Laxatives: which kind for which constipation — and how fast does each one work?
Bulk, water, a push or a softener — and from below, a rescue that works in minutes.
Select any card for the reasoning, what to watch for and an exam tip.
Laxatives
Key points
The order of play: diet and fluid, then a bulk-former; if stools stay hard, add or switch to an osmotic (a macrogol first, lactulose next); if they are soft but hard to pass, add a stimulant.
Opioid-induced constipation: an osmotic plus a stimulant, never a bulk-former; naloxegol or naldemedine only when adequately titrated laxatives have failed.
Onset shapes the timing: stimulants work overnight, so give them at bedtime; lactulose can take two days, bulk-formers three; suppositories and enemas work in minutes.
Any laxative in excess costs potassium, and magnesium salts build up when the kidneys are poor.
Coming off: reduce one laxative at a time, and the stimulant first.
When to refer
Key takeaway
Start with fibre, fluid and a bulk-former; add an osmotic if the stools stay hard, and a stimulant if they are soft but won’t come. Opioid constipation needs an osmotic plus a stimulant, never bulk — and new constipation over 50, blood or weight loss needs investigating, not a laxative.
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