Lower GI Pharmacology: Constipation, Diarrhea, and IBS

Osmotic laxatives retain water in the intestinal lumen through osmotic pressure, increasing luminal fluid volume, softening stool, and stimulating propulsive contractions. Polyethylene glycol 3350 is a high-molecular-weight polymer that passes through the gastrointestinal tract intact without absorption or metabolism, dragging an obligate water load. It is the preferred osmotic agent for chronic constipation — negligible systemic absorption, no significant electrolyte shifts at standard doses, and appropriate for elderly patients and those with renal impairment. Lactulose is a non-absorbable disaccharide fermented by colonic bacteria to short-chain fatty acids, lowering pH and increasing osmotic pressure; fermentation-related bloating limits tolerability. Magnesium hydroxide and magnesium citrate act osmotically and also stimulate cholecystokinin release; they should be used cautiously in renal impairment because magnesium is partially absorbed.

Senna and bisacodyl stimulate myenteric plexus neurons and inhibit colonic fluid absorption. Sennosides are prodrugs activated by colonic bacteria to anthranoid metabolites. Bisacodyl is converted by intestinal esterases to its active form, which directly stimulates submucosal secretomotor neurons. Both have a 6 to 12 hour oral onset. The historical concern that chronic use causes permanent enteric nerve damage (cathartic colon) has not been supported by prospective data; current evidence supports their safety for long-term use when needed.

Lubiprostone activates type-2 chloride channels on the apical surface of intestinal epithelial cells, driving chloride and water secretion into the lumen. It is approved for chronic idiopathic constipation, irritable bowel syndrome with predominant constipation, and opioid-induced constipation. Minimal systemic absorption confines activity to the intestinal lumen. Nausea occurs in approximately 30 percent of patients and is reduced by taking with food. Linaclotide and plecanatide are guanylate cyclase-C agonists — discussed further in Section 4 in the context of irritable bowel syndrome.

Opioid-induced constipation arises because mu-opioid receptors in the enteric nervous system — present at the highest density of any site outside the central nervous system — are activated by opioid analgesics, producing reduced propulsive motility, increased segmental contractions, reduced secretion, and increased anal sphincter tone. Conventional laxatives do not address this receptor-mediated mechanism and are frequently inadequate. Peripherally acting mu-opioid receptor antagonists were developed to block gastrointestinal opioid effects without reversing central nervous system analgesia.

Methylnaltrexone is a quaternary ammonium derivative of naltrexone; the permanent positive charge substantially limits central nervous system penetration, confining antagonism to peripheral mu-opioid receptors. Naloxegol is a polyethylene glycol-conjugated naloxol derivative that is a P-glycoprotein substrate at the blood-brain barrier, reducing central nervous system exposure to less than 1 percent of plasma concentrations. Naldemedine has a bulky side chain reducing blood-brain barrier penetration and once-daily oral dosing. All three reverse opioid-induced constipation within 24 to 48 hours without compromising analgesia. They are contraindicated when gastrointestinal obstruction is known or suspected. Naloxegol is a cytochrome P450 3A4 substrate; strong cytochrome P450 3A4 inhibitors are contraindicated and moderate inhibitors require dose reduction.

Oral vancomycin 125 milligrams four times daily for 10 days achieves high intraluminal concentrations — exceeding 1000 micrograms per milliliter in stool — with negligible systemic absorption, avoiding the nephrotoxicity and ototoxicity of intravenous vancomycin. Fidaxomicin 200 milligrams twice daily for 10 days is a macrolide antibiotic minimally absorbed from the gastrointestinal tract that inhibits bacterial RNA polymerase at a site distinct from rifamycins. In two phase 3 randomized controlled trials, fidaxomicin produced similar clinical cure rates to vancomycin but significantly lower recurrence rates for non-hypervirulent strains — approximately 40 percent lower recurrence — attributed to its narrower spectrum and relative sparing of colonization-resistance organisms that vancomycin suppresses. Fidaxomicin is now the preferred first-line agent for non-severe Clostridioides difficile infection per major guidelines, though cost limits access in some settings.

Metronidazole has been downgraded by the Infectious Diseases Society of America guidelines and is no longer recommended as first-line for any severity category. Head-to-head trials show inferior cure rates and higher recurrence compared with vancomycin. Metronidazole is now an alternative only when vancomycin and fidaxomicin are unavailable or cost-prohibitive.

Severe Clostridioides difficile infection — defined by white blood cell count greater than 15,000 cells per microliter or serum creatinine greater than 1.5 milligrams per deciliter — is treated with oral vancomycin 125 milligrams four times daily, as available data favor vancomycin over fidaxomicin for severe disease. Fulminant disease with hypotension, ileus, or toxic megacolon requires vancomycin 500 milligrams four times daily orally or via nasogastric tube plus intravenous metronidazole 500 milligrams every 8 hours, with immediate surgical consultation.

Fecal microbiota transplant transfers screened donor stool into the gastrointestinal tract of patients with recurrent Clostridioides difficile infection, restoring colonization resistance through microbiome reconstitution. Meta-analyses of randomized controlled trials demonstrate resolution rates of 80 to 90 percent for multiply recurrent infection, substantially higher than antibiotic therapy alone. The FDA has approved standardized fecal microbiota products for recurrent Clostridioides difficile infection: Rebyota (fecal microbiota, live-jslm) as a rectal formulation and Vowst (fecal microbiota spores, live-brpk) as oral capsules approved in 2023.

Bezlotoxumab is a human monoclonal antibody against Clostridioides difficile toxin B given as a single intravenous infusion of 10 milligrams per kilogram during standard antibiotic treatment. It does not treat active infection but reduces recurrence risk by approximately 10 percentage points compared with placebo — from approximately 26 percent to 17 percent — with the greatest benefit in high-risk patients including those aged 65 or older, immunocompromised patients, and patients with prior Clostridioides difficile episodes. Bezlotoxumab carries a black box warning for heart failure exacerbation and should be used cautiously in patients with pre-existing heart failure.