Sulfasalazine is actually a chemically linked combination of Mesalamine (5-ASA) and Sulfapyridine.  This combination of Mesalamine and Sulfapyridine is known as a Functional Group in the study of chemistry.  Mesalamine is an anti-inflammatory drug and Sulfapyridine is an antibacterial agent.  Paired together, the compounds create Sulfasalazine, which is cheaper than Mesalamine by itself, but has a less favourable side-effect profile.  Notably, possible blockage of the urinary tract.  To monitor other potential side effects a patient should routinely have their blood tested while taking this drug.

Mesalamine (5-ASA)

Mesalamine is an anti-inflammatory drug that is used to treat mild to moderate Crohn's Disease and Ulcerative Colitis.  It is the portion of Sulfasalazine that reduces inflammation, but alone is more expensive than Sulfasalazine.  Mesalamine acts locally in the intestinal tract, and is more or less a topical drug that reduces inflamed portions of the gut as it washes over the surface.  Mesalamine carries with it a small risk of kidney, liver, or blood disorder and so a patient should always be monitored with periodic blood counts while taking the medication.