Nausea is the side effect everyone is warned about. It is also the one that reliably improves.
Constipation is the one people are not warned about, do not raise because it is embarrassing, and are still dealing with in month nine. In my experience it accounts for more quiet discontinuations than any other single complaint in this drug class.
Three causes stacked on top of each other
The drug slows the whole gut, not just the stomach. Slower transit means more time for water to be reabsorbed from stool, which is the direct mechanical cause.
But two consequences of appetite suppression are doing at least as much work.
You are eating less bulk. Stool is largely composed of what you ate plus bacterial mass. Cut intake by half and there is straightforwardly less to pass, less frequently.
You are drinking less. A substantial share of daily fluid normally arrives inside food, and thirst signalling drops alongside appetite. Most people on these drugs are mildly dehydrated and unaware of it — which is also the route to more serious kidney trouble.
Fibre intake usually collapses too, because when you can only manage a small plate, vegetables and legumes are the first things people drop in favour of protein.
24% vs 11%
Constipation on semaglutide 2.4 mg versus placebo in STEP 1
NEJM, 2021
Fewer than 3
Bowel movements per week — the clinical definition of constipation
~17 g
Typical daily psyllium husk dose, taken with a full glass of water
The ladder, in order
Work through these in sequence rather than starting at the bottom.
1. Fluid, properly. This is not optional and it is where most people fail. Aim for pale straw-coloured urine. If you have been relying on thirst to tell you, it has stopped working reliably.
2. Soluble fibre, built up slowly. Psyllium husk — Metamucil or a generic — is the best-evidenced option. Two cautions that determine whether it helps or makes things worse: it must be taken with a full glass of water, and the dose must be built up over one to two weeks. A large fibre load dropped onto a slow gut produces bloating and a harder problem than you started with.
3. Food sources, if you can manage the volume. Kiwi fruit has surprisingly good trial evidence for constipation. Prunes work. Chia seeds soaked into yoghurt deliver soluble fibre in a small volume, which is the constraint that matters here. More on building a small plate that still does this job in what to eat on a GLP-1.
4. Magnesium. Magnesium citrate or oxide draws water into the bowel and is gentle enough for regular use in most people. Avoid it if you have significant kidney impairment.
5. Polyethylene glycol. MiraLAX and its generics. This is the workhorse: osmotic, non-stimulant, safe for daily long-term use, and it does not depend on the gut contracting harder. For persistent GLP-1 constipation this is usually the answer rather than a stopgap.
6. Stimulant laxatives. Senna, bisacodyl. Effective, and fine occasionally. Daily use on an already slowed gut is a poor long-term plan and tends to produce cramping.
If you are weighing the various supplement routes, the comparison is laid out in the best constipation supplements for GLP-1 users and, for the gut-flora angle, the probiotic roundup.
The mistakes that make it worse
Insoluble fibre alone. Bran and raw vegetable bulk without adequate fluid, on a slow gut, produces distension rather than relief. Soluble beats insoluble here.
Fibre without water. Psyllium taken dry is genuinely counterproductive.
Waiting. Constipation compounds. A week of no action turns a manageable problem into impaction, which is a different and much less pleasant situation.
Cutting protein. People sometimes blame their protein shake and stop it. Protein is not the cause, and abandoning it costs you lean mass to solve a problem fluid and fibre would have fixed.
Assuming it will pass. It often does at a given dose. It comes back at the next one. Having a routine that you resume before each dose increase is better than restarting the problem-solving each time — the escalation schedule tells you exactly when to expect it.
Nausea makes people miserable for six weeks. Constipation makes people quit in month nine, and nobody puts it on the discontinuation form.
When it is not just constipation
Rare, but this is the reason to pay attention.
The routine worth setting up on day one
Rather than reacting in week three: start the fluid habit before you need it, add psyllium during your first month at a low dose and build up, keep polyethylene glycol in the cupboard, and walk after meals.
People who do this describe constipation as a minor feature of their year. People who wait until it is a problem spend considerably longer fixing it.
The uncomfortable summary
This side effect is unglamorous, under-reported, and the most likely reason you will eventually stop a medication that is working.
It is also almost entirely manageable with water, psyllium, a cheap osmotic laxative and a walk. The gap between those two sentences is filled with people who never mentioned it to anyone.