Guide

Constipation on a GLP-1: Mechanism, Incidence and the Line to Watch

Constipation was reported by 17% of participants at tirzepatide 15 mg in SURMOUNT-1. It follows directly from slowed gastric emptying and reduced food volume, and it is u

What was recorded

In SURMOUNT-1 at 15 mg, constipation was reported by 17% of participants, alongside diarrhoea at 23% and dyspepsia at 10%. Both directions of bowel change occur, sometimes in the same person at different points in titration.

Why it happens

Two mechanisms combine. These drugs slow gastric emptying, which slows transit through the digestive tract. And reduced appetite means less food volume and often less fibre and fluid, which independently slows things further.

So it is partly pharmacological and partly a downstream consequence of eating less — which is why the useful conversation covers both dose and intake rather than only one.

The line that matters

The prescribing information for these drugs notes ileus — a functional obstruction where the bowel stops propelling contents forward. It has been reported in postmarketing use.

Ordinary constipation is uncomfortable and slow. Ileus presents differently: significant abdominal distension, severe pain, persistent vomiting, and an inability to pass stool or gas at all. That combination is a reason for urgent assessment rather than patience.

We are drawing this line explicitly because the two are described with the same everyday word, and the distinction is the clinically important part of this page.

Expected, contact, urgent

Expected: less frequent bowel movements, harder stool, some straining, particularly after a dose increase and while intake is lower than usual.

Contact your prescriber: constipation persisting beyond a few days despite the usual measures, significant discomfort, or a pattern that worsens with each dose increase.

Urgent: severe abdominal pain with distension, vomiting, and inability to pass stool or gas.

What we will not do here

Recommend specific laxatives, fibre products or doses. Those interact with other medications, some are inappropriate with certain conditions, and the choice depends on what is actually causing the problem in your case. A pharmacist can advise on this quickly and knows what else you take.

What is worth saying generally is that adequate fluid intake matters more than usual when transit is slowed, and that this is harder when appetite suppression has reduced everything you consume, including drinks.

What to track

1. Whether it started or worsened at a specific dose step.
2. Whether it alternates with diarrhoea, which is common and worth mentioning.
3. How much you are drinking, honestly.
4. Whether anything else changed — new medications, particularly iron or opioids, compound this significantly.

Questions readers actually ask