What are the different classifications of laxatives?

Classification of Laxatives

Laxatives are classified into six major categories based on their mechanism of action: bulk-forming agents, osmotic laxatives, stimulant laxatives, stool softeners/detergents, lubricants, and newer prokinetic/secretagogue agents. 1

1. Bulk-Forming Laxatives

• Mechanism: Increase stool mass and water content by absorbing fluid in the intestinal lumen 1
• Examples: Unprocessed wheat bran, oat bran, methylcellulose, ispaghula (psyllium), and sterculia 1, 2
• Clinical use: Effective for patients who cannot tolerate adequate dietary fiber, but methylcellulose also functions as a fecal softener 1
• Critical limitation: Require adequate fluid intake to work effectively; generally not recommended in advanced disease or opioid-induced constipation due to fluid requirements 1

2. Osmotic Laxatives

These are strongly endorsed as first-line agents for chronic constipation and are preferred in clinical practice. 1

Macrogols (Polyethylene Glycol/PEG)

• Mechanism: Inert polymers that sequester fluid in the bowel without net gain or loss of sodium and potassium 1, 3
• Advantages: Can be used safely long-term without tolerance, highly effective, and well-tolerated 1
• Dosing: 17g daily at approximately $1/day 4

Lactulose

• Mechanism: Semi-synthetic disaccharide not absorbed from the GI tract; produces osmotic diarrhea of low pH and prevents ammonia-producing organisms 1
• Limitations: 2-3 day latency before effect; common intolerance due to sweet taste, nausea, flatulence, bloating, and abdominal discomfort 1
• Alternative dosing: 30-60 mL BID-QID 4

Magnesium and Sulfate Salts

• Mechanism: Primarily osmotic action; useful for rapid bowel evacuation 1
• Critical warning: Excessive doses can cause hypermagnesemia; use cautiously in renal impairment 1
• Alternative: Magnesium hydroxide 1 oz twice daily 4

Sodium Salts

• Should be avoided as they may cause sodium and water retention 1

3. Stimulant Laxatives

Add stimulant laxatives if inadequate response to osmotic laxatives occurs. 1

Anthraquinone Compounds (Senna, Aloe, Cascara)

• Mechanism: Converted by colonic bacteria to active sennosides; stimulate myenteric plexus and inhibit colonic water absorption 1
• Site of action: Primarily descending and sigmoid colon 1
• Timing: Best taken in evening or bedtime for morning bowel movement 1
• Important note: Despite historical concerns, no clinical or animal evidence supports that prolonged use damages intestinal muscle or myenteric neurons 1
• Limitation: Stimulating effect may be too great for weak or debilitated patients 1

Diphenylmethane Derivatives (Bisacodyl, Sodium Picosulfate)

• Mechanism: Stimulate sensory nerves in proximal colon; increase sodium and water movement into colonic lumen 1
• Use: Short-term use recommended in refractory constipation 1

General Warnings for Stimulants

• Increase intestinal motility and often cause abdominal cramps 1
• Contraindicated in intestinal obstruction 1
• Excessive use causes diarrhea and hypokalemia 1
• Dantron should be avoided due to potential carcinogenicity 1

4. Stool Softeners/Detergents

Docusate Sodium

• Mechanism: Probably acts both as stimulant and softening agent; stimulates fluid secretion by small and large intestine 1
• Critical limitation: Use in palliative care is based on inadequate experimental evidence 1
• Generally not recommended in advanced disease 1

5. Lubricant Laxatives

Mineral Oil (Liquid Paraffin)

• Mechanism: Lubricates the lining of the gut to facilitate defecation 5
• Serious risks: Aspiration may cause lipoid pneumonia, anal seepage, skin excoriation, and foreign body reaction if anorectal mucosa is broken 1
• Efficacy: Less effective than PEG 1
• Recommendation: Generally not recommended for advanced disease 1
• Enemas carry risks of intestinal perforation, rectal mucosal damage, and bacteremia 5

Castor Oil

• Mechanism: Principal effect on small bowel fluid secretion 1
• Current use: Rarely used 1

6. Newer Prokinetic and Secretagogue Agents

5-HT4 Receptor Agonists (Prucalopride)

• Mechanism: Selective serotonin 5-HT4 receptor agonist with prokinetic properties 1
• Indication: Licensed for chronic constipation when other laxatives fail 1
• Side effects: Headache and GI symptoms (abdominal pain, nausea, diarrhea) are most frequent but rare; generally occur at treatment start and are transient 1

Guanylate Cyclase-C Agonists (Linaclotide)

• Mechanism: 14-amino acid peptide acting on intestinal GC-C, generating cyclic GMP, stimulating chloride secretion, increasing luminal fluid and accelerating transit 1
• Additional benefit: May have visceral analgesic activity 1

Lubiprostone

• Cost consideration: Newer agents (lubiprostone, linaclotide, prucalopride) cost $7-9 daily compared to $1/day for PEG 4
• Indication: Consider if traditional laxatives fail 4

Peripherally Acting Mu-Opioid Receptor Antagonists (Methylnaltrexone)

• Mechanism: Blocks opioid-induced dysmotility without altering central analgesic effects 1
• Indication: Licensed for opioid-induced constipation in palliative care when response to other laxatives is inadequate 1
• Use: Should be used as adjunct to existing laxative therapy 1
• Dosing: Given subcutaneously on alternate days 1

7. Rectal Therapies (Suppositories and Enemas)

Suppositories

• Types: Glycerine, bisacodyl, CO2-releasing compounds 1
• Mechanism: Stool softeners and stimulants for rectal motility 1
• Indication: Preferred first-line therapy when digital rectal exam identifies full rectum or fecal impaction 1
• Advantage: Work more quickly than oral laxatives 1

Enemas

• Indication: Used only if oral treatment fails after several days to prevent fecal impaction 1
• Risks: Intestinal perforation (suspect if abdominal pain occurs), rectal mucosal damage, bacteremia 1
• Contraindications: Neutropenia, thrombocytopenia, paralytic ileus, intestinal obstruction, recent colorectal or gynecological surgery 1
• Bleeding risk: Patients on anticoagulation or with coagulation/platelet disorders risk bleeding complications or intramural hematomas 1

Clinical Algorithm for Laxative Selection

Start with osmotic laxatives (preferably PEG) as first-line therapy 1, 4, then escalate to stimulant laxatives if inadequate response 1. Reserve bulk-forming agents for patients with adequate fluid intake 1, avoid stool softeners and lubricants in advanced disease 1, and consider newer prokinetic/secretagogue agents only after traditional laxatives fail 4. Use rectal therapies for fecal impaction or full rectum on examination 1.