This study aimed to investigate the effectiveness and safety of glucosamine, chondroitin, the two in combination, or celecoxib in the treatment of knee osteoarthritis (OA). PubMed, Embase and Cochrane Library were searched through from inception to February 2015. A total of 54 studies covering 16427 patients were included. Glucosamine plus chondroitin, glucosamine alone, and celecoxib were all more effective than placebo in pain relief and function improvement. Specifically, celecoxib is most likely to be the best treatment option, followed by the combination group. All treatment options showed clinically significant improvement from baseline pain, but only glucosamine plus chondroitin showed clinically significant improvement from baseline function. In terms of the structure-modifying effect, both glucosamine alone and chondroitin alone achieved a statistically significant reduction in joint space narrowing. Although no significant difference was observed among the five options with respect to the three major adverse effects (withdrawal due to adverse events, serious adverse events and the number of patients with adverse events), the additional classical meta-analysis showed that celecoxib exhibited a higher rate of gastrointestinal adverse effect comparing with the placebo group. The present study provided evidence for the symptomatic efficacy of glucosamine plus chondroitin in the treatment of knee OA.