  We calculate finite-temperature corrections to the decay rate of a generic
neutral (pseudo)scalar particle that decays into (pseudo)scalars or
fermion-antifermion pairs. The ratio of the finite-temperature decay rate to
the zero-temperature decay rate is presented. Thermal effects are largest in
the limit where the decaying particle is nonrelativistic but with a mass well
below the background temperature, but significant effects are possible even
when we relax the former assumption. Thermal effects are reduced for the case
of nonzero momentum of the decaying particle. We discuss cosmological scenarios
under which significant finite-temperature corrections to the decay rate can be
achieved.
