Nabumetone Side Effects. Nabumetone is the generic form of the brand-name drug Relafen, a prescription medicine used to treat pain and inflammation caused by osteoarthritis, rheumatoid arthritis, or gout. Nabumetone is in a class of medications called nonsteroidal anti-inflammatory drugs (NSAIDs).It works by blocking an enzyme that creates a hormone (prostaglandin) that causes inflammation and pain. Reviews. Nabumetone is the generic form of the brand-name drug Relafen, a prescription medicine used to treat pain and inflammation caused by osteoarthritis, rheumatoid arthritis, or gout. Nabumetone is in a class of medications called nonsteroidal anti-inflammatory drugs (NSAIDs).