Gastrointestinal & Hepatic · Topic 21 of 26

Constipation, Diarrhea & Gastroparesis

Gastrointestinal · motility and bowel function

Constipation, Diarrhea & Gastroparesis

Start a laxative with the first opioid dose, keep antimotility drugs away from bloody diarrhea, and prove gastroparesis with a 4-hour emptying study.

Key numbers

Gastroparesis: delayed emptying, no obstruction> 10% retained at 4 h

Metoclopramide: approved, but limit to 12 weeks≤ 12 weeks

Stool osmotic gap below 50 means secretorygap < 50 mEq/L

01 · GiveStart a laxative with every opioid

Patients rarely become tolerant to opioid slowing of the gut: start a daily stimulant (senna) and/or osmotic laxative (polyethylene glycol) when opioids begin, especially in older or immobile patients; fiber and fluids alone rarely suffice.

02 · AvoidNo antimotility drugs in bloody diarrhea

Avoid loperamide and other antimotility drugs in bloody or febrile diarrhea and in suspected C. difficile or Shiga toxin–producing E. coli: they can worsen C. difficile colitis (toxic megacolon) and raise the risk of HUS.

03 · Defined byGastroparesis: delayed emptying, no obstruction

Gastroparesis = symptomatic delayed gastric emptying of solids without mechanical obstruction: exclude obstruction with upper endoscopy or imaging, then confirm with solid-meal scintigraphy (more than 10% retained at 4 hours).

Presentation

What you'll see

Constipation red flags suggest obstructionSevere or persistent pain, a distended tympanitic abdomen, vomiting, blood in stool, weight loss, or new severe constipation in an older adult point to a serious cause such as mechanical obstruction or colon cancer.

Diarrhea red flags: blood, fever, dehydrationBlood or pus in stool, fever, dehydration, nocturnal diarrhea, weight loss or duration over 4 weeks suggest an organic cause and need stool testing and a directed workup.

Diagnosis & monitoring

Work-up

Stool osmotic gap below 50 means secretoryStool osmotic gap = 290 − 2 × (stool Na⁺ + stool K⁺). A gap below 50 mEq/L indicates secretory diarrhea; a larger gap suggests osmotic diarrhea (laxatives, sorbitol, carbohydrate malabsorption).

Management

Orders

Add a PAMORA for refractory opioid constipation must knowFor opioid-induced constipation despite laxatives, add a peripherally acting mu-opioid receptor antagonist — methylnaltrexone (subcutaneous or oral), naloxegol or naldemedine (oral); do not use with known or suspected bowel obstruction (perforation risk).

New hospital diarrhea: test for C. difficile must knowFor new diarrhea in a hospitalized patient, especially after antibiotics, test for C. difficile and review the medication list — laxatives, sorbitol-containing liquid drugs, magnesium antacids and antibiotics are common culprits.

Metoclopramide: approved, but limit to 12 weeksMetoclopramide is the only FDA-approved drug for gastroparesis and is suggested for refractory symptoms; its boxed warning for tardive dyskinesia restricts use to under 12 weeks.

Erythromycin works short term onlyErythromycin (a motilin agonist) speeds gastric emptying but is used for 1–4 weeks at a time because tachyphylaxis develops.

Jejunal feeding when oral intake failsIf oral intake cannot maintain nutrition in gastroparesis, feed through a jejunostomy tube (bypassing the stomach); parenteral nutrition is rarely needed. A venting gastrostomy is a second-line option.

Fecal impaction: enemas, then manual disimpactionTreat fecal impaction with tap-water enemas, then small hypertonic enemas; if these fail, fragment and remove the mass manually after topical anesthetic (sedation if needed).

Rehydrate first; IV fluids if severeTreat diarrhea first with fluid and electrolytes: an oral glucose-electrolyte solution when diarrhea is mild and vomiting minimal; IV saline with potassium (bicarbonate or acetate if HCO₃⁻ is below 15 mEq/L) when severe.

Red flags & don'ts

Avoid harm

Do not treat gastroparesis pain with opioids avoidPotent opioids slow gastric and pyloric function and are associated with worse gastroparesis; gastroparesis pain should not be treated with opioids, including tramadol and tapentadol.

No antibiotics for Shiga toxin E. coli avoidDo not give antibiotics for STEC O157 or other Shiga toxin 2–producing E. coli infection; most acute infectious diarrhea needs only supportive care.

Osmotic or secretory diarrhea?

Commonly confused

Stool osmotic gap	Large gap
Typical causes	PEG, magnesium, lactulose, sorbitol, lactose intolerance
Hidden cause to test for	Stool magnesium (covert laxative), hydrogen breath test

Teach & follow up

Patient education

Small-particle diet for gastroparesisFirst-line gastroparesis management is dietary — a small-particle diet — plus optimal glucose control in diabetic gastroparesis, because acute hyperglycemia itself delays gastric emptying.

More detail

Reference

Show 7 reference facts

Docusate (stool softener) is ineffective for severe constipation; stimulant or osmotic agents are preferred.

Alvimopan, an oral mu-opioid antagonist, is restricted to short-term in-hospital use in surgical patients to treat postoperative ileus; it carries a possible myocardial infarction risk.

Before a gastric emptying study, stop opioids, cannabinoids, prokinetics, antiemetics and other drugs that alter emptying for 48 hours.

Unexplained secretory diarrhea warrants endocrine testing (gastrin, calcitonin, VIP, histamine, urine 5-HIAA) and a laxative assay; osmotic diarrhea suggests covert magnesium laxatives (stool magnesium) or carbohydrate malabsorption (hydrogen breath test).

In gastroparesis, antiemetics relieve symptoms but do not speed emptying; central neuromodulators are not recommended, and intrapyloric botulinum toxin was ineffective in randomized trials (G-POEM is the pyloric option).

Chronic diarrhea (over 4 weeks) workup includes stool occult blood, fecal fat or elastase, Giardia testing, CBC, celiac serology (IgA tTG), TSH and fecal calprotectin to screen for IBD.

Secretory diarrhea comes mostly from infection (enterotoxins), plus unabsorbed fat and bile acids, drugs and endocrine secretagogues; osmotic diarrhea comes from unabsorbed solutes — PEG, magnesium, lactulose, sorbitol, or lactose in lactase deficiency.

Pearl

The colon never becomes tolerant to opioids — so the bowel regimen starts with the first opioid dose, not after the first missed stool.

Sources · Merck Manual Professional — Constipation · Merck Manual Professional — Treatment of Pain (opioid adverse effects and constipation prevention) · Merck Manual Professional — Diarrhea · Shane AL et al. 2017 IDSA Clinical Practice Guidelines for the Diagnosis and Management of Infectious Diarrhea. Clin Infect Dis 2017;65:e45–e80 · Merck Manual Professional — Overview of Gastroenteritis · Camilleri M et al. ACG Clinical Guideline: Gastroparesis. Am J Gastroenterol 2022;117:1197–1220 · Merck Manual Professional — Enteral Nutrition (sorbitol-containing liquid medications and diarrhea) · DailyMed — RELISTOR (methylnaltrexone bromide) tablets and injection: prescribing information (contraindications) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).