Drinking enough on a weight loss jab, when you never feel thirsty

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

Why fluid drops on the medicine

The medicines make you feel full, and that feeling does not tell the difference between food and drink. People drink less without noticing. Add a week of nausea, a bout of vomiting or diarrhoea, and the fact that a lot of the water you used to get came from food you no longer eat, and fluid intake can halve.

The result shows up as headaches, tiredness, dizziness when you stand, dark urine, constipation and a dry mouth. Most of those also appear on the side effect list for the medicine, so they get blamed on the drug when a glass of water would have fixed them.

How much

The NHS Eatwell guidance suggests 6 to 8 glasses a day, which is 1.5 to 2 litres. Water, lower-fat milk, sugar-free drinks, tea and coffee all count. In hot weather, when you exercise, or on a day you are being sick, you need more.

The simplest check is urine colour. Pale straw means you are fine. Dark yellow means drink.

Fitting it in when you feel full

• Small and often. A quarter of a glass every half hour gets to 2 litres by evening without one uncomfortable moment.
• Drink between meals, not with them. A full glass with food fills the stomach that the medicine has already slowed, and makes nausea worse.
• A bottle you can see. Fill a 1 litre bottle in the morning, finish it by lunch, fill it again.
• Warm drinks. Herbal tea, warm water with lemon, and clear broth go down more easily than cold water on a queasy stomach.
• Foods with water in them. Cucumber, melon, soup, yoghurt and oranges all count towards the total.
• Set a reminder. Thirst is not a reliable prompt on these medicines, so the phone does the job instead.

Electrolytes

If you have vomited or had diarrhoea, water alone can leave you short of salts. The NHS dehydration page recommends oral rehydration sachets from the pharmacy in that case. They cost a pound or two and taste better than they used to. Sports drinks work in a pinch but carry sugar. On a normal day, with normal meals, you do not need electrolyte powders.

Caffeine and fizzy drinks

Tea and coffee hydrate. The caffeine effect on urine is small at normal intake. Fizzy drinks hydrate too, but the gas sits in a slow stomach and causes bloating and burping, which is why many people on the medicines go off them within a week. Still water or squash is easier.

Alcohol

Alcohol pulls water out of the body. On a smaller appetite it also hits harder, and nausea the morning after is worse. Our alcohol guide covers the detail. A glass of water for every drink is the oldest advice and still the best.

Why it links to kidneys

The medicine leaflets warn about kidney problems, and the warning is about dehydration. Severe vomiting or diarrhoea that goes on for days, with no fluid going in, can strain the kidneys. Keep fluids up through a bad patch and that risk falls away. If you cannot keep fluids down, that is the point to call the prescriber or 111.

Sources

1. Water, drinks and hydration (NHS)
2. Dehydration (NHS)
3. Tirzepatide: side effects (NHS)
4. Wegovy patient information leaflet (Novo Nordisk, via the electronic medicines compendium)