Can the arthritis drug sulfasalazine cause mouth ulcers and a sore mouth?

Occasionally, yes.: Sulfasalazine and other sulfonamide medications occasionally provoke ulcerative conditions, such as erythema multiforme, and occasionally, more severe, life threatening conditions like stevens johnson syndrome or toxic epidermal necrolysis. If you are experiencing ulcerations in your eyes, mouth, skin, or elsewhere, you should consult your doctor to see if your medication regimen needs changing.

Check with doc: Check with your doc, you may be allergic to it-try switching to a different med.