Treatment of Constipation
Start with polyethylene glycol (PEG) 17g once daily as first-line therapy, combined with lifestyle modifications including increased fluid intake and physical activity when appropriate.
The American Gastroenterological Association recommends PEG as the preferred first-line pharmacological agent for constipation, though stimulant laxatives (senna or bisacodyl 10-15mg, 2-3 times daily) are equally appropriate alternatives, particularly for opioid-induced constipation. 1
Critical Initial Assessment Before Treatment
Before starting any laxative therapy, you must rule out several conditions:
- Perform a digital rectal examination to exclude fecal impaction 1
- Obtain plain abdominal imaging if bowel obstruction is suspected clinically 2, 1
- Check for metabolic causes: hypercalcemia, hypokalemia, hypothyroidism, and diabetes mellitus 2, 1
- Review and discontinue constipating medications when feasible, including antacids, anticholinergics (antidepressants, antispasmodics, phenothiazines, haloperidol), and antiemetics 2, 1
- A complete blood count is the only laboratory test routinely needed; metabolic panels have low diagnostic utility unless other clinical features warrant them 1
Stepwise Treatment Algorithm
First-Line Options (Choose One):
- Polyethylene glycol (PEG) 17g once daily - preferred by the American Gastroenterological Association 1
- Senna or bisacodyl 10-15mg, 2-3 times daily - particularly effective for opioid-induced constipation 2, 1
- Milk of magnesia 1 oz twice daily - inexpensive alternative osmotic agent with comparable efficacy 1
- All first-line agents cost approximately $1 or less per day 1
Important caveat: Do NOT add stool softeners like docusate to stimulant laxatives, as evidence shows no additional benefit 1. This is a common pitfall in clinical practice.
Lifestyle Modifications (Concurrent with Pharmacotherapy):
- Increase fluid intake to at least 2 liters daily 2, 1
- Encourage physical activity when appropriate 2, 1
- Dietary fiber should only be added if the patient has adequate fluid intake (at least 2 liters daily); supplemental medicinal fiber like psyllium is unlikely to control medication-induced constipation 1
Treatment Goal
Aim for one non-forced bowel movement every 1-2 days, not necessarily daily bowel movements. 2, 1 This is an important distinction that prevents overtreatment.
Second-Line Treatment (If Constipation Persists):
Add one of the following agents to your first-line therapy:
- Rectal bisacodyl once daily 1
- Lactulose 2, 1
- Magnesium hydroxide or magnesium citrate (avoid in renal impairment due to hypermagnesemia risk) 2, 1
- Switch to or add PEG if not already using it 1
Third-Line Treatment (If Gastroparesis Suspected):
- Add metoclopramide 10-20mg, 2-3 times daily as a prokinetic agent 2, 1
- This is particularly relevant for patients on GLP-1 agonists (like Mounjaro) which slow gastric emptying 1
Fourth-Line Treatment (Refractory Constipation):
For persistent constipation unresponsive to standard laxatives, consider newer secretagogues:
- Linaclotide - FDA-approved for IBS-C, CIC, and functional constipation in pediatric patients 6-17 years 3
- Lubiprostone 24 mcg twice daily (8 mcg twice daily for IBS-C) - take with food and water, swallow whole 4
- Plecanatide 1
Special Consideration: Opioid-Induced Constipation
All patients receiving opioid analgesics should be prescribed a prophylactic laxative concomitantly, unless contraindicated by pre-existing diarrhea. 1 This is non-negotiable.
- Start with stimulant laxatives (senna or bisacodyl) or osmotic laxatives (PEG) - these are preferred over fiber for opioid-induced constipation 1
- For refractory opioid-induced constipation: Consider methylnaltrexone 0.15 mg/kg every other day (maximum once daily) - a peripherally acting μ-opioid receptor antagonist that relieves constipation while maintaining pain control 2, 1
- Limitation: Effectiveness in patients taking diphenylheptane opioids (e.g., methadone) has not been established 4
Management of Fecal Impaction
If impaction is present on digital rectal exam:
- Administer glycerine suppositories 2
- Perform manual disimpaction if necessary 2
- Consider mineral oil or warm water enemas 5
Key Clinical Pitfalls to Avoid
- Do not rely on fiber supplements alone - they are ineffective for medication-induced constipation without adequate hydration (at least 2 liters daily) 1
- Do not add stool softeners to stimulant laxatives - evidence shows no additional benefit 1
- Do not use magnesium-based laxatives long-term - potential for toxicity, especially in renal impairment 1, 5
- Reassess for impaction or obstruction if constipation persists despite treatment 1
- Do not forget prophylactic laxatives when starting opioid therapy - this is the most common preventable error 1