The aim of this study was to present the current state of knowledge regarding the use of the ketogenic diet in the treatment of obesity in adults, with particular emphasis on its safety, indications, and contraindications, and to relate these findings to its established use in drug-resistant epilepsy and other clinical conditions. Despite growing interest in its clinical application, the effectiveness and safety of this intervention remain a matter of debate. A narrative review of the literature available in PubMed and Google Scholar from 2021 to 2024 was conducted using the keywords “ketogenic diet,” “obesity,” “epilepsy,” “type 2 diabetes,” “cardiovascular diseases,” “PCOS,” “neurological diseases,” and “psychiatric disorders,” as well as their combinations with AND/OR operators. Meta-analyses, randomized controlled trials, systematic reviews, and selected clinical trials evaluating the effectiveness and safety of the ketogenic diet were included in the analysis. The ketogenic diet demonstrates effectiveness in short-term weight loss and improvement of selected metabolic parameters in patients with obesity. However, its long-term effects remain equivocal and do not appear to offer significant advantages over other dietary strategies. The diet is associated with a risk of adverse effects and requires appropriate clinical monitoring. While it is a recognized and effective therapeutic option for drug-resistant epilepsy, its use in other conditions has not been established as a standard therapeutic approach. The ketogenic diet may be considered as part of obesity treatment in carefully selected patients, provided that potential benefits and risks are individually assessed. Further high-quality studies are needed to determine its long-term effectiveness and safety.