I've heard (anecdotally) that the methane producing bacteria cause constipation, and the hydrogen producing bacteria produce diarrhea...but don't quote me on that, it was just from some non-expert comment somewhere that I read.
I was dx with the methane "flavor" of sibo and had been dealing with chronic constipation for about 6 or 7 months. Took rifaximan and neomycin. Waiting on results from follow up test to see if I need another round of treatment.
Like magic, nothing else worked. Started out using it a couple of times a day then had to back off more and more as time went by and now just use it about 3 times a month.