  We argue that at subgap energies quantum behavior of superconducting weak
links can be exactly accounted for by an effective Hamiltonian for a Josephson
particle in a quantum dissipative environment formed by Andreev levels. This
environment can constitute an important source for intrinsic inelastic
relaxation and dephasing in highly transparent weak links. We investigate the
problem of macroscopic quantum tunneling in such weak links demonstrating that
-- depending on the barrier transmission -- the supercurrent decay can be
described by three different regimes: ($i$) weak intrinsic dissipation, ($ii$)
strong intrinsic dissipation and ($iii$) strong capacitance renormalization.
Crossover between quantum and thermally-assisted supercurrent decay regimes can
also be strongly affected by the Andreev level environment.
