Used primarily in the treatment of feline infectious peritonitis (FIP), a fatal viral disease in cats caused by a mutation of feline coronavirus. It works by inhibiting viral RNA replication, effectively stopping the spread of the virus in the body. The compound has shown high efficacy in both wet and dry forms of FIP, leading to remission in a significant number of treated cats. It is administered via subcutaneous injection or oral formulation over several weeks. Due to lack of regulatory approval in many countries, it is often used under compassionate use or off-label conditions. Research is ongoing into its potential antiviral activity against other RNA viruses.