I take a low dose of sulfasalazine on top of my 3g pentasa. I had to get blood tests every 2 weeks while I built up my dose from one pill/day to my current dose of 3 pills/day. They were checking liver function, because this medicine can adversely affect the liver. I took blood test for about 3 months and then when all was fine I was able to skip the blood tests.
The only slightly negative side effect I've had is really yellow urine. There's an increased risk of headache, so I make sure I take my larger dose at night when I'm going to sleep anyway. I've been getting frequent headaches as part of perimenopause, and I haven't noticed them getting any worse with this drug.
I credit sulfazine with significantly reducing the arthritis I was having in my hands and wrists.