Purpose <p>Complex peptide injectables represent an advanced class of pharmaceuticals with high therapeutic potential. These peptides, which mimic or modulate naturally occurring peptides in the body, offer targeted and effective treatment for various diseases. However, their structural complexity—influenced by length, sequence, and post-translational modifications—presents challenges in formulation, stability, and delivery. This review explores the development, challenges, and strategies for optimizing complex peptide injectables, with case studies highlighting their clinical applications.</p> Methods <p>The review examines key formulation and delivery strategies employed to enhance the stability and bioavailability of complex peptides. Approaches such as nanoparticle encapsulation, carrier molecule conjugation, and sustained-release formulations are analyzed to determine their effectiveness in overcoming enzymatic degradation and ensuring targeted delivery. Case studies from various therapeutic areas, including oncology, metabolic disorders, and infectious diseases, are used to illustrate real-world applications and advancements.</p> Results <p>Despite formulation challenges, complex peptide injectables demonstrate high specificity and potency, enabling precise targeting with minimal off-target effects. Their unique ability to interact with biological targets inaccessible to small molecules expands therapeutic options, making them highly promising for next-generation drug development. Innovations in formulation technology have led to improved stability, controlled release, and enhanced efficacy, as evidenced by successful applications in cancer therapy, metabolic regulation, and antimicrobial treatments.</p> Conclusion <p>This review provides a comprehensive overview of peptide drug development, associated challenges, and real-world case studies, contributing to the growing body of knowledge in peptide-based therapeutics.</p>