Escape of a particle from a metastable potential, whose motion is governed by the generalized Langevin equation, is a common model of many chemical and physical activated processes in condensed phase. In the intermediate-to-strong damping regime the rate of escape is controlled by the particle dynamics near the barrier top. Since Kramers, the parabolic barrier approximation is commonly used to get the expression for the rate in this regime. We consider the influence of anharmonic corrections to the potential barrier on the quantum rate and get leading order corrections in terms of the inverse barrier height. New terms appearing in the quantum expression for the rate are associated with tunneling through the barrier and become important at low temperatures. The analytic theory is compared with recent numerically exact quantum simulations [M. Topaler and N. Makri, J. Chem. Phys. 101, 7500 (1994)].