Laxatives

Laxatives are medications used to promote defecation. Most often, laxatives are used to treat constipation or for bowel preparation for certain procedures. There are 4 main classes of laxatives: bulk-forming, stimulant, osmotic, and emollient. Often, a combination of these medications is required to have the desired effect. All laxatives can result in bloating. Laxative overuse can induce diarrhea and dehydration. Non–anion gap metabolic acidosis occurs owing to bicarbonate loss in the feces, while metabolic alkalosis occurs because of volume contraction.

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Overview

Definition

Laxatives are medications that increase the frequency of bowel movements.

Classification

  • Bulk-forming laxatives
  • Stimulant laxatives
  • Osmotic laxatives
  • Emollient laxatives

General indications

  • Constipation: when conservative measures (e.g., fiber, hydration, exercise) fail
  • Bowel regimen for patients on opioid therapy
  • Bowel preparation before a rectal examination or procedure involving the colon (e.g., colonoscopy)

General side effects

  • Bloating
  • Diarrhea 
  • Dehydration
  • Non–anion gap metabolic acidosis due to loss of bicarbonate through feces
  • Metabolic alkalosis due to volume contraction

Bulk-forming Laxatives

Chemistry

Bulk-forming laxatives tend to be either:

  • Natural or synthetic polysaccharides
  • Cellulose derivatives

Agents

  • Psyllium husk (e.g., Metamucil)
  • Methylcellulose 
  • Polycarbophil

Mechanism of action

  • Absorbs water → forms a bulky gel
  • Added bulk distends the colon → promotes peristalsis
  • Allows for material to more freely flow through the intestine

Pharmacokinetics

  • Absorption: none
  • Metabolism: none
  • Onset of action: 12–72 hours

Indications

  • Constipation
  • Fiber supplement
  • Irritable bowel syndrome

Contraindications

  • Fecal impaction
  • GI obstruction

Adverse effects

Adverse effects tend to be minimal with these agents but may include:

  • Abdominal cramps
  • Diarrhea
  • Intestinal obstruction
  • Anaphylaxis (associated with psyllium in susceptible individuals)

Stimulant Laxatives

Agents

  • Senna 
  • Bisacodyl
  • Castor oil

Mechanism of action

  • Myenteric plexus stimulation through direct action on the intestinal mucosa or nerve plexus → colonic contractions
  • Alter water and electrolyte secretion in the intestinal mucosa

Pharmacokinetics

  • Absorption: none
  • Metabolism: none
  • Onset of action: 
    • 6–12 hours for oral administration
    • Approximately 1 hour for rectal administration

Indications

  • Constipation
  • Clearing the colon before colonoscopy

Adverse effects

  • Melanosis coli:
    • Brown or black discoloration of colonic mucosa on colonoscopy 
    • Due to pigmented macrophage deposition in lamina propria
  • Hypokalemia
  • Metabolic acidosis or alkalosis
  • Abdominal cramps
  • Diarrhea
  • Nausea
  • Vomiting
Melanosis coli associated with stimulant laxative use

Melanosis coli is a black discoloration of the colon associated with stimulant laxative use.

Image: “Melanosis coli in the elderly: a look beyond what can be seen” by BMC Surgery. License: CC BY 2.0

Osmotic Laxatives

Chemistry

Osmotic laxatives are often:

  • Nonabsorbable sugars
  • Nonabsorbable salts

Agents

  • Magnesium citrate
  • Magnesium sulfate
  • Polyethylene glycol
  • Sorbitol
  • Lactulose

Mechanism of action

  • General:
    • Osmotically active substances
    • Draws water into the GI lumen → soft stools
  • Lactulose:
    • Bacteria degrade lactulose → lactic acid
    • ↓ pH → promotes ammonia conversion to ammonium → excretion

Pharmacokinetics

  • Absorption: none
  • Metabolism: none
  • Onset of action: 
    • 12–72 hours for oral administration 
    • < 1 hour for rectal administration

Indications

  • Constipation
  • Preparation for colonoscopy 
  • Lactulose is indicated for hepatic encephalopathy

Contraindications

Lactulose should not be used in patients with galactosemia.

Adverse effects

  • Bloating
  • Flatulence
  • Diarrhea
  • Nausea
  • Dehydration
  • Hypermagnesemia (in patients with renal impairment who take magnesium-containing laxatives)

Emollient Laxatives

Agents

Docusate (Colace) is a common emollient laxative, also known as a “stool softener.”

Mechanism of action

  • ↓ Surface tension along oil–water interface of stool
  • Incorporates water and fat in stool → soft stool

Pharmacokinetics

  • Absorption: none
  • Metabolism: none
  • Onset of action: 12–72 hours

Indications

  • Constipation
  • Cerumen impaction (off-label)

Contraindications

  • GI obstruction
  • Concomitant mineral oil use

Adverse effects

Side effects are generally mild but may include:

  • Diarrhea
  • Abdominal cramps
  • Throat irritation
  • Dehydration

Comparison of Laxative Medications

The following table compares and contrasts members of the laxative drug class:

Table: Comparison of laxative medications
Class Examples Mechanism of action Side effects
Bulk-forming laxatives
  • Methylcellulose
  • Psyllium husk (e.g., Metamucil)
  • Calcium polycarbophil
  • Water-soluble fibers draw water into GI lumen
  • Formation of viscous mass easily excreted during defecation
  • Abdominal cramps
  • Diarrhea
  • Intestinal obstruction
  • Anaphylaxis
Stimulant laxatives
  • Senna
  • Bisacodyl
  • Castor oil
Myenteric plexus stimulation resulting in colonic contractions
  • Melanosis coli
  • Diarrhea
  • Nausea
  • Vomiting
  • Hypokalemia
  • Metabolic acidosis or alkalosis
  • Abdominal cramps
Osmotic laxatives
  • Magnesium citrate
  • Magnesium hydroxide
  • Polyethylene glycol
  • Lactulose
Draws water into GI lumen, softening the stools
  • Bloating
  • Flatulence
  • Diarrhea
  • Nausea
  • Dehydration
  • Hypermagnesemia
Emollient laxatives Docusate
  • Reduces surface tension along oil–water interface of stool
  • Incorporates water and fat in stool, resulting in soft stools
  • Diarrhea
  • Abdominal cramps
  • Throat irritation
  • Dehydration

References

  1. Wald, A. (2021). Management of chronic constipation in adults. UpToDate. Retrieved May 29, 2021, from https://www.uptodate.com/contents/management-of-chronic-constipation-in-adults
  2. Rao, S.S. (2020). Constipation in the older adult. UpToDate. Retrieved May 29, 2021, from https://www.uptodate.com/contents/constipation-in-the-older-adult
  3. Farinde, A. (2020). Laxatives, stool softeners, and prokinetic agents: laxatives, stool softeners, and prokinetic agents. Medscape. Retrieved May 29, 2021, from https://emedicine.medscape.com/article/2172208-overview
  4. McQuaid, K. R. (2012). Drugs used in the treatment of gastrointestinal diseases. In Katzung, B. G., et al. (Eds.), Basic & Clinical Pharmacology. New York: McGraw-Hill Medical, pp. 1092–1093.
  5. Bashir, A., Sizar, O. (2020). Laxatives. StatPearls. Retrieved June 6, 2021, from https://www.ncbi.nlm.nih.gov/books/NBK537246/

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