Ulcerative colitis and Crohn's disease affecting the colon, both to settle a flare and to prevent the next one, and rheumatoid arthritis and some forms of spondyloarthritis where it slows joint damage.

Gut bacteria split the molecule in the colon into two halves. One half, 5-aminosalicylic acid, stays in the bowel and damps down inflammation in the lining. The other, sulfapyridine, is absorbed into the blood and is thought to carry the effect on inflamed joints.

Nausea, loss of appetite, headache, rash and a harmless orange colour to the urine, skin, and soft contact lenses. Low mood and dizziness occur in some people.

Regular blood tests are required, especially in the first months, because sulfasalazine can suppress the bone marrow and affect the liver. Report a sore throat, fever, unexplained bruising or bleeding, or extreme tiredness straight away. Not for people allergic to sulfonamides or aspirin. It can reduce sperm count temporarily, which reverses after stopping. Folic acid is often prescribed alongside it. It is a prescription medicine in Pakistan.