In Japan, fourteen-membered ring macrolides, antibacterial agents, and S-carboxymethylcysteine (SCMC; carbocisteine), a mucolytic, are commonly used to treat chronic rhinosinusitis (CRS), and they are also used in combination. However, no large-scale randomized study has examined the effects of these pharmacotherapies. The aim of this study is to evaluate the effect of combined administration of clarithromycin (CAM), a fourteen-membered ring macrolide, and SCMC, compared with CAM single therapy.