Constipation on a weight loss jab: fibre, fluid and what works

SASian Anderson, Writer and editor, MyGLP. Sources checked 21 October 2026.

Why it happens

Constipation is listed as a common side effect for all the weight loss medicines. The NHS tirzepatide page has it alongside nausea and tiredness. The medicine slows the whole gut, not only the stomach. At the same time you eat less, so there is less bulk to move, and most people drink less because they feel full. Three causes, one result.

The fibre target

The UK target is 30g of fibre a day for adults, set by the Scientific Advisory Committee on Nutrition. The average UK woman eats about 17g. On a weight loss medicine, with a smaller plate, it is easy to fall below that. The NHS fibre page sets out the food groups.

Two kinds of fibre do two jobs. Soluble fibre, in oats, beans, lentils, chia and psyllium, soaks up water and softens the stool. Insoluble fibre, in wholegrain bread, bran and vegetable skins, adds bulk. You want both, with soluble doing more of the work when your gut is slow.

Fibre on a small plate

Add fibre slowly, a few grams a week. A jump from 15g to 30g in a day gives you wind and bloating on top of a stomach that already feels full. Chia and psyllium need water or they make things worse.

Fluid

Fibre without water sets like cement. The NHS suggests 6 to 8 glasses a day, which is about 1.5 to 2 litres, and more in hot weather or if you exercise. Our hydration guide has ways to get there when a full glass feels like too much.

Movement

A ten-minute walk after a meal moves the gut along. So does squatting, which is why some people find a small step under the feet on the toilet helps. The walking guide is the simplest place to start.

When food is not enough

If you have gone three days with no bowel movement, or passing a stool hurts, use a laxative from the pharmacy rather than waiting it out. The NHS constipation page lists the types. Macrogol sachets, which draw water into the stool, are the usual first choice and are safe to take for a few weeks. Senna works by stimulating the bowel and is better kept for short runs. Ask the pharmacist which suits you. Prescribers at the weight loss pharmacies deal with this question every day and will not be surprised by it.

A routine that works

1. Chia or psyllium stirred into yoghurt or water at breakfast.
2. Beans or lentils at one meal a day.
3. Fruit with skin, two portions, as snacks.
4. A glass of water before each meal and one between.
5. A walk after the largest meal.
6. Macrogol at the first sign of trouble, not day four.

Sources

1. How to get more fibre into your diet (NHS)
2. Carbohydrates and health report (30g fibre recommendation) (Scientific Advisory Committee on Nutrition)
3. Constipation (NHS)
4. Tirzepatide: side effects (NHS)
5. Composition of foods integrated dataset (McCance and Widdowson) (Public Health England)