An 85-year-old man in palliative care on morphine is dehydrated. Which medicine may predispose to faecal impaction?

• A. Senna
• B. Docusate
• C. Lactulose
• D. Ispaghula husk

Answer: (D)

In this scenario, the key idea is how bulk-forming laxatives behave when there isn’t enough fluid in the gut. Ispaghula husk (psyllium) works by absorbing water and swelling to form a bulky, soft stool that is easier to pass. However, this action depends on adequate hydration. When a patient is dehydrated and has reduced gut motility from opioid use (morphine), the bulk-forming material can swell without enough fluid to dissociate and move along the intestines, increasing the risk of forming a hard mass that becomes difficult to pass—facilitating a faecal impaction.

The other options act by different mechanisms that are less likely to predispose to impaction in this context: senna stimulates intestinal contractions to move stool along; docusate lowers surface tension to soften stool, usually helping passage rather than causing a blockage; lactulose draws water into the bowel to soften and loosen stool. None of these rely on swelling a mass that needs adequate hydration, so they’re less likely to cause impaction under dehydration.

So, the reason this choice is best is that it highlights the need for sufficient fluids when using bulk-forming laxatives, a condition not met in a dehydrated patient on opioids, which raises the risk of faecal impaction.