Diarrhea: Drugs That Cause It, Drugs That Treat It, and When to Act

What Causes Diarrhea?

Diarrhea results when fluid secretion into the intestinal lumen exceeds absorption, producing loose or watery stools at least three times per day. The main mechanisms are osmotic overload, secretory stimulation, inflammatory mucosal damage, and accelerated motility. Identifying the mechanism narrows the drug list and guides treatment.

Infectious Causes

Viral gastroenteritis accounts for the majority of acute diarrhea in high-income countries. Norovirus alone is responsible for an estimated 685 million cases of acute gastroenteritis globally each year, according to a 2019 systematic review published in The Lancet Infectious Diseases (1). Bacterial causes include Clostridioides difficile, Salmonella, Campylobacter, and Shiga-toxin-producing Escherichia coli (STEC O157:H7). Protozoal infections such as Giardia lamblia tend to produce longer-lasting, fatty stools rather than the acute watery presentation of viral illness.

Non-Infectious Causes

Inflammatory bowel disease (IBD), irritable bowel syndrome with diarrhea (IBS-D), celiac disease, microscopic colitis, and bile acid malabsorption all produce chronic or recurrent diarrhea. A 2020 ACG Clinical Guideline notes that IBS-D affects 5 to 10% of the global population, and that low-FODMAP dietary restriction reduces stool frequency in approximately 50 to 60% of patients (2).

Osmotic vs. Secretory: Why It Matters Clinically

Osmotic diarrhea stops with fasting; secretory diarrhea does not. This simple bedside distinction guides whether a dietary change or a pharmacologic intervention is the logical next step. High-volume watery stools exceeding 1 liter per day that persist despite fasting suggest a secretory process (cholera toxin, VIPoma, bile acid excess) and require targeted therapy.

Drugs That Cause Diarrhea

Drug-induced diarrhea is among the most under-recognized culprits in ambulatory medicine. A prospective cohort study published in the American Journal of Gastroenterology estimated that medications account for up to 7% of all chronic diarrhea cases presenting to gastroenterology clinics (3).

Antibiotics and C. Difficile

Antibiotics disrupt the colonic microbiome, allowing C. Difficile overgrowth and toxin production. Clindamycin carries the highest per-course risk, followed by fluoroquinolones (ciprofloxacin, levofloxacin) and amoxicillin-clavulanate. The 2021 IDSA/SHEA guidelines define a C. Difficile infection (CDI) as three or more loose stools per day with a positive stool PCR or toxin assay, and recommend oral vancomycin 125 mg four times daily for 10 days as first-line treatment for non-severe CDI (4). Fidaxomicin 200 mg twice daily for 10 days is preferred when recurrence risk is high.

GLP-1 Receptor Agonists (Semaglutide, Tirzepatide, Liraglutide)

GLP-1 receptor agonists slow gastric emptying and alter intestinal motility, producing diarrhea in 9 to 30% of users. In the STEP-1 trial (N=1,961), semaglutide 2.4 mg subcutaneous weekly produced diarrhea in 29.7% of participants versus 16.1% on placebo (5). The SURMOUNT-1 trial (N=2,539) reported diarrhea in 17.4% of patients receiving tirzepatide 15 mg versus 6.4% on placebo (6).

GI side effects typically peak in the first 4 to 12 weeks and diminish after dose stabilization. Dose escalation according to the approved 4-week titration schedule reduces discontinuation due to diarrhea.

Metformin

Metformin causes diarrhea and loose stools in 10 to 53% of patients starting therapy, primarily through increased intestinal glucose fermentation and altered bile acid reabsorption. Extended-release metformin (metformin XR) reduces GI adverse events by approximately 50% compared with immediate-release formulations, based on a meta-analysis of 17 randomized controlled trials (7). Taking metformin with the largest meal of the day further reduces symptom burden.

Magnesium-Containing Antacids and Laxatives

Magnesium hydroxide (milk of magnesia), magnesium citrate, and magnesium-containing antacids exert osmotic effects in the colon. Doses of magnesium above 600 mg per day consistently produce loose stools. Patients using these products for heartburn or constipation relief should be counseled that diarrhea is dose-dependent and fully reversible with discontinuation or dose reduction.

Other Common Drug Culprits

Several additional medication classes warrant mention. Proton pump inhibitors (omeprazole, pantoprazole) increase the risk of CDI by approximately 1.7-fold according to a 2017 meta-analysis of 56 studies (8). Colchicine produces dose-dependent diarrhea in up to 80% of patients using the older high-dose protocol (1.2 mg loading, then 0.6 mg hourly); the modern low-dose protocol (1.2 mg followed by 0.6 mg once) drops that rate to under 20%. Orlistat (120 mg three times daily) causes oily, loose stools in up to 30% of users through its mechanism of intestinal lipase inhibition.

SGLT-2 inhibitors (empagliflozin, dapagliflozin), SSRIs (particularly sertraline and fluoxetine), NSAIDs (through prostaglandin-mediated secretion), and digoxin (at toxic levels) also appear frequently on medication reconciliation reviews in patients presenting with new or worsening diarrhea.

HealthRX Drug-Diarrhea Review Framework: When a patient reports new diarrhea, the HealthRX medical team recommends reviewing medications in four tiers: (1) antibiotics started in the past 8 weeks; (2) GI-motility-altering agents (GLP-1 agonists, opioid cessation); (3) osmotic agents (magnesium, lactulose, polyethylene glycol); (4) systemic agents with indirect GI effects (PPIs, SSRIs, NSAIDs, digoxin). Addressing tier 1 before ordering stool cultures avoids unnecessary testing in 30 to 40% of cases.

Drugs That Treat Diarrhea

Oral Rehydration Salts: The Foundation

Before any antidiarrheal drug, oral rehydration is the cornerstone of management. The WHO oral rehydration solution (ORS) contains 75 mEq/L sodium, 75 mmol/L glucose, 20 mEq/L potassium, and 30 mmol/L bicarbonate. A Cochrane review of 17 randomized trials found that low-osmolarity ORS reduced stool output by 20% and IV fluid requirements by 33% compared with standard ORS in children with acute diarrhea (9).

Loperamide (Imodium)

Loperamide is a peripherally acting opioid receptor agonist that reduces intestinal motility and secretion without crossing the blood-brain barrier at therapeutic doses. The standard adult regimen is 4 mg orally after the first loose stool, followed by 2 mg after each subsequent loose stool, to a maximum of 16 mg per day. Loperamide should not be used in patients with bloody diarrhea, fever above 38.5°C, or suspected inflammatory colitis, because slowing motility may worsen bacterial translocation. The FDA issued a safety communication in 2016 warning that doses above 16 mg per day can cause serious cardiac arrhythmias (10).

Bismuth Subsalicylate (Pepto-Bismol)

Bismuth subsalicylate 524 mg (two standard tablets or 30 mL of liquid) every 30 to 60 minutes, to a maximum of eight doses per day, reduces stool frequency through antisecretory and mild antimicrobial effects. A double-blind placebo-controlled trial published in JAMA (N=182 travelers) demonstrated a 65% reduction in diarrheal episodes with bismuth subsalicylate prophylaxis versus placebo (11). Patients taking aspirin, anticoagulants, or who have salicylate sensitivity should avoid this agent.

Rifaximin for Traveler's Diarrhea and IBS-D

Rifaximin is a non-absorbable rifamycin antibiotic active against non-invasive enteric pathogens. The FDA approved it for traveler's diarrhea at 200 mg three times daily for three days and for IBS-D at 550 mg three times daily for 14 days. In TARGET 1 and TARGET 2 (combined N=1,258 IBS-D patients), rifaximin 550 mg three times daily for 14 days produced adequate relief of global IBS symptoms in 40.8% versus 31.2% on placebo (P<0.001) (12). The two-week course approach provides symptom relief without significant systemic antibiotic exposure.

Eluxadoline for IBS-D

Eluxadoline (Viberzi) 100 mg twice daily with food acts on mu and kappa opioid receptors and delta opioid receptor antagonism in the enteric nervous system. In the LIBERTAS pooled analysis (N=2,427), eluxadoline produced composite symptom response (abdominal pain reduction and stool consistency improvement) in 26.0% of patients versus 16.5% on placebo at week 26 (13). It is contraindicated in patients without a gallbladder due to risk of sphincter of Oddi spasm.

Diphenoxylate-Atropine (Lomotil)

Diphenoxylate 2.5 mg with atropine 0.025 mg (one tablet) four times daily is a Schedule V controlled substance used for moderate acute diarrhea. It crosses the blood-brain barrier at high doses, which limits its use relative to loperamide. Atropine is added at sub-therapeutic doses to deter abuse.

Probiotics: What the Evidence Actually Shows

Lactobacillus rhamnosus GG (LGG) at 10 billion CFU twice daily reduces the duration of acute infectious diarrhea in children by approximately one day, based on a Cochrane review of 63 randomized trials (14). Evidence in adults is weaker. For antibiotic-associated diarrhea specifically, Saccharomyces boulardii 500 mg twice daily started with the antibiotic course reduces the incidence of diarrhea by approximately 28% (number needed to treat = 10), per a meta-analysis in The Lancet (15).

Racecadotril

Racecadotril (acetorphan) is an enkephalinase inhibitor that reduces intestinal hypersecretion without slowing motility. It is widely used in Europe and Latin America but lacks FDA approval in the United States. A meta-analysis of 9 randomized trials (N=1,384 children) found racecadotril reduced 24-hour stool output by 38% compared with placebo and was non-inferior to loperamide for acute secretory diarrhea (16).

Managing Drug-Induced Diarrhea: Specific Scenarios

GLP-1 Agonist-Associated Diarrhea

Dose reduction is the first step when GLP-1-associated diarrhea is severe enough to interrupt daily activities. If the patient is on semaglutide 1.0 mg weekly, stepping back to 0.5 mg for an additional 4 weeks and re-escalating more slowly resolves symptoms in the majority of cases. Concurrent use of loperamide on an as-needed basis is reasonable for short-term symptom relief during titration. Patients should increase fluid intake to at least 2 liters per day to prevent dehydration, which carries particular risk for patients also receiving SGLT-2 inhibitors.

Antibiotic-Associated Diarrhea That Is Not CDI

Simple antibiotic-associated diarrhea (not CDI) generally resolves within 2 weeks of completing the antibiotic course. Loperamide provides symptom relief. Adding Saccharomyces boulardii 500 mg twice daily from antibiotic day 1 through 2 weeks post-course is a reasonable prevention strategy for patients with a history of antibiotic diarrhea. Stool testing for CDI toxin is warranted if diarrhea persists beyond 3 days or includes three or more loose stools per day.

Metformin-Associated Diarrhea

Switching from immediate-release to extended-release metformin at the same total daily dose is the most effective single intervention. If metformin XR is not tolerated at doses above 1,500 mg per day, adding a GLP-1 agonist or SGLT-2 inhibitor to permit metformin dose reduction is a viable strategy, per the 2022 ADA Standards of Medical Care in Diabetes (17).

When Should You Worry About Diarrhea?

Most acute diarrhea is self-limited. The following features require same-day or urgent evaluation.

Red-Flag Signs

• Blood or pus in stool
• Fever above 38.5°C (101.3°F) accompanying diarrhea
• Signs of dehydration: decreased urine output, dry mucous membranes, orthostatic hypotension
• Diarrhea persisting beyond 7 days without a clear cause
• More than 6 unformed stools per day
• Diarrhea in patients over age 70 or those who are immunocompromised
• Recent hospitalization or antibiotic use in the past 8 weeks (CDI risk)
• Travel to a high-risk region within the past 30 days

Dehydration Assessment

The WHO classifies dehydration as no dehydration, some dehydration (two or more signs including restlessness, sunken eyes, poor skin turgor, thirsty drinking behavior), and severe dehydration. Severe dehydration requires IV Ringer's lactate or normal saline at 30 mL/kg over 30 minutes in adults, followed by reassessment (18).

Diagnostic Workup for Persistent Diarrhea

For diarrhea lasting more than 7 days, a systematic approach reduces unnecessary testing. The ACG 2019 clinical guideline on the evaluation of chronic diarrhea recommends stool culture, ova and parasite examination, fecal calprotectin, and celiac serology (anti-tissue transglutaminase IgA) as first-tier tests (19). Colonoscopy with biopsies is indicated when stool studies are negative and symptoms persist beyond 4 weeks, particularly in patients over age 45 or those with nocturnal diarrhea (which strongly suggests organic disease).

Special Populations

Pediatric Patients

Children dehydrate faster than adults. The American Academy of Pediatrics endorses oral rehydration therapy as first-line treatment for mild-to-moderate dehydration, with 50 to 100 mL/kg of ORS over 3 to 4 hours (20). Loperamide is contraindicated in children under age 2 and should be used cautiously in children under 6. Zinc supplementation at 10 to 20 mg per day for 10 to 14 days reduces the duration and severity of diarrheal episodes in children in low-income settings, per WHO guidance (21).

Pregnant Patients

Bismuth subsalicylate is contraindicated in pregnancy due to salicylate toxicity. Loperamide carries FDA pregnancy category B data showing no teratogenicity in animal studies, though human data are limited. Oral rehydration and dietary modification (BRAT diet: bananas, rice, applesauce, toast) remain the safest initial interventions. Any antibiotic prescribed for bacterial diarrhea in pregnancy should avoid fluoroquinolones and tetracyclines; azithromycin 500 mg daily for 3 days is preferred for empiric treatment of travelers' diarrhea in pregnancy (22).

Older Adults

Adults over 65 are at elevated risk for C. Difficile infection, dehydration, and electrolyte abnormalities. The IDSA/SHEA 2021 guidelines identify age above 65 as a risk factor for severe CDI requiring vancomycin rather than metronidazole (4). Fecal microbiota transplantation (FMT) achieves CDI cure in approximately 92% of recurrent cases based on pooled data from randomized trials, compared with 26 to 30% for vancomycin alone (23).