Sulfasalazine (SSZ), marketed under the trade name Azulfidine among others, is a medication used to treat rheumatoid arthritis. It was believed at the time that bacterial infections were the cause of rheumatoid arthritis. Sulfasalazine is a sulfa drug, a derivative of mesalazine. It was developed in the 1950s. It is on the World Health Organization's List of Essential Medicines, a list of the most important medication needed in a basic health system.