Constipation affects approximately 15% of the general population and is among the most common gastrointestinal (GI) complaints encountered in clinical practice. Pharmacological management spans multiple mechanistic classes, each acting at distinct points in intestinal fluid and motility regulation. Selecting the appropriate agent requires understanding not only mechanism but also the clinical context, particularly whether constipation is functional, medication-induced, or a manifestation of an underlying motility disorder.
Osmotic laxatives work by retaining water within the intestinal lumen through osmotic pressure, increasing luminal fluid volume, softening stool, and stimulating propulsive colonic contractions. Polyethylene glycol (PEG) is a high-molecular-weight polymer that is neither absorbed nor metabolized; it passes through the GI tract intact, dragging an obligate water load. PEG 3350 (available as Miralax and as osmotic bowel preparation formulations) is highly effective for chronic constipation and has a favorable safety profile, with negligible systemic absorption and no significant electrolyte shifts at standard doses, making it appropriate for elderly patients and those with renal impairment.1 Lactulose is a non-absorbable disaccharide that is fermented by colonic bacteria to short-chain fatty acids and gases, lowering luminal pH and increasing osmotic pressure. Its onset is slower than PEG (24–48 hours), and fermentation-related bloating and flatulence limit tolerability. Magnesium salts (magnesium hydroxide, magnesium citrate) act both osmotically and by stimulating cholecystokinin release, which increases intestinal motility; they are effective but should be used cautiously in patients with renal impairment because magnesium is partially absorbed.
Stimulant laxatives act directly on intestinal epithelial cells and the enteric nervous system to increase propulsive motility and reduce fluid absorption. Senna (sennosides A and B) and bisacodyl are the two primary agents in this class. Sennosides are prodrugs metabolized by colonic bacteria to active anthranoid metabolites that stimulate the myenteric plexus and inhibit colonic water absorption. Bisacodyl is converted by intestinal esterases to its active form, which directly stimulates secretomotor neurons in the submucosal plexus and increases propulsive motility. Both agents have onset of action of 6–12 hours orally and are appropriate for short-term use and as rescue therapy. The traditional concern that chronic stimulant laxative use causes cathartic colon or permanent enteric nerve damage has not been substantiated by prospective data, and current evidence supports their safety in long-term use for refractory chronic constipation.2
Bulk-forming laxatives (psyllium, methylcellulose, calcium polycarbophil) increase stool bulk by absorbing water and expanding within the colon, stretching the colonic wall and stimulating peristalsis. They require adequate fluid intake to function safely; insufficient hydration can cause luminal obstruction, particularly in patients with motility disorders or strictures. Their onset is slow (12–72 hours), and they are best suited for mild functional constipation and as long-term maintenance agents. Docusate sodium, a stool softener, acts as a surfactant that allows water and lipids to penetrate the stool mass. Despite widespread use, its clinical efficacy in controlled trials is modest, and it is generally considered less effective than osmotic agents for established constipation.3
Secretagogue laxatives represent a mechanistically distinct class that acts on intestinal epithelial transporters to increase luminal fluid secretion. Lubiprostone is a bicyclic fatty acid derived from prostaglandin E1 that activates type-2 chloride channels (ClC-2) on the apical surface of intestinal epithelial cells. ClC-2 activation drives chloride and secondarily water secretion into the lumen, increasing intraluminal fluid and accelerating transit. Lubiprostone is approved for chronic idiopathic constipation, IBS with predominant constipation (IBS-C), and opioid-induced constipation (OIC). It has minimal systemic absorption, and its primary adverse effect is nausea (occurring in approximately 30% of patients), which is reduced by taking it with food. Lubiprostone is contraindicated in patients with known or suspected mechanical GI obstruction.
Opioid-induced constipation (OIC) warrants specific attention because its mechanism is distinct from functional constipation and standard laxatives are frequently inadequate. Opioid analgesics bind to mu-opioid receptors (MORs) throughout the GI tract, particularly in the myenteric and submucosal plexuses of the enteric nervous system, producing reduced propulsive motility, increased non-propulsive segmental contractions, reduced intestinal secretion, and increased anal sphincter tone. The enteric nervous system contains the highest density of mu-opioid receptors in the body outside the central nervous system (CNS).4 Because conventional laxatives do not address the underlying receptor-mediated mechanism, they are often insufficient for OIC. Peripherally acting mu-opioid receptor antagonists (PAMORAs) were developed specifically to block opioid effects in the GI tract without reversing CNS-mediated analgesia, because they are either excluded from the CNS by the blood-brain barrier (BBB) or are substrates for P-glycoprotein efflux at the BBB.
Three PAMORAs are currently approved for OIC. Methylnaltrexone is a quaternary ammonium derivative of naltrexone; the quaternary nitrogen carries a positive charge that substantially limits CNS penetration, confining its antagonist activity to peripheral MORs in the GI tract. It is available subcutaneously and orally, with the subcutaneous route used for patients on palliative opioid therapy who require rapid relief. Naloxegol is a PEGylated derivative of naloxol; PEGylation increases its molecular size and makes it a P-glycoprotein substrate at the BBB, reducing CNS exposure to less than 1% of plasma concentrations at therapeutic doses. Naldemedine is a naltrexone derivative with a bulky side chain that reduces BBB penetration; it has a half-life of approximately 11 hours and once-daily oral dosing. All three PAMORAs effectively increase spontaneous bowel movements within 24–48 hours without compromising opioid analgesia as demonstrated in placebo-controlled trials, and they do not precipitate opioid withdrawal when used at approved doses.5
PAMORAs should not be given to patients with known or suspected GI obstruction and are not appropriate when constipation is unrelated to opioid therapy. A theoretical concern is precipitation of opioid withdrawal if CNS opioid receptor blockade occurs; this risk is low at approved doses given the peripheral selectivity of all three agents. However, patients on high opioid doses or with blood-brain barrier disruption (CNS tumor, meningitis, prior radiation) may have increased CNS exposure. Signs of opioid withdrawal should prompt discontinuation. Methylnaltrexone requires dose reduction in severe renal impairment (creatinine clearance less than 30 mL/min). Naloxegol is a CYP3A4 substrate; concomitant strong CYP3A4 inhibitors (ketoconazole, clarithromycin) are contraindicated, and moderate inhibitors require dose reduction to 12.5 mg daily.
Mild functional constipation, long-term maintenance: PEG 3350 first-line; psyllium as adjunct for stool consistency. Both are safe for long-term use.
Acute constipation, rapid relief needed: bisacodyl or senna (onset 6–12 h oral) or magnesium citrate for more urgent situations.
Renal impairment: avoid magnesium-containing laxatives (magnesium accumulation risk). PEG preferred; lactulose acceptable.
Opioid-induced constipation refractory to standard laxatives: PAMORA (methylnaltrexone SC for rapid effect; naloxegol or naldemedine orally for outpatient maintenance). Confirm no GI obstruction before initiating.
Constipation-predominant IBS or chronic idiopathic constipation refractory to osmotic/stimulant agents: lubiprostone, linaclotide, plecanatide, or tenapanor (see Section 4 for full secretagogue discussion in IBS context).
Diarrhea management requires distinguishing acute infectious diarrhea, which is usually self-limited and may worsen with motility-inhibiting agents, from chronic diarrhea where underlying causes such as bile acid malabsorption, small intestinal bacterial overgrowth, or functional disorders guide pharmacological selection. The primary antidiarrheal agents act either by reducing intestinal motility or by modifying intestinal secretion, and their appropriate use depends entirely on the clinical setting.
Loperamide is a synthetic opioid that acts as a potent agonist at mu-opioid receptors (MORs) in the myenteric plexus of the intestinal wall. Activation of these peripheral MORs reduces the release of acetylcholine from the myenteric plexus, decreasing propulsive peristalsis and increasing segmental non-propulsive contractions, thereby slowing intestinal transit. Loperamide also reduces intestinal secretion by inhibiting calmodulin-dependent phosphodiesterase, and it increases anal sphincter tone. Unlike morphine or codeine, loperamide does not cross the blood-brain barrier (BBB) under normal circumstances because it is an extremely high-affinity P-glycoprotein substrate at the BBB; at standard doses, central nervous system (CNS) concentrations are negligible. This peripheral selectivity is the basis for its antidiarrheal efficacy without CNS opioid effects or analgesic action.6 At supratherapeutic doses (the quantities found in recreational misuse), P-glycoprotein efflux can be saturated, and cardiac arrhythmias – including QT (corrected QT interval) prolongation and potentially fatal ventricular arrhythmias – have been reported, which led to FDA packaging restrictions on high-unit loperamide sales.
Loperamide is appropriate for acute non-inflammatory diarrhea (traveler’s diarrhea without dysentery features, acute viral gastroenteritis after ruling out bloody diarrhea or high fever) and for chronic functional diarrhea and diarrhea-predominant IBS (IBS-D). It should not be used when infectious diarrhea with invasive pathogens is suspected (blood in stool, fever above 38.5 degrees Celsius, dysenteric presentation) because reducing motility can impair pathogen clearance and prolong illness with organisms such as Shigella, Salmonella, or Clostridioides difficile. Loperamide is also contraindicated in CDI (Clostridioides difficile infection), where it may precipitate toxic megacolon by suppressing the motility that is partially responsible for toxin clearance.
Bismuth subsalicylate (BSS) has multiple mechanisms: the bismuth ion has direct antimicrobial activity against enteropathogenic bacteria including enterotoxigenic Escherichia coli (ETEC) and Helicobacter pylori; salicylate inhibits intestinal prostaglandin synthesis, reducing secretion; and bismuth may bind bacterial toxins. BSS is effective for traveler’s diarrhea prophylaxis and treatment and for non-specific acute diarrhea. It turns stools and tongue black due to bismuth sulfide formation, which should be noted for patients to avoid confusion with gastrointestinal (GI) bleeding. The salicylate component is systemically absorbed and represents a clinically meaningful salicylate dose; BSS should not be used in children or teenagers with viral illnesses (Reye syndrome risk), in patients on anticoagulants (salicylate-warfarin interaction), or in patients with salicylate hypersensitivity.
Rifaximin is a minimally absorbed rifamycin derivative (<0.4% systemic bioavailability) that achieves high intraluminal antibiotic concentrations against enteric pathogens while producing negligible systemic antibiotic effects. It is approved for traveler’s diarrhea caused by non-invasive strains of ETEC and for reducing recurrence of hepatic encephalopathy (discussed in Gastro-07). For traveler’s diarrhea, a 3-day course of rifaximin 200 mg three times daily reduces illness duration significantly compared to placebo and is comparable in efficacy to systemic fluoroquinolones while avoiding fluoroquinolone systemic adverse effects and resistance pressure. Rifaximin is not appropriate when invasive enteric pathogens are likely (fever, dysentery features), as its intraluminal-only activity does not treat bacteremic or invasive disease.14
Bile acid diarrhea (BAD), also called bile acid malabsorption (BAM), occurs when excessive bile acids reach the colon and stimulate colonic secretion and motility through activation of TGR5 (Takeda G protein-coupled receptor 5) and direct mucosal effects. It is commonly seen after ileal resection or disease (Crohn’s disease involving the terminal ileum), post-cholecystectomy, and as an idiopathic primary condition that may be misdiagnosed as IBS-D. Cholestyramine, a bile acid sequestrant (anion exchange resin), binds bile acids in the intestinal lumen and prevents their colonic delivery. It is effective for BAD but poorly tolerated because of its gritty texture and GI adverse effects (constipation, bloating). Colesevelam and colestipol are better-tolerated alternatives. An important practical point is that cholestyramine binds numerous other drugs (thyroxine, warfarin, digoxin, fat-soluble vitamins, certain antibiotics) in the intestinal lumen; all co-medications should be taken at least 1–2 hours before or 4 hours after cholestyramine administration.
Loperamide is contraindicated in: (1) confirmed or suspected CDI – risk of toxic megacolon; (2) dysenteric diarrhea (blood, mucus, fever above 38.5 degrees Celsius) – invasive pathogens require pathogen clearance, not motility suppression; (3) antibiotic-associated diarrhea of uncertain etiology pending CDI testing; (4) acute severe ulcerative colitis flare – risk of toxic megacolon. In children under two years, avoid loperamide entirely. Oral rehydration solution (ORS) remains the cornerstone of management for secretory diarrhea regardless of antidiarrheal use; antidiarrheal agents reduce stool frequency but do not address dehydration.
Clostridioides difficile infection (CDI) is the leading cause of healthcare-associated infectious diarrhea in the developed world, with an estimated 500,000 cases annually in the United States. Antibiotic selection for CDI has been substantially revised over the past decade, with high-quality clinical trial data supporting a hierarchy that elevates fidaxomicin and oral vancomycin while removing metronidazole from the first-line position it held for decades. Recurrent CDI remains a major clinical challenge, with recurrence rates of 20–30% after a first episode and rising with each subsequent recurrence.
The pathophysiology of CDI centers on toxin-mediated colonic injury. Following antibiotic disruption of the normal colonic microbiome, Clostridioides difficile colonizes the colon and produces two major virulence toxins: toxin A (TcdA) and toxin B (TcdB), both of which are glucosyltransferases that inactivate Rho family GTPases within colonocytes, disrupting the actin cytoskeleton, disrupting tight junctions, and causing apoptosis and inflammatory cell recruitment. TcdB is the primary virulence determinant in human disease. Hypervirulent strains of the NAP1 (North American pulsotype 1, also known as ribotype 027) lineage produce binary toxin (CDT) in addition to TcdA and TcdB, and their emergence in the 2000s was associated with increased severity and recurrence rates. Diagnosis is made by nucleic acid amplification test (NAAT) for the toxin genes tcdA and tcdB, or by enzyme immunoassay for the toxin proteins; NAAT is more sensitive but may detect colonization without active disease, so clinical context is essential.
Oral vancomycin (125 mg four times daily for 10 days) is effective for non-severe CDI and remains a standard first-line option. Because vancomycin is minimally absorbed from the gastrointestinal (GI) tract when given orally, it achieves very high intraluminal concentrations (>1000 mcg/mL in stool) while producing negligible systemic exposure, which avoids the nephrotoxicity and ototoxicity associated with intravenous vancomycin. For severe CDI (leukocyte count greater than 15,000 cells/mcL or serum creatinine greater than 1.5 mg/dL), oral vancomycin is preferred over fidaxomicin based on available data, with 125–500 mg four times daily used for severe cases. For fulminant CDI (hypotension, ileus, megacolon), oral or nasogastric vancomycin 500 mg four times daily combined with intravenous metronidazole 500 mg every 8 hours is the standard approach, with surgical consultation required if there is inadequate response within 24–48 hours.8
Fidaxomicin (200 mg twice daily for 10 days) is a macrolide antibiotic with a narrow spectrum of activity that is highly concentrated in the colon and minimally absorbed systemically. Its mechanism of action – inhibition of bacterial RNA (ribonucleic acid) polymerase at a binding site distinct from rifamycins – is bactericidal against C. difficile. The principal clinical advantage of fidaxomicin over oral vancomycin is demonstrated in two phase 3 randomized trials: fidaxomicin produces similar rates of clinical cure but significantly lower rates of recurrence for non-hypervirulent strains, with a recurrence rate approximately 40% lower than vancomycin in per-protocol analyses.15 This recurrence advantage is attributed to fidaxomicin’s narrower spectrum and relative sparing of Bacteroides species and other colonization-resistance organisms that are suppressed by vancomycin. The recurrence advantage is not observed in hypervirulent NAP1/ribotype 027 strains, for reasons that remain incompletely understood. Fidaxomicin is now recommended as the preferred agent for non-severe CDI by major guidelines including the Infectious Diseases Society of America (IDSA) and American College of Gastroenterology (ACG), though cost remains a limiting factor in some healthcare systems.
Metronidazole, long considered first-line for CDI, has been downgraded to an alternative agent for non-severe CDI only in settings where vancomycin and fidaxomicin are unavailable or cost-prohibitive. The IDSA/SHEA (Society for Healthcare Epidemiology of America) 2021 guidelines no longer recommend metronidazole as first-line for any severity category. The evidence for this change includes head-to-head trials demonstrating inferior clinical cure rates and higher recurrence rates with metronidazole compared to vancomycin, particularly for non-severe disease.8 For patients with a first recurrence, oral vancomycin (if initial treatment was metronidazole) or a fidaxomicin course is recommended; for second and subsequent recurrences, extended pulsed-tapered vancomycin regimens or fecal microbiota transplant (FMT) are the preferred approaches.
Fecal microbiota transplant (FMT) involves the transfer of stool from a healthy screened donor into the GI tract of a patient with recurrent CDI, with the goal of restoring colonization resistance through microbiome reconstitution. The rationale is that the CDI cycle of antibiotic treatment followed by recurrence reflects persistent microbiome depletion that allows C. difficile to re-establish; FMT abruptly restores the diverse microbiome that competitively excludes C. difficile. Meta-analyses of randomized controlled trials demonstrate CDI resolution rates of 80–90% with FMT for multiply recurrent CDI, substantially higher than antibiotic therapy alone.10 FMT can be delivered colonoscopically, via enema, via nasojejunal tube, or via oral encapsulated preparations. The FDA has approved standardized FMT products: Rebyota (fecal microbiota, live-jslm) for recurrent CDI in adults, and Vowst (fecal microbiota spores, live-brpk) as an oral capsule preparation approved in 2023. These regulated products represent an evolution from clinician-prepared donor stool preparations, offering standardized donor screening, pathogen testing, and reproducible product composition.
Bezlotoxumab is a human monoclonal antibody directed against C. difficile toxin B (TcdB) that is given as a single intravenous infusion (10 mg/kg) during antibiotic treatment for CDI. It does not treat active CDI but reduces the risk of recurrence by neutralizing TcdB in the colon during and after antibiotic therapy. In the MODIFY (Monoclonal Antibodies for Prevention of Recurrent CDI) I and MODIFY II trials, bezlotoxumab reduced CDI recurrence by approximately 10 percentage points compared to placebo (from approximately 26% to 17%) in patients receiving standard-of-care antibiotics, with the greatest absolute benefit in high-risk patients (age 65 or older, immunocompromised, severe CDI, hypervirulent strain, prior CDI episode).9 Bezlotoxumab has a black box warning for heart failure exacerbation, as trial data showed increased rates of heart failure-related events in patients with pre-existing heart failure; it should be used with caution in this population. Its routine use is not recommended for all CDI episodes; current IDSA guidance supports its use in patients with high recurrence risk factors.
Non-severe CDI (WBC less than 15,000, creatinine less than 1.5 mg/dL): fidaxomicin 200 mg twice daily for 10 days (preferred) OR vancomycin 125 mg four times daily for 10 days. Severe CDI (WBC 15,000 or higher, or creatinine 1.5 mg/dL or higher): vancomycin 125 mg four times daily for 10 days. Fulminant CDI (hypotension, ileus, or toxic megacolon): vancomycin 500 mg four times daily (oral/NG) plus IV metronidazole 500 mg every 8 hours; surgical consultation. First recurrence: fidaxomicin preferred if initial treatment was vancomycin; vancomycin if initial was metronidazole. Second or subsequent recurrence: extended pulsed-tapered vancomycin or FMT. Bezlotoxumab for high-risk patients during any antibiotic course. WBC = white blood cell count; NG = nasogastric.
Irritable bowel syndrome (IBS) is a disorder of gut-brain interaction defined by the Rome IV diagnostic criteria as recurrent abdominal pain at least one day per week for the preceding three months, associated with at least two of three features: pain related to defecation, change in stool frequency, or change in stool form. Pharmacological management is subtype-driven: IBS with predominant constipation (IBS-C) and IBS with predominant diarrhea (IBS-D) have distinct drug armamentaria, and central neuromodulators targeting visceral hypersensitivity are used across subtypes. Understanding the mechanisms underlying each drug class is essential for rational selection and for managing patients who fail initial therapy.
Linaclotide and plecanatide are guanylate cyclase-C (GC-C) agonists approved for both IBS-C and chronic idiopathic constipation. GC-C is expressed on the luminal surface of intestinal epithelial cells, and both linaclotide and plecanatide bind to it as structural mimetics of the endogenous ligands guanylin and uroguanylin. GC-C activation increases intracellular cyclic guanosine monophosphate (cGMP), which activates the cystic fibrosis transmembrane conductance regulator (CFTR) channel, driving chloride and bicarbonate secretion into the intestinal lumen. Luminal cGMP also directly inhibits pain-sensing neurons (nociceptors) in the submucosal plexus, which is the mechanism underlying their analgesic benefit in IBS beyond the prokinetic effect.11 The key pharmacokinetic feature of both agents is negligible systemic absorption (<1% bioavailability), which confines their activity to the intestinal lumen and minimizes systemic adverse effects.
Linaclotide 290 mcg daily is approved for IBS-C; the 72 mcg and 145 mcg doses are approved for chronic idiopathic constipation. Plecanatide 3 mg daily is approved for both indications. The primary adverse effect of both agents is diarrhea, which is the pharmacodynamic extension of their mechanism; it is the most common reason for discontinuation and is dose-dependent. Both agents carry an FDA black box warning against use in patients younger than 2 years (risk of dehydration from severe secretory diarrhea in immature GI physiology) and should not be used in patients with known or suspected GI obstruction.
Tenapanor is an inhibitor of the sodium/hydrogen exchanger isoform 3 (NHE3) on intestinal epithelial cells, approved for IBS-C. NHE3 is the primary intestinal sodium absorption transporter; its inhibition reduces sodium and water absorption from the intestinal lumen, increasing luminal fluid and accelerating transit. Tenapanor also reduces paracellular phosphate absorption in the intestine and is separately approved for hyperphosphatemia in patients with chronic kidney disease on dialysis. Its systemic bioavailability is negligible (<3%), and its principal adverse effect is diarrhea. Unlike GC-C agonists, tenapanor does not have a direct analgesic effect on intestinal nociceptors, though luminal distention relief may contribute to pain improvement.
Alosetron is a potent, selective antagonist of the 5-hydroxytryptamine type 3 (5-HT3) receptor, which is expressed on intrinsic and extrinsic primary afferent neurons in the GI tract and plays a central role in mediating intestinal motility, secretion, and visceral pain perception. Blocking 5-HT3 receptors slows colonic transit, increases colonic compliance, and reduces visceral afferent signaling, collectively reducing stool frequency, urgency, and pain in IBS-D. Alosetron was approved in 2000, voluntarily withdrawn in 2000 due to reports of serious GI adverse effects, and reintroduced in 2002 under a restricted prescribing program. It is now available only through a Risk Evaluation and Mitigation Strategy (REMS) program, is approved only for women with severe IBS-D who have not responded to conventional therapies, and is prescribed only by enrolled prescribers. The serious adverse effects that triggered its withdrawal were ischemic colitis (estimated incidence approximately 1 in 1,000 patients per year) and severe constipation (approximately 1 in 1,000 cases required hospitalization); patients must be counseled on these risks and instructed to stop alosetron immediately if constipation or rectal bleeding develops.12
Eluxadoline is a mixed opioid receptor modulator approved for IBS-D that acts as a mu-opioid receptor (MOR) agonist, a kappa-opioid receptor (KOR) agonist, and a delta-opioid receptor (DOR) antagonist. The MOR and KOR agonism reduces intestinal secretion and motility; the DOR antagonism is intended to attenuate the constipating and nausea-inducing effects of full MOR agonism and reduce the potential for systemic opioid adverse effects. Eluxadoline is minimally absorbed systemically and acts primarily in the enteric nervous system. It is contraindicated in patients without a gallbladder (post-cholecystectomy) because of a significant risk of sphincter of Oddi spasm leading to acute pancreatitis in this population; this is attributed to unopposed MOR agonism at the sphincter of Oddi after loss of gallbladder-mediated bile flow regulation. It is also contraindicated in patients with known or suspected biliary duct obstruction, alcoholism, or those who consume more than three alcoholic drinks per day. The mechanism underlying pancreatitis risk in cholecystectomized patients represents one of the clearest examples in GI pharmacology of how anatomical variation changes drug safety profile.
Rifaximin (550 mg three times daily for 14 days) is approved for IBS-D in patients without constipation. The rationale for its use in IBS-D is based on the hypothesis that small intestinal bacterial overgrowth (SIBO) or microbiome dysbiosis contributes to IBS symptoms through excessive gas production, altered motility, and immune activation. In the TARGET (Targets for Adverse GI symptoms and Evaluating Rifaximin Treatment) 1 and TARGET 2 trials, rifaximin produced modest but statistically significant improvements in global IBS symptoms and bloating compared to placebo, with response rates approximately 10–12 percentage points higher than placebo.7 A distinguishing feature of rifaximin in this context is its retreatability; patients who respond initially and relapse can be retreated, with sustained response rates similar to the initial treatment, unlike most antibiotics for infectious indications. Its minimal systemic absorption means it does not substantially alter systemic microbiome and has a low risk of selecting for systemic antibiotic resistance.
Antispasmodics target the smooth muscle component of IBS pain. Dicyclomine and hyoscine (scopolamine) are anticholinergic agents that block muscarinic M3 (muscarinic acetylcholine receptor subtype 3) receptors on intestinal smooth muscle, reducing colonic spasm. Their efficacy in IBS is modest and limited by anticholinergic adverse effects (dry mouth, urinary retention, blurred vision, constipation, cognitive effects in elderly patients). They are better tolerated as short-term or intermittent agents for acute pain episodes. Peppermint oil acts as a calcium channel blocker in intestinal smooth muscle and has some evidence for efficacy in IBS with an improved tolerability profile relative to anticholinergics, though the evidence base is less robust. Mebeverine and alverine, available in some international markets but not FDA-approved in the United States, have direct smooth muscle relaxant activity with fewer systemic anticholinergic effects.
Low-dose tricyclic antidepressants (TCAs) and selective serotonin reuptake inhibitors (SSRIs) are used as central neuromodulators for IBS across subtypes, targeting visceral hypersensitivity rather than motility. TCAs (amitriptyline, nortriptyline, desipramine) reduce visceral afferent signaling through multiple mechanisms: sodium channel blockade on sensory afferents, antihistamine effects reducing central sensitization, and enhanced descending pain inhibition. At the low doses used in IBS (10–50 mg at bedtime, compared to antidepressant doses of 75–300 mg), the GI transit-slowing effect of TCAs is clinically useful in IBS-D, where constipation is a beneficial side effect. Meta-analyses support tricyclic antidepressant (TCA) efficacy for global IBS symptom improvement with an NNT (number needed to treat) of approximately 4–5.13 SSRIs have a more evidence-limited role in IBS; they accelerate GI transit (5-HT reuptake inhibition increases serotonin at 5-HT4 receptors, which promote motility), making them better suited to IBS-C than IBS-D, though their analgesic mechanism may be beneficial across subtypes. Serotonin-norepinephrine reuptake inhibitors (SNRIs) may offer complementary analgesic benefit through norepinephrine-mediated descending pain inhibition and have emerging evidence in IBS with predominant pain.
Visceral hypersensitivity – heightened perception of normal or subthreshold intestinal stimuli – is a central pathophysiological feature of IBS, detectable by rectal balloon distension studies showing reduced pain thresholds in IBS patients compared to controls. It arises from altered central pain processing (central sensitization), altered peripheral nociceptor signaling, and dysregulation of the brain-gut axis including the hypothalamic-pituitary-adrenal axis and autonomic nervous system. Low-dose TCAs reduce visceral afferent transmission and enhance descending inhibition; their efficacy in IBS is mechanistically independent of their antidepressant action, which is why doses much lower than antidepressant doses are effective. When initiating a TCA for IBS, counsel patients that the indication is pain modulation, not mood treatment, to improve medication acceptance and adherence.
IBS-C first-line secretagogues: linaclotide 290 mcg daily (GC-C agonist; also reduces visceral pain via cGMP); plecanatide 3 mg daily (GC-C agonist; pH-activated in proximal intestine). Lubiprostone 8 mcg twice daily if GC-C agonists not tolerated. Tenapanor 50 mg twice daily as alternative (NHE3 inhibitor).
IBS-D agents: rifaximin 550 mg TID for 14 days (retreatable; targets microbiome dysbiosis). Eluxadoline 100 mg twice daily with meals (contraindicated post-cholecystectomy). Alosetron 0.5–1 mg twice daily (women with severe IBS-D only; REMS required).
Neuromodulators (any subtype): low-dose TCA (desipramine or nortriptyline preferred for lower anticholinergic burden; amitriptyline at bedtime). SSRI for IBS-C predominant pain; SNRI for pain-predominant IBS with comorbid anxiety/depression.
Antispasmodics for acute pain: dicyclomine or hyoscine short-term; peppermint oil as better-tolerated alternative for frequent use. TID = three times daily.
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