What are the differences in action, use, and ingredients between Colace (Docusate), Dulcolax (Bisacodyl), Miralax (Polyethylene Glycol), Milk of Magnesia (Magnesium Hydroxide), and Magnesium Citrate?

Comparison of Common Laxatives: Action, Use, and Ingredients

The main differences between these laxatives are their mechanisms of action, with polyethylene glycol (Miralax) being the most effective and best-tolerated osmotic laxative, while stimulant laxatives like bisacodyl (Dulcolax) are best for short-term use, and stool softeners like docusate (Colace) have limited efficacy. 1, 2

Mechanism of Action

Osmotic Laxatives

• Polyethylene Glycol (Miralax)

  • Active ingredient: Polyethylene glycol 3350, 17g per dose 3
  • Mechanism: Draws water into the intestinal lumen, increasing stool bulk and softness
  • No net gain or loss of sodium and potassium 1
  • Response has been shown to be durable over 6 months 1

• Milk of Magnesia

  • Active ingredient: Magnesium hydroxide 2,400mg per 30mL 4
  • Mechanism: Osmotic effect that draws water into the intestine
  • Also has some stimulant properties 1
  • Caution in renal insufficiency due to risk of hypermagnesemia 1, 2

• Magnesium Citrate

  • Active ingredient: Magnesium citrate
  • Mechanism: Similar to milk of magnesia but typically produces more rapid results
  • More potent than milk of magnesia, often used for bowel prep 2
  • Also requires caution in renal insufficiency 1, 2

Stimulant Laxatives

• Bisacodyl (Dulcolax)
  • Active ingredient: Bisacodyl 5mg per tablet
  • Mechanism: Stimulates intestinal motility and increases secretions
  • Recommended for short-term use or rescue therapy 1
  • Works directly on the colon to increase peristalsis 1

Stool Softeners

• Docusate (Colace)
  • Active ingredient: Docusate sodium 100mg per capsule 5
  • Mechanism: Detergent action that allows water to penetrate stool
  • Limited evidence for effectiveness in palliative care 1
  • Not recommended as primary treatment for chronic constipation 2

Clinical Use and Effectiveness

Polyethylene Glycol (Miralax)

• First-line treatment for chronic idiopathic constipation 1, 2
• Recommended initial dose: 17g daily 1
• Cost-effective ($10-$45 monthly) 1
• Better accepted than magnesium hydroxide in pediatric studies 6
• Superior to lactulose in increasing stool frequency (0.95 more stools per week) 7
• Superior to milk of magnesia with small but significant increase in stool frequency 7

Milk of Magnesia

• Effective osmotic laxative for constipation 1
• Recommended initial dose: 400-500mg daily 1
• Should be used with caution in renal insufficiency 1, 2
• Less effective than PEG in pediatric studies 7
• Less well-accepted than PEG due to taste 6

Magnesium Citrate

• More potent than milk of magnesia 2
• Often used for bowel preparation before procedures 8
• Can cause significant fluid shifts 8
• Typical dose: 8 oz daily 2

Bisacodyl (Dulcolax)

• Recommended for short-term use or rescue therapy 1
• Initial dose: 5mg daily, maximum 10mg daily 1
• Can cause cramping and abdominal discomfort 1
• Often used when osmotic laxatives fail 2
• Can be administered orally or rectally (suppository) 2

Docusate (Colace)

• Limited evidence for effectiveness 1
• Not recommended as primary treatment for chronic constipation 2
• May have some benefit for prevention but not treatment 2
• Generally not recommended in advanced disease 1

Comparative Effectiveness

• For Chronic Constipation:

  • PEG is superior to placebo, lactulose, and milk of magnesia 7
  • PEG is better tolerated than most other laxatives 6, 9
  • Stimulant laxatives (bisacodyl) are effective but recommended for short-term use 1
  • Docusate has limited evidence supporting its effectiveness 1

• For Bowel Preparation:

  • PEG-based preparations are more effective than MiraLAX alone 10
  • Combination of stimulant laxatives with PEG can be effective and safe 8

Clinical Considerations and Pitfalls

• Renal Impairment:

  • Avoid or use caution with magnesium-based products (milk of magnesia, magnesium citrate) 1, 2
  • PEG is generally safe in renal impairment 1

• Pregnancy:

  • Lactulose is the only osmotic agent well-studied in pregnancy 1
  • Use caution with magnesium products 1

• Opioid-Induced Constipation:

  • Bulk laxatives not recommended 1
  • Consider combination of stimulant and osmotic laxatives 2

• Common Side Effects:

  • PEG: Bloating, abdominal discomfort, cramping 1
  • Magnesium products: Risk of hypermagnesemia, especially with renal impairment 1
  • Bisacodyl: Cramping, abdominal discomfort 1
  • Docusate: Generally well-tolerated but limited efficacy 1

Algorithmic Approach to Laxative Selection

1. First-line treatment:

   • Polyethylene glycol (Miralax) 17g daily 1, 2
   • If not tolerated or contraindicated, consider magnesium hydroxide (milk of magnesia) 400-500mg daily 1

2. If inadequate response:

   • Add stimulant laxative (bisacodyl 5-10mg) 2
   • Consider increasing PEG dose 1

3. For acute situations or bowel prep:

   • Consider magnesium citrate for more rapid effect 2, 8
   • Bisacodyl can be used as rescue therapy 1

4. For prevention only:

   • Docusate may have limited role but not recommended as primary treatment 1, 2