Acute Diarrhea in Adults

Diarrhea is defined by the Rome IV criteria as loose or watery stools, without predominant abdominal pain or bothersome bloating, occurring in more than 25% of bowel movements — clinically this corresponds to Bristol Stool Form Scale types 5-7.^[1] Acute diarrhea lasts under 14 days; persistent diarrhea lasts 14-30 days; chronic diarrhea lasts more than 30 days and needs a different, more extensive diagnostic workup that is outside the scope of this chapter. Most acute diarrhea is infectious in origin, self-limited, and does not require a specific diagnosis or antimicrobial therapy — for the large majority of patients the priority is preventing and correcting dehydration, not identifying a pathogen. The core clinical task covered here is distinguishing this large majority of uncomplicated, self-limited presentations from the minority that signal an invasive/inflammatory process, a high-risk host, or Clostridioides difficile infection, each of which changes management substantially.^[1]

Typical presentation

An increase in stool frequency and/or liquidity relative to the patient's normal pattern, often with urgency, abdominal cramping, and variable nausea. Confirm this is a genuine change in stool water content and weight — patients with irritable bowel syndrome or another motility disturbance often describe "diarrhea" that is really an increased frequency of small, normal-weight bowel movements, which is managed very differently from true diarrhea.

• Recent antibiotic use — the leading risk factor for Clostridioides difficile infection and antibiotic-associated diarrhea generally.
• Recent hospitalization or other health-care facility exposure.
• Foodborne exposure — undercooked meat or poultry, unpasteurized dairy, shellfish, or contaminated water.
• Recent travel to a region with a higher risk of infectious diarrhea (a positive travel history points toward traveller's diarrhea rather than this general approach).
• Daycare attendance or employment, or living/working in an institutional setting — nursing home, correctional facility, psychiatric facility.
• Sick contacts with similar symptoms, suggesting a common-source or person-to-person outbreak.
• Immunocompromise — HIV/AIDS, chemotherapy, immunosuppressive therapy, transplant recipients.
• Inflammatory bowel disease, where distinguishing a flare from superimposed infection is important.
• Extremes of age — young children and older adults carry a substantially higher dehydration risk.
• Medications that commonly cause diarrhea — acarbose, magnesium-containing antacids, antibiotics, cholinergic drugs, colchicine, fluorouracil, irinotecan, laxatives, metformin, promotility agents, prostaglandins, proton pump inhibitors, NSAIDs, immune checkpoint inhibitors (e.g., pembrolizumab, nivolumab, ipilimumab), and tyrosine kinase inhibitors (e.g., erlotinib).

History

• Onset and duration — abrupt or gradual, and how many days it has been present, since this defines acute versus persistent versus chronic.
• Stool characteristics — watery, bloody, mucousy, or greasy/oily and foul-smelling and difficult to flush (suggests steatorrhea and points away from a typical acute infectious cause).
• Frequency and approximate volume of stool.
• Symptoms of volume depletion — thirst, tachycardia, orthostatic symptoms, decreased urination, lethargy, decreased skin turgor.
• Associated systemic symptoms and their severity — nausea, vomiting, abdominal pain/cramping, fever, myalgias, altered mental status.
• Recent dietary changes.
• Full medication history, including nonprescription drugs, herbal/natural health products, and supplements — many common medications cause diarrhea (see Risk Factors).
• Recent travel history — a positive travel history within the relevant incubation window points toward traveller's diarrhea rather than this general approach.
• Occupation and setting — food handling, health-care work, daycare attendance/employment — and whether co-workers, classmates, or household members have similar symptoms.
• Sexual activity — diarrhea occurs more often in people living with HIV/AIDS.
• Family history of inflammatory bowel disease, colorectal cancer, or celiac disease if the presentation is atypical for a simple acute infectious process.

Etiology of acute diarrhea

Identifying the likely cause guides which patients need testing and which can be managed supportively.^[1]

When to test, and when not to

Selective testing pearls

• Consider Escherichia coli O157:H7 in a patient with a history of undercooked ground beef ingestion and acute bloody diarrhea or hemolytic uremic syndrome, or in a patient with marked abdominal pain and bloody diarrhea but without high fever — do not give empiric antibiotics or antimotility agents while this is suspected, since antibiotic treatment of Shiga toxin-producing E. coli can increase the risk of hemolytic uremic syndrome.
• Consider Vibrio parahaemolyticus in a patient who has eaten shellfish within the 3 days before illness onset.
• Send a C. difficile toxin assay routinely as part of stool testing whenever stool testing is indicated at all.
• Test patients with inflammatory bowel disease in whom distinguishing a flare from a superimposed infection is critical — for example, to rule out cytomegalovirus colitis.
• Fecal specimens from a patient whose diarrhea develops after 3 days of hospitalization have a low diagnostic yield when cultured for standard bacterial pathogens or examined for ova and parasites — direct testing toward C. difficile and medication causes instead in this setting.

• Prevent and correct dehydration and electrolyte depletion — the single highest-priority goal for essentially every patient.
• Reduce symptom severity and duration, and restore usual stool frequency and quality of life.
• Identify and treat the specific cause when a specific treatment genuinely changes outcome — for example, confirmed Clostridioides difficile infection or a specific bacterial pathogen requiring antibiotics.
• Avoid unnecessary antimicrobial exposure when diarrhea is self-limited, since this adds cost, adverse-effect risk, and antimicrobial resistance pressure without benefit.
• Prevent complications — severe dehydration, electrolyte disturbance, hemorrhoidal irritation from frequent stooling, and, in the invasive/CDI setting, toxic megacolon.
• Prevent transmission to others, particularly for contagious causes (norovirus, C. difficile, Shigella) in food handlers, health-care workers, daycare settings, and institutional residents.

• Discontinue medications that commonly cause diarrhea when clinically feasible — see Risk Factors for the full list.
• Stop ingestion of poorly absorbed carbohydrates — dietetic candies/gum and jams containing sorbitol, mannitol, or xylitol; beverages and foods high in fructose; and lactose-containing dairy products if lactose intolerance is suspected. A 2-week trial of a lactose-restricted diet is a reasonable, low-cost way to assess this without a formal workup.
• Reintroduce a bland, low-fat, low-complex-carbohydrate diet once stool frequency is subsiding — bananas, rice, unsweetened applesauce, and clear soups are well-tolerated examples; there is no need to remain nil-by-mouth.
• Maintain adequate fluid and electrolyte intake throughout with a balanced-electrolyte oral rehydration solution — see Pharmacologic Treatment Options and Drug Tables for exact composition and dosing.

• Food handlers, health-care workers with direct patient contact, daycare attendees/employees, and residents or staff of an institutional facility (nursing home, correctional facility, psychiatric facility) who develop diarrhea should be tested for bacterial, parasitic, or viral pathogens and may need to be excluded from work/attendance per local public health direction, since their illness may be an early indicator of a wider outbreak.
• Report a suspected outbreak to local public health authorities and support their investigation.

Pregnancy

1st line bulk-forming agents such as psyllium are the preferred first choice for diarrhea in pregnancy given minimal systemic absorption.

2nd line loperamide has limited human pregnancy data but has not shown a consistent increased risk of major malformations across available studies — a reasonable second-line option when a bulk-forming agent is insufficient.

Avoid diphenoxylate/atropine (teratogenic in animal studies; the active metabolite is likely excreted in breast milk) and bismuth subsalicylate (salicylate exposure is associated with adverse perinatal outcomes, including premature closure of the fetal ductus arteriosus).

For CDI in pregnancy: metronidazole is not associated with congenital malformations and can be used; vancomycin has not shown adverse pregnancy effects and can be used. Fidaxomicin has limited pregnancy-specific data — use per specialist direction when it would otherwise be the preferred agent.

Breastfeeding

Metronidazole is transferred into breast milk — suspend breastfeeding for the duration of a metronidazole course and resume once treatment is complete. Vancomycin has limited breastfeeding data but is considered probably compatible given minimal oral absorption. Cholestyramine can be used with caution in pregnancy/breastfeeding — monitor for fat-soluble vitamin deficiency and coagulopathy with prolonged use.

Pediatrics

Oral rehydration therapy is the priority intervention and should be started early — children and infants become dehydrated faster than adults. Confirm current product-specific age restrictions before recommending loperamide, bismuth subsalicylate, or diphenoxylate/atropine in a child. Bismuth subsalicylate must specifically be avoided in children or teenagers with chickenpox or a flu-like illness because of Reye syndrome risk.

Older adults

Dehydration develops faster and more often presents atypically — confusion, falls, or reduced oral intake rather than classic thirst — so have a lower threshold to start oral rehydration therapy and to escalate to IV fluids if oral intake cannot keep pace. Older adults are more susceptible to the sedating and anticholinergic adverse effects of codeine and diphenoxylate/atropine specifically — favour loperamide when an opioid-class antimotility agent is appropriate at all. Clostridioides difficile infection is both more common and more severe in this population — monitor closely and have a lower threshold to manage as severe.

Renal impairment

Use bismuth subsalicylate with caution in renal insufficiency given reduced clearance of the salicylate component. Oral vancomycin for CDI needs no renal dose adjustment in typical use, since it is poorly absorbed and acts locally in the gut.

Reassess hydration status at every contact — ask about urine output, thirst, and, in caregivers of young children or frail older adults, alertness and skin turgor — and escalate to IV rehydration if oral intake cannot keep pace with losses. Reassess within 48-72 hours if symptomatic treatment has not improved a presumed self-limited illness, and re-screen for red flags that may not have been present initially (new blood in stool, new fever, worsening pain). For confirmed CDI, monitor clinical response — stool frequency/consistency, resolution of fever and pain — rather than repeat stool testing: a stool toxin/PCR test can remain positive for weeks after successful treatment, and routine test-of-cure is not recommended.^[4] Monitor renal function and white blood cell count in patients treated for severe or fulminant CDI. Confirm that any causative medication was actually stopped, and confirm resolution of associated electrolyte disturbances, particularly in older adults or anyone who required significant rehydration.

1. Confirm adherence to oral rehydration therapy and symptomatic treatment, and confirm no red-flag features were missed on reassessment — new blood, new fever, worsening pain.
2. If a presumed self-limited illness has not improved within 48-72 hours, or persists beyond 7 days, send stool culture and a C. difficile toxin assay rather than continuing empiric symptomatic care.
3. If Clostridioides difficile infection is confirmed and not improving on initial therapy, reassess severity (non-severe, severe, or fulminant) and escalate the regimen accordingly rather than simply extending the same course.
4. Manage a CDI recurrence based on which agent was used initially and how many prior recurrences have occurred — see the Clinical Decision Algorithm and Drug Tables for the specific recurrence ladder.
5. Reassess the full medication list for a causative drug that has not yet been stopped.
6. If diarrhea persists beyond 2-4 weeks despite a negative infectious workup and appropriate supportive care, this is no longer "acute diarrhea" — refer for a chronic diarrhea workup rather than continuing to manage it as an acute presentation.

• Using loperamide, diphenoxylate/atropine, or another antimotility agent in bloody diarrhea, high fever, or suspected CDI/invasive infection — toxic megacolon risk.
• Reaching for empiric antibiotics for routine, self-limited watery diarrhea.
• Using metronidazole as first-line CDI therapy — it is now reserved for non-severe disease when vancomycin and fidaxomicin are both unavailable.
• Recommending a sports drink, soft drink, or juice as "rehydration" — the sodium-to-glucose ratio is wrong for oral rehydration and can worsen diarrhea.
• Missing a medication cause of diarrhea (see Risk Factors) and treating the presentation as infectious instead.
• Ordering repeat C. difficile stool testing to confirm cure — a positive result can persist for weeks after successful treatment and does not on its own indicate treatment failure.
• Testing or treating an asymptomatic C. difficile carrier.
• Giving antibiotics for suspected E. coli O157:H7 — this specifically increases the risk of hemolytic uremic syndrome rather than helping.
• Under-recognizing dehydration in an older adult who presents atypically (confusion, falls) rather than with classic thirst.
• Relying on probiotics as routine primary therapy for acute infectious diarrhea, or as monotherapy for CDI — current evidence does not support either use.^[5]
• Missing supratherapeutic/misuse-pattern loperamide dosing as a cause of unexplained cardiac dysrhythmia.^[6]

How to use this table: Doses are adult doses and require patient-specific confirmation, including renal/hepatic adjustment. This chapter's clinical sections focus on acute diarrhea in adults; the alpha2-adrenergic-agonist and somatostatin-analogue entries below are relevant mainly to chronic/secretory diarrhea and are included for completeness, since the source drug class list spans both. Select any drug name to open its complete Pharmasist Drug Database page.

Oral Rehydration Therapy

Intestinal Adsorbents

Opioid Antimotility Agents

Hydrophilic Bulking Agents

Antibacterial Agents for Clostridioides difficile Infection

Resins

Alpha2-adrenergic Agonists (chronic/secretory diarrhea)

Somatostatin Analogues (chronic/secretory diarrhea)