1. The study evaluated the use of excipient combinations to reduce the viscosity of highly concentrated monoclonal antibody formulations for subcutaneous injection. 2. Results showed that certain excipient combinations reduced viscosity in a synergistic manner, allowing formulations with viscosities low enough for subcutaneous delivery. Individual excipients had only minor effects on viscosity. 3. Forced degradation studies found that some excipient combinations maintained protein stability similar to formulations without excipients, while other combinations reduced stability. The best combinations balanced viscosity reduction and stability.