Three days without going to the toilet, then four. Your belly feels hard and heavy, and you end up eating even less in case it makes things worse. On a GLP-1 treatment, everyone talks about nausea. Constipation arrives more quietly, and it often lasts longer.
The treatment, how it is adjusted and how long you stay on it are for your doctor to decide. Here I explain what is happening in your gut, and the basics I give everyone when digestion slows down.
A hormone that puts the brakes on the stomach
GLP-1 is a hormone your own gut makes. The end of the small intestine and the colon release it when food reaches them. It is part of what researchers call the ileal brake, the signal that tells the stomach to empty more slowly while the intestine still has work to do. It relaxes the upper stomach, calms its contractions and tightens its outlet (Marathe et al., 2011).
The treatments mimic this hormone, at much higher levels and for much longer. At the Mayo Clinic in the United States, 40 adults with obesity took a treatment from this family or a placebo for sixteen weeks. After five weeks, it took a median of 70 minutes longer for half a solid meal to leave the stomach. By sixteen weeks, that delay had dropped to about half an hour. And the people whose stomach had slowed the most at the start were the ones who had lost the most weight (Halawi et al., 2017).
The slowdown carries on into the gut
In Japan, fourteen people with diabetes swallowed a capsule fitted with a camera, before and after starting a GLP-1 treatment. In those whose nerves had not been damaged by diabetes, the capsule stayed about an hour in the stomach before treatment and two and a half hours after. The trip through the small intestine went from under four hours to nearly seven (Nakatani et al., 2017).
For the colon, studies in humans are still missing. What we do know is that the colon forms stools by taking water out of what it receives, and that the speed of transit changes how they turn out. In Bristol, researchers slowed down volunteers' transit with an anti-diarrhoea medicine. Their stools became harder, smaller and less frequent (Lewis and Heaton, 1997).
Less water, less movement
The medicine is not the whole story. When your stomach stays full, you feel less thirsty and drink less. A panel of Spanish experts, including endocrinologists, GPs and diabetes nurses, lists this among the causes of constipation on treatment. Their first advice is to move more, drink more and increase fibre (Gorgojo-Martínez et al., 2022). Most of these experts declare links with the companies that make these treatments.
What the big trials show
One analysis pooled three large 68-week trials, with 2,117 adults with overweight or obesity on treatment and 1,262 on placebo. Constipation affected 24% of the people treated, against 11% on placebo. It lasted longer than the other digestive problems, a median of 47 days against 35 on placebo. Almost all of these problems were mild or moderate, and they became less common over time in those who stayed on treatment (Wharton et al., 2022).
The placebo figure matters too. One person in ten became constipated without the medicine, with the same diet and activity advice as everyone else. In my guides, I warn that digestion can slow down in the first few days of a change in diet, while the gut gets used to different kinds of fibre.
The basics for your digestion
When digestion slows down, on treatment or not, I always go back to the same basics. I check them before anything else, and they do not replace your doctor's advice.
Water first. My marker is about 500 ml for every 15 kg of body weight, so 2.5 litres for someone who weighs 75 kg, spread over the day, and more with exercise or heat. When your stomach fills up fast, the Spanish experts suggest drinking in small sips throughout the day, without getting to the point of feeling too full.
Then cooked vegetables, and unsweetened apple compote. Compote and very ripe bananas bring soluble fibre, which soaks up water and forms a kind of gel. The fibre in broccoli does not soak up water. You need both, and too much of either can block you up as easily as loosen things. I have gone through the amounts and the sources without grains in the article on fibre.
Magnesium in the evening. I recommend the bisglycinate form, with dinner or at bedtime, and I remind people not to skip it when digestion slows down. On treatment, ask your doctor before adding any supplement.
And walking, which the experts put at the top of their list. When your appetite drops to the point of skipping meals, your muscle is at stake too. I cover that in the article on what to eat on a GLP-1.
When to call your doctor
If the constipation settles in, makes you skip meals or stops you drinking, tell your doctor. It is up to them to decide whether you need something for the constipation or a change to your follow-up.
Severe belly pain, vomiting, or a belly that swells with no wind and no stools needs medical advice straight away. In a large American database of people treated for weight loss, bowel obstruction was rare, but it was about four times more common than in people taking another weight-loss medicine (Sodhi et al., 2023).
The Spanish experts also suggest keeping a diary. Write down every day what you drink, what you eat and when you go to the toilet. At your next appointment, those few lines will tell your doctor more than a vague "it's not great".
This article is for information only and does not replace medical advice.