Sulfasalazine and sulfites are two different substances. While it is possible for there to be a connection (they're both sulfur containing substances), it is more likely something else caused your sulfite sensitivity. Vitamin B12, magnesium, zinc, iron, and molybdenum are nutrients that are needed to offset the sulfite sensitivity (these nutrients help the enzyme that processes sulfites to work better). Taking these nutrients should increase the amount of sulfite you can tolerate before experiencing a reaction - IF the problem is due to a deficiency in the sulfite processing enzyme activity. Vitamin B12 also directly binds to sulfites, rendering them harmless to the body, so if you do get exposed to sulfites accidentally, you can take oral Vitamin B12 to reduce the symptoms of the sulfite reaction. IF you are allergic to sulfites, the nutrients should still help, but won't help as much.
I happen to also be allergic to sulfasalazine and have sulfite sensitivity, but I tend to not do well with anything that contains sulfur (this is not the norm for people with sulfite sensitivity - usually, sulfa-drug allergy and sulfite sensitivity don't occur together).
I'm aware they're not the same substance. But I have often wondered about
the sulfur connection.
Coincidentally, I just bought some sublingual B12/folic acid to try to see if it might help boost my energy level. I need to also find a good multi-vitamin with minerals that I can tolerate.