Medications for IBS-C include osmotic laxatives and guanylate cyclase-c agonists, both of which can increase water retention in the bowel and stimulate intestinal motility. Antidepressants and antispasmodics may also be used to reduce abdominal pain.[1]
There’s some evidence that methane could slow intestinal transit, and antibiotic treatment of methane-positive small-intestinal bacterial overgrowth (SIBO) improved constipation in a small number of studies.[2]