We develop a perturbation theory for slow-light photonic-crystal waveguides engineered to suppress group-velocity
dispersion, and predict that weak material loss (gain) is enhanced proportionally to the slow-down
factor, whereas the attenuation (amplification) rate saturates for loss (gain) exceeding a certain threshold. This
happens due to hybridization of propagating and evanescent modes which allows significant intensity enhancement
observed in our numerical simulations for photonic crystal waveguides even under strong material losses.