while oral 5-asa have relatively few side effects (compared to drugs like Pred or immunosuppressants), and are said to act mostly topically, there is some systematic absorption. I have been told that the systemic absorption of the enemas is LESS, so that would mean less side effects.
However, I had some increase in bleeding and pain after the 1st week of Rowasa. Guess that counts as a side effect. My GI said this sometimes happens, and added hydrocortisone enemas for alternate nights or for several nights weekly. (I ended up taking a mix of the two most nights, and seems to work well).
Initially retention was an issue. The hydorcortisone reduced inflammation and really helped me tolerate/retain the Rowasa. Now after a few months, retaining all night and getting good sleep is just not an issue, but it can be a hassle at first if the condition of the rectum and sigmoid is poor.
If the UC is mostly in the sigmoid, then I think it makes sense to taper down the oral meds to maintenance levels, where they can insure your UC does not spread to transverse or right colon. Possibly switch to Lialda and only do one or two tablets daily.
I would give some pause to enemas and slipper elm together. The 5-asa needs to act topically and you may not get full benefit of the enemas if you mucosal lining is coated with slippery elm. I don't this this potential interaction has been clinically studied.