Previcox and Rimadyl are both nonsteroidal anti-inflammatory drugs (NSAIDs) that are commonly used in dogs to relieve pain and inflammation. These are not the same medication, despite some similarities. The active ingredient in Previcox is firocoxib, while the active ingredient in Rimadyl is carprofen. Both drugs prevent the synthesis of prostaglandins, which are in charge of producing inflammation and discomfort.