Constipation: What Could Be Causing It

How Common Is Constipation, and How Is It Defined?

Constipation is one of the most frequent gastrointestinal complaints in primary care. A 2020 systematic review estimated the global prevalence of functional constipation at 15.3% using Rome criteria and up to 21.7% when self-reported definitions were included 1. The Rome IV criteria define functional constipation as fewer than three spontaneous complete bowel movements per week, with at least two of the following present for at least three months: straining, lumpy or hard stools (Bristol types 1 or 2), sensation of incomplete evacuation, sensation of anorectal obstruction, and manual maneuvers to support defecation 2.

Frequency alone does not tell the whole story. Some patients move their bowels daily yet strain for 20 minutes and pass pellet-like stool. Others go every three days without any distress. The Rome IV framework captures both patterns. Prevalence rises steeply after age 60, and women are affected roughly twice as often as men according to a large U.S. population-based study (N=10,018) published in the American Journal of Gastroenterology 3.

Functional (Primary) Constipation: The Most Likely Cause

For the majority of patients, no underlying disease explains the problem. Functional constipation is a diagnosis of exclusion, but it accounts for roughly 95% of chronic cases seen in gastroenterology clinics 4. Three overlapping subtypes exist: normal-transit constipation, slow-transit constipation, and defecatory disorders (also called dyssynergic defecation or pelvic floor dysfunction).

Normal-transit constipation is the single most common subtype. Stool moves through the colon at a standard rate, yet the patient reports constipation symptoms. It responds best to dietary fiber, adequate hydration (1.5 to 2 liters of fluid daily), and osmotic laxatives such as polyethylene glycol (PEG) 3350 5.

Slow-transit constipation involves measurably delayed colonic motility confirmed by radiopaque marker studies or wireless motility capsule. It is more common in young women. A 2004 study using radiopaque markers found that slow-transit constipation accounted for approximately 13% of patients referred for chronic constipation 6.

Dyssynergic defecation, where the pelvic floor muscles contract paradoxically during attempted evacuation, is present in up to 40% of patients referred to motility centers. Biofeedback therapy produces symptom improvement in roughly 70% to 80% of dyssynergia patients, a rate superior to laxatives in randomized controlled trials 7. The American Gastroenterological Association (AGA) recommends anorectal manometry and a balloon expulsion test as first-line investigations when a defecatory disorder is suspected.

Dietary and Lifestyle Factors That Slow the Gut

Low fiber intake is the first variable most clinicians address. The average American consumes about 15 grams of fiber per day, roughly half the recommended 25 to 30 grams 8. A meta-analysis of five RCTs (N=389) found that dietary fiber supplementation increased stool frequency by 1.3 bowel movements per week compared with placebo 9.

Not all fiber is equal. Soluble, gel-forming fibers such as psyllium have better evidence for improving stool consistency than insoluble fibers like wheat bran, which can worsen bloating in some patients. Dr. Satish Rao, a gastroenterologist at Augusta University and a leading pelvic floor researcher, has noted: "Psyllium is my first-line recommendation for functional constipation because it normalizes both hard and loose stools without the gas and distension patients often get from bran."

Physical inactivity also plays a role. Bed-bound or sedentary patients have slower colonic transit times. Observational data from the Nurses' Health Study (N=62,036) showed that women who exercised daily had a 44% lower risk of constipation compared with those who exercised less than once per week 10. Dehydration compounds the problem by allowing the colon to extract more water from stool, producing harder, more difficult-to-pass feces.

Ignoring the urge to defecate, a habit common among shift workers and school-age children, can blunt the gastrocolic reflex over time and contribute to rectal hyposensitivity.

Medications: A Frequently Overlooked Cause

Drug-induced constipation is the second most common cause after functional etiologies, and it is the single most reversible. Clinicians should perform a thorough medication reconciliation before ordering imaging or specialty referrals. The major drug classes implicated include:

Opioids. Opioid-induced constipation (OIC) affects 40% to 80% of patients on chronic opioid therapy 11. Mu-opioid receptors in the enteric nervous system slow peristalsis, increase fluid absorption, and raise anal sphincter tone. Unlike other opioid side effects, OIC rarely improves with tolerance. The AGA recommends peripherally acting mu-opioid receptor antagonists (PAMORAs) such as naloxegol (Movantik) or methylnaltrexone (Relistor) when conventional laxatives fail 12.

Anticholinergics. Tricyclic antidepressants (amitriptyline, nortriptyline), first-generation antihistamines (diphenhydramine), antipsychotics (olanzapine, quetiapine), and overactive bladder drugs (oxybutynin, tolterodine) all reduce colonic motility through muscarinic receptor blockade. A 2019 population-based study in JAMA Internal Medicine (N=284,343) linked cumulative anticholinergic exposure to higher rates of constipation and other adverse outcomes 13.

Calcium-channel blockers. Verapamil is the worst offender among antihypertensives, causing constipation in up to 25% of patients. Nifedipine and amlodipine carry a lower but still measurable risk.

Iron supplements. Ferrous sulfate, the most commonly prescribed form, causes constipation in roughly 20% of users. Switching to ferrous bisglycinate or intravenous iron often resolves the symptom.

Other contributors. Aluminum-containing antacids, bile acid sequestrants (cholestyramine), 5-HT3 antagonists (ondansetron), and certain supplements (high-dose calcium) round out the list.

Endocrine and Metabolic Conditions

When constipation persists despite adequate fiber, hydration, and medication review, endocrine screening is warranted.

Hypothyroidism slows gastrointestinal motility through reduced smooth-muscle contractility and decreased mucosal secretion. Constipation is reported by 20% to 40% of hypothyroid patients. A simple TSH measurement identifies the condition 14. Thyroid hormone replacement with levothyroxine typically restores normal bowel function within weeks.

Diabetes mellitus causes constipation through autonomic neuropathy affecting the enteric nervous system. Up to 60% of patients with longstanding type 2 diabetes report constipation, according to a cross-sectional analysis published in Diabetes Care (N=423) 15. Poor glycemic control accelerates neuropathic damage, making HbA1c optimization a first-line intervention for diabetic constipation.

Hypercalcemia, regardless of cause, reduces colonic motility. Primary hyperparathyroidism is the most common outpatient etiology. The 2022 Endocrine Society guidelines recommend serum calcium measurement as part of the initial constipation workup in patients over 50 16.

Hypokalemia and hypomagnesemia impair smooth-muscle contractility. Diuretic use and poor dietary intake are frequent culprits. Correcting the electrolyte deficit often resolves the constipation without additional laxatives.

Neurological Conditions

The gut requires intact neural circuitry from the brain to the enteric nervous system. Several neurological diseases disrupt this pathway.

Parkinson disease causes constipation in 50% to 80% of patients, and bowel symptoms may precede motor diagnosis by a decade or more 17. Alpha-synuclein deposits in the enteric nervous system slow colonic transit and impair rectal sensation. Dr. Michael Camilleri, a gastroenterologist at Mayo Clinic and one of the foremost motility researchers, has stated: "Constipation in Parkinson disease is not a nuisance symptom. It reflects the same neurodegenerative process that affects the basal ganglia, and it deserves aggressive management."

Multiple sclerosis causes constipation in approximately 40% to 50% of patients, primarily through disrupted suprasegmental control of the defecation reflex.

Spinal cord injury above the sacral segments produces upper motor neuron bowel, characterized by slow transit and a hyperreflexic pelvic floor. Structured bowel programs timed to the gastrocolic reflex are the standard of care 18.

Hirschsprung disease presents almost exclusively in neonates and young children. Absence of ganglion cells in the distal colon creates a functional obstruction. It is rare in adults but not impossible. A rectal suction biopsy is diagnostic.

Structural and Mechanical Causes

Obstructive lesions must be excluded when alarm features are present. Colorectal cancer is the primary concern, particularly in patients over 50 with new-onset constipation, iron-deficiency anemia, or a family history of colorectal malignancy. The U.S. Preventive Services Task Force recommends screening colonoscopy beginning at age 45 for average-risk individuals 19.

Strictures from inflammatory bowel disease (particularly Crohn disease), post-surgical adhesions, and extrinsic compression from pelvic masses (ovarian tumors, uterine fibroids) can all produce progressive, worsening constipation.

Rectocele, an anterior herniation of the rectal wall into the vagina, causes a pocket where stool becomes trapped. It affects up to 80% of parous women on defecography, though many are asymptomatic. Symptomatic rectoceles may respond to pelvic floor physiotherapy or, in refractory cases, surgical repair 20.

Rectal intussusception and rectal prolapse also impede defecation. Dynamic MR defecography is the preferred imaging modality when pelvic floor anatomy needs clarification.

Constipation During Pregnancy and in Older Adults

Pregnancy-related constipation affects 25% to 40% of pregnant women, driven by rising progesterone levels (which relax smooth muscle), uterine compression of the colon, and prenatal iron supplementation 21. PEG 3350 and psyllium are considered safe in pregnancy. Stimulant laxatives (senna, bisacodyl) are generally reserved for short-term use when osmotic agents fail.

Older adults face a convergence of risk factors: polypharmacy, reduced mobility, decreased dietary intake, weakened abdominal musculature, and age-related slowing of colonic transit. A study in the Journal of the American Geriatrics Society (N=3,166) found that adults over 65 using three or more constipating medications had a threefold higher risk of chronic constipation compared with age-matched controls on zero constipating drugs 22. Deprescribing anticholinergics and switching to less constipating alternatives should precede any laxative escalation.

Red Flags: When Constipation Needs Urgent Evaluation

Not all constipation is benign. The following features should prompt timely investigation:

• New-onset constipation after age 50 without an obvious precipitant
• Blood in the stool (hematochezia or positive fecal occult blood)
• Unintentional weight loss exceeding 5% of body weight over six months
• Progressive narrowing of stool caliber
• Family history of colorectal cancer or inflammatory bowel disease
• Iron-deficiency anemia
• Palpable abdominal or rectal mass
• Severe, acute constipation with vomiting, distension, and absence of flatus (suggesting bowel obstruction)

The American College of Gastroenterology (ACG) 2021 clinical guideline on chronic constipation states that colonoscopy is indicated when alarm features are present, but routine colonoscopy for chronic constipation without red flags is not recommended 23.

First-Line Treatment Approaches by Cause

Treatment depends entirely on etiology. For functional constipation, the ACG guideline recommends a stepwise approach:

Step 1: Lifestyle modification. Increase dietary fiber to 25 to 30 grams per day (psyllium preferred), ensure fluid intake of 1.5 to 2 liters daily, and maintain regular physical activity.

Step 2: Osmotic laxatives. PEG 3350 (17 grams daily) is the best-studied osmotic agent, with a number needed to treat (NNT) of 3 in a Cochrane meta-analysis of 10 RCTs (N=868) 24. Lactulose is an alternative but produces more bloating.

Step 3: Secretagogues. Linaclotide (Linzess, 145 mcg daily) and plecanatide (Trulance, 3 mg daily) activate guanylate cyclase-C receptors, increasing intestinal chloride and water secretion. In a phase III trial (N=1,272), linaclotide produced at least 3 complete spontaneous bowel movements per week in 21.3% of patients vs. 6.0% with placebo 25. Lubiprostone (Amitiza, 24 mcg twice daily) activates type-2 chloride channels and is FDA-approved for chronic idiopathic constipation.

Step 4: Prokinetics. Prucalopride (Motegrity, 2 mg daily), a selective 5-HT4 receptor agonist, is effective for slow-transit constipation. A pooled analysis of three RCTs (N=1,791) showed that 24% of patients achieved at least 3 spontaneous complete bowel movements per week vs. 12% with placebo 26.

For opioid-induced constipation specifically, conventional laxatives should be tried first. If they fail, naloxegol 25 mg daily or methylnaltrexone 12 mg subcutaneously every other day are the next steps 12.

For dyssynergic defecation, biofeedback therapy is the treatment of choice. Laxatives alone do not correct the underlying pelvic floor incoordination.

Diagnostic Workup: What to Expect

The initial evaluation is straightforward. A detailed history covering stool frequency, consistency (using the Bristol Stool Scale), straining, medication list, diet, and fluid intake captures most of the relevant information. A digital rectal exam assesses resting sphincter tone, squeeze pressure, and the presence of rectal masses or impaction.

Basic laboratory studies include TSH, serum calcium, fasting glucose or HbA1c, and a complete blood count to screen for anemia. If these are unrevealing and symptoms persist despite empiric treatment, the next tier involves anorectal manometry, a balloon expulsion test, and possibly a colonic transit study with radiopaque markers or a wireless motility capsule 23.

Colonoscopy is reserved for patients with alarm features or those who are due for age-appropriate colorectal cancer screening. A 2019 retrospective cohort (N=4,882) published in Gastroenterology found that the yield of colonoscopy for organic disease in patients with chronic constipation and no alarm features was only 2.3%, comparable to asymptomatic screening populations 27.