Parenteral administration is an effective route for fast-acting treatments and conditions that demand high bioavailability or require avoidance of hepatic first-pass metabolism. However, developing injectable drugs with poorly water-soluble drugs finds several challenges, including precipitation risk, stability issues, and possible toxic effects. Injectable nanocrystals (NCs) provide a pathway to enhance bioavailability by addressing these limitations and allow the drug to be released in a controlled manner without organic solvents. Parenteral NCs showed superior pharmacokinetic profiles, prolonged therapeutic effects, and optimised patient adherence to chronic condition treatments. Intramuscular administration of NCs allows controlled and sustained drug release by forming depots that maintain therapeutic levels for weeks or months, with granulomatous inflammation playing a key role in modulating release. Despite their potential, NCs face obstacles in sterilisation, standardisation for industrial scale, and approval by regulatory agencies, requiring advances in nanotechnology, materials science, and quality control. Current technologies, such as artificial intelligence and targeted delivery systems, can directly impact the optimisation of the development and approval of these drugs. This chapter explores the scientific, clinical, and industrial implications of NCs, emphasising their revolutionary potential in treating acute and chronic diseases while highlighting the need for an interdisciplinary alliance to overcome existing challenges.