Introduction

 

Laxatives are one of the most widely prescribed drugs, both on the wards in hospital as well as in general practice. Most doctors will have had to prescribe it at least once in their career. This makes them quite an important drug to be aware of.

Laxatives, cathartics and purgatives... what are they on about?

 

Annoyingly, these terms can be found used interchangeably, and thereby can cause some confusion! In essence though, they work to the same effect: to increase intestinal emptying. The main difference between them is potency; cathartics are the most potent, followed by purgatives and then laxatives.

 

Laxatives have 3 uses in particular:

  1. Constipation - a difficult symptom as it can be quite subjective!
  2. Bowel preparation - to evacuate the bowel in preparation for a radiological investigation or for an endoscopy
  3. Bowel preparation prior to surgery

 

    What I find helpful when it comes to deciding which laxative would be most appropriate for each situation is understanding how the gut works normally, knowing why the laxative is required and then fitting in where each type of laxative exerts its action.

     

    Normal gut motility

    Normal gut motility depends on several factors:

    • Normal structure of the intestines
    • Intact nervous supply - both intrinsic nerves of the intramural plexus as well as the extrinsic autonomic nerves (collectively these form the enteric nervous system)
    • Bulk of contents (and therefore distension of the intestines, which together with mucosal irritation, stimulates peristalsis)
    • Hardness of stool

     

      These factors lead to peristalsis, a vital process that leads to defecation. Any disruption can lead to an alteration of bowel habit -and essentially, that's what the laxatives do!

      Now a little on constipation...

      Constipation has a wide variety of causes, and the laxative prescribed often depends on the cause. Also, before prescribing a laxative, it is important to first rule out a treatable cause.

      For example, patients who have constipation secondary to poor diets should first be encouraged to increase their fibre and fluid intakes before going on to a laxative. In addition, because the different laxatives have different mechanisms of action, some will be more appropriate for the cause of constipation than another, e.g. peripheral opioid receptor antagonists will be best for constipated patients who have no choice but to take opioids chronically.

       

      Some causes of constipation are listed here:

      • Immobility
      • Age
      • Drugs (this is a big cause, always remember to look at what drugs the patient is on first)
      • Pregnancy
      • Poor dietary intake of fibre
      • Poor fluid intake
      • Surgery
      • Neurodegenerative diseases such as MS, Parkinsons
      • Malignancy in the bowel
      • Spinal Cord Injury

       

       

      The Laxatives

      There are 3 main types of laxatives, as depicted here in the diagram below:



      What these laxatives do in the colon:

      Action of the main laxatives on the colon

      Summary tables of individual drugs

      Summary tables with details for some of the more common drugs in each type are provided below for quick reference:

      Stimulant Laxatives



      Bulk Laxatives



      Osmotic Laxatives



      The other laxatives to know about:

      Faecal softeners: act to soften and lubricate faeces

      • Sodium docusate (also has stimulant effect and so sometimes grouped with stimulant laxatives)
      • Parafin oil

       

      Peripheral opioid receptor antagonists: blocks opioid receptors

      • Methylnaltrexone bromide

       

      5HT4- receptor agonists: selectively stimulates serotonin receptors to increase colonic motility

      • Prucalopride

       

       

      Again, summary tables with details of some of the individual drugs in each type are provided below.

       

      Faecal Softeners



      Peripheral Opioid Receptor Antagonists



      5HT4- receptor agonists



      Final notes on prescribing laxatives:

      Pregnancy: Most laxatives are safe to use in pregnancy, although manufacturers advise caution and not to prescribe more than needed. The main laxative to avoid in pregnancy is the 5HT4 receptor agonist prucalopride.

       

      Eating Disorders: It is quite easy to forget but laxatives can be abused by those desiring to lose weight. Ask about laxative use if you suspect an eating disorder.

       

      Always remember that laxatives are available OTC, and always check with the patient if they are on any of those before prescribing them some.

       

      Laxative overuse can result in diarrhoea, electrolyte disturbances and dehydration and therefore even though it is considered quite a safe drug, the prescriber must always be cautious!

       

      References

      1. Shanbhag T V. Prep Manual for Undergraduates: Pharmacology. India: Elsevier. 2008.
      2. British Medical Association and the Royal Pharmaceutical Society of Great Britain. British National Formulary. 62nd ed. UK: BMJ Publishing Group. 2011.
      3. Rang H P, Dale M M, Ritter J M, Flower R J. 6th Ed. Philedelphia: Churchill Livingstone Elsevier. 2007.
      4. Smith M E, Morton D G. The Digestive System. 2nd Ed. Churchill Livingstone Elsevier. 2010.
      5. Travis S P L, Taylor R H, Misiewicz. Gastroenterology Pocket Consultant. Oxford: Blackwell Scientific Publications. 1991.
      6. Reid J L, Rubin P C, Walters M R. Lecture Notes: Clinical Pharmacology & Therapeutics. 7th ed. Blackwell Publishing. 2006
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