Intravenous (IV) therapy constitutes a foundational and indispensable modality in contemporary medical practice, offering direct and controlled systemic delivery of therapeutic agents with unparalleled pharmacokinetic precision. This comprehensive review synthesizes the physiological principles, clinical evidence, and practical protocols governing the three primary domains of modern IV therapy: fluid resuscitation, iron replacement, and vitamin/nutrient administration. We detail the evidence-based application of crystalloid solutions—isotonic, hypotonic, and hypertonic—for targeted volume and tonicity management. The analysis highlights the paradigm shift toward IV iron formulations, such as ferric carboxymaltose, for efficiently correcting iron deficiency anemia, particularly in inflammatory states or malabsorption, supported by survey data showing high rates of biochemical correction (87%) and symptomatic improvement (66%). Furthermore, the review critically delineates established, life-saving applications of IV vitamins (e.g., thiamine in Wernicke’s encephalopathy) from the evidence-light landscape of wellness-oriented nutrient infusions. Emphasis is placed on the critical importance of rigorous patient selection, adherence to sterile compounding standards (USP <797>), utilization of smart infusion technology, and vigilant monitoring to ensure safety and efficacy. The article concludes by identifying key research priorities, including the development of safer formulations and biomarker-stratified trials, to advance personalized IV therapy. Ultimately, IV therapy remains a cornerstone of medical intervention, whose optimal utility is realized through a meticulous, evidence-driven, and patient-centered approach.