The aim of the present double-blind, placebo-controlled, parallel-group, randomized, phase III clinical trial is to assess the efficacy, safety, and tolerability of Sinupret extract (3 x 160 mg = 480 mg daily) taken for up to 16-weeks compared with placebo in the treatment of chronic rhinosinusitis (CRS) in adults. In addition, in an exploratory approach to identify potential pharmacological modes of action underlying the expected treatment benefit, the anti-inflammatory activity of Sinupret extract will be assessed in a subset of CRS patients.