Constipation when starting GLP-1: the fix

Constipation is the most common gut side effect of GLP-1 drugs. It's also one of the most misunderstood, most people throw bran at it and make it worse. Here's a sequenced plan that actually works.

GLP-1 receptor agonists slow gastric emptying and reduce overall gut motility. This is the central mechanism for appetite suppression, food sits longer, fullness signals last longer, you eat less. The downside is that stools dry out by the time they reach the colon (more time means more water absorbed back into the bloodstream), and bowel movements become less frequent.

On top of this, most people on GLP-1s eat much less, so there's less stool volume to push through, and they often drink less because the appetite drop reduces fluid intake too.

The fastest fix. Most people drink less once their appetite drops, so this needs deliberate attention rather than trusting thirst. Around 2 to 2.5 litres a day suits most adults, though if you have been given a fluid limit for heart or kidney reasons, that limit wins and this is a conversation for whoever set it. Spread it through the day rather than in large volumes at meals, which only adds to the fullness. Adding electrolytes, a pinch of salt and half a lemon, helps if plain water makes you nauseous.

Add psyllium husk: 5-10 g in a glass of water once a day, separate from your main meals. Or: 30-50 g of oats in the morning. Or: 2 kiwi fruit (with skin if tolerated) at breakfast. These specifically help GLP-1 constipation because they form a gel that softens stool without adding bulk.

Avoid wheat bran and other purely insoluble fibres at this stage, they add bulk but can make slow-transit constipation worse.

Two caveats on fibre, both of which matter more on a GLP-1 than they otherwise would. Take it with a full glass of water and keep drinking through the day: bulking fibre without enough fluid makes a slow gut slower. And if you are not passing anything at all, do not add fibre. Fibre needs a moving gut to work with, and adding bulk to one that has stopped makes things worse rather than better. That situation needs a doctor, not more oats.

Walk 30 minutes a day, ideally including a 10-minute walk after dinner. Movement directly stimulates the gut. Regular physical activity such as 30 minutes of brisk walking a day speeds up colonic transit and improves the bowel pattern in people with chronic constipation, so it's worth building in even though there's no GLP-1-specific percentage.

If the first three steps have not shifted things after a week, the next layer is an osmotic laxative, meaning something that holds water in the colon so the stool stays soft. Two things here are worth knowing, because a lot of what circulates about magnesium is the wrong way round.

Macrogol, sold as Movicol or Laxido in the UK and as polyethylene glycol or Miralax in the US, has the strongest evidence of anything in this section. The joint American Gastroenterological Association and American College of Gastroenterology guideline gives it a strong recommendation for chronic constipation. It is not a last resort, and there is no reason to work through gentler-sounding options first.

Magnesium is also reasonable, and the form to use is magnesium oxide. The same guideline recommends it, conditionally, meaning the evidence is decent but thinner. This is the opposite of the advice you will usually see online, which steers people away from the oxide towards citrate or glycinate. The trials were done on the oxide.

On how much: supplemental magnesium has an upper limit, and diarrhoea is the effect that sets it. UK guidance puts 400 mg a day as the level not expected to cause problems, and the US limit for supplements is 350 mg. Going above that is using magnesium deliberately as a laxative, which is a decision to make with a pharmacist or your prescriber rather than off a website, and particularly so if your kidneys are anything other than normal, because magnesium is cleared by the kidneys and builds up when they are not.

1. Still constipated after a couple of weeks: lactulose is another osmotic option, available from a pharmacy. Ask the pharmacist how much to take and for how long.
2. If that is not enough, this is the point to speak to your prescriber rather than keep layering things on. There are prescription options beyond the osmotics, and which one suits depends on what you have already tried.
3. Ask about pausing the next dose increase, or stepping back to the previous dose, if constipation is what is making the drug hard to live with. Some people need an extra month at a lower dose before going up.
4. Do not stop the GLP-1 abruptly on your own. Most constipation is dose-related, and stepping back rather than stopping usually keeps the appetite benefit.
5. Stimulant laxatives such as senna or bisacodyl are useful for rescue, but tolerance builds with daily use, so they are not the thing to settle into.

• Wheat bran or All-Bran cereal. Insoluble fibre on a slow-transit gut often worsens bloating without improving stool form.
• Aloe vera juice or 'detox teas' with senna. Stimulant action causes cramping and tolerance.
• Probiotics for constipation. Modest effect at best in this specific context.
• Prune juice every day. Works once or twice but loses effect quickly.
• Coffee enemas. No evidence, real risks.

This article is about comfort and everyday patterns, not medical guidance. Your doctor and pharmacist know you and your medicine; if anything about how you feel worries you, speak to them promptly, and if you ever feel seriously unwell, treat it as an emergency. The medicine's own patient information leaflet carries the important safety information and is worth reading in full.

The whole point of these drugs is that your stomach empties more slowly. That has two consequences well outside the subject of constipation, and both come from prescribing information and current guidance rather than anything speculative.

If you take the contraceptive pill and you are on Mounjaro: slowed emptying means an oral pill may not be absorbed properly. The tirzepatide prescribing information says to switch to a non-oral method, or add a barrier method, for four weeks after starting and for four weeks after every dose increase. The effect is largest after the first dose and lessens after that. This warning is specific to tirzepatide and is not on the semaglutide labelling, but if you rely on an oral pill and you are on any of these drugs, it is worth a conversation.

If you are booked for surgery, an endoscopy, or anything involving sedation: food can still be in the stomach after the usual fasting period, which matters to whoever is putting you to sleep. Tell them you are on a GLP-1, which one, and when your last dose was. Current multi-society guidance from 2024 is that most people should carry on taking it, reversing earlier advice to stop, but the team will want to adjust how they do things. That might mean liquids only for 24 hours beforehand, or scanning your stomach before they start. The risk is manageable when they know about it.