Drug-resistant epilepsy (DRE) affects 20-30% of patients with epilepsy who fail to achieve seizure control with antiseizure medications, posing a significant therapeutic challenge. In this narrative review, we examine the clinical efficacy and safety of the classic ketogenic diet (cKD) and its variants, including the modified Atkins diet (MAD), medium-chain triglyceride diet (MCTD), and low glycemic index treatment (LGIT), in patients with genetically confirmed drug-resistant epilepsy. These diets induce a metabolic shift from glucose to ketones, enhance mitochondrial function, modulate neurotransmitter balance, and exert anti-inflammatory effects. However, genetic factors strongly influence the efficacy and safety of the cKD, with absolute indications including glucose transporter type 1 deficiency syndrome (GLUT1DS) and pyruvate dehydrogenase complex deficiency (PDCD). Preferred adjunctive applications of the KD include genetic epilepsies, such as SCN1A-related Dravet syndrome, TSC1/TSC2-related tuberous sclerosis complex, and UBE3A-related Angelman syndrome. However, because of the risk of metabolic decompensation, the cKD is contraindicated in patients with pathogenic variants of pyruvate carboxylase and SLC22A5. Recent advancements in precision medicine suggest that genetic and microbiome profiling may refine patient selection and optimize KD-based dietary interventions. Genome-wide association studies and multiomics approaches have identified key metabolic pathways influencing the response to the cKD, and these pave the way for individualized treatment strategies. Future research should integrate genomic, metabolomic, and microbiome data to develop biomarker-driven dietary protocols with improved efficacy and safety. As dietary therapies continue to evolve, a personalized medical approach is essential to maximize their clinical utility for genetic epilepsy and refractory epilepsy syndromes.