  The role of strong absorption of particles in intermidiate and final states
has been considered. The range of applicability of phenomenological model of
absorption has been studied. This model is nonuniversal. Its applicability
depends on the type of interaction Hamiltonian and matrix element used. We also
demonstrate that the violation of the unitarity condition can produce a
qualitative error in the results. The absorption (decay) in the final state
does not tend to suppress the total process probability as well as the
probability of the channel corresponding to absorption. This is true for the
reactions, decays and $n\bar{n}$ conversion in the medium.
