ObjectiveTo systematically review the clinical effects of non-humidified versus humidified low-to-moderate flow oxygen inhalation therapy via nasal cannula. MethodsRandomized controlled trials (RCTs), clinical controlled trials (CCTs) and cross-over studies about the clinical effects of non-humidified versus humidified low-to-moderate flow nasal cannula oxygen inhalation therapy in hospitalized adult patients were searched in The Cochrane Library (Issue 3, 2016), The Joanna Briggs Institute Evidence Based Practice (EBP) Database, EMbase, PubMed, Web of Science, CBM, CNKI, VIP and WanFang Data from inception to March 2016. Three reviewers independently screened literature, extracted data and assessed the risk of bias of included studies. Then meta-analysis was performed using RevMan 5.3 software. ResultsA total of nine RCTs and three CCTs involving 3 756 patients were finally included. The results of meta-analysis indicated that: non-humidified was superior to humidified oxygen therapy in reducing bacterial contamination of oxygen apparatus (P < 0.05) and the time of daily replacement of sterile water oxygen humidifier bottles (P < 0.05), while the two groups were alike in relieving nasal dryness (RR=1.08, 95%CI 0.91 to 1.29, P=0.37), nasal bleeding (RR=1.17, 95%CI 0.66 to 2.08, P=0.59) and discomfort (RR=0.80, 95%CI 0.56 to 1.14, P=0.22). ConclusionCurrent evidence indicates that there is no significant difference between non-humidified and humidified low-to-moderate flow nasal cannula oxygen inhalation therapy in relieving patients' nasal dryness, nasal bleeding and discomfort. But non-humidified oxygen therapy can reduce bacterial contamination of oxygen apparatus, simplify the operation procedures and lessen nurses' operation time.