Sulfasalazine, sold under the brand name Azulfidine among others, is a medication used to treat rheumatoid arthritis, ulcerative colitis, and Crohn's disease.

[5] One proposed mechanism is the inhibition of prostaglandins, resulting in local anti-inflammatory effects in the colon.

[12] Sulfasalazine inhibits dihydropteroate synthase, and can cause folate deficiency and megaloblastic anemia.

[18] Sulfasalazine may cause stomach upset, nausea, vomiting, loss of appetite, headache, dizziness, or unusual tiredness.

[citation needed] Around 90% of a dose of sulfasalazine reaches the colon, where most of it is metabolized by bacteria into sulfapyridine and mesalazine (also known as 5-aminosalicylic acid or 5-ASA).

[3] A mix of unchanged, hydroxylated, and glucuronidated sulfapyridine is eliminated in urine, as is acetylated mesalazine and unmetabolized sulfasalazine.

[3][2] The mechanism of action is not clear, but it appears that sulfasalazine and its metabolites have immunosuppressive, antibacterial, and anti-inflammatory effects.

[22] It is commonly used in treating inflammatory bowel disease in part due to its cost effectiveness.