected to undergo mass
renormalization at one loop order. Typically these corrections should contain
imaginary parts, indicating that the states are unstable against decay into
lighter particles. However in such cases, direct computation of the
renormalized mass using superstring perturbation theory yields divergent
result. Previous approaches to this problem involve various analytic
continuation techniques, or deforming the integral over the moduli space of the
torus with two punctures into the complexified moduli space near the boundary.
In this paper we use insights from string field theory to describe a different
approach that gives manifestly finite result for the mass shift satisfying
unitarity relations. The procedure is applicable to all states of
(compactified) type II and heterotic string theories. We illustrate this by
computing the one loop correction to the mass of the first massive state on the
leading Regge trajectory in SO(32) heterotic string theory.
