Complex sentence: However, adherent mucus does not appear to offer much protection against exogenous damaging agents such as alcohol and aspirin, which permeate the adherent mucus layer, damaging the underlying epithelium.
Split into simpler sentences: However, adherent mucus does not appear to offer much protection against exogenous damaging agents, e.g. alcohol and aspirin. These agents permeate the adherent mucus barrier, damaging the underlying epithelium.