Sulfasalazine was one of the first drugs used to treat Crohn's disease and is still used for treating Crohn's disease that is restricted to the colon. Sulfasalazine usually begins to reduce symptoms within a few days, but its full effect may require up to four weeks of treatment. A problem with sulfasalazine is that it can cause an allergic reaction in a minority of patients and can cause headaches, particularly when given in high doses.
Aha! The reason for my constant headaches.