There is ongoing debate concerning non-nutritive sweeteners, their usage, and their effects on metabolism. The association between non-nutritive sweeteners consumption, development of metabolic diseases, and changes in appetite-regulating hormones is not clear. The aim of this article is to present an overview of non-nutritive sweeteners and to examine the scientific evidence of their effects on glucose metabolism and appetite-regulating hormones. Some observational studies suggest an association between non-nutritive sweeteners consumption and development of metabolic diseases; however, adiposity is a confounder frequently found in these studies. Results of the available clinical trials are heterogeneous and not comparable because of major differences between them. Future controlled studies evaluating specific non-nutritive sweeteners, with an appropriate sample size, including a uniform study group, with sufficient exposure time, and considering adjustment for confounder variables, such as anthropometric characteristics, previous consumption of non-nutritive sweeteners, and coexistence of significant metabolic comorbidities, are needed.