Hydration on GLP-1 Medication

GLP-1 medicines such as Mounjaro (tirzepatide) and Wegovy (semaglutide) do not drain fluid from the body in the way a diuretic does, but dehydration is a genuine risk on treatment. Nausea, vomiting and diarrhoea are among the most common side effects, and feeling full can mean you drink far less than usual. The product information for both medicines warns that fluid loss from these side effects can cause dehydration, and for Mounjaro, that this could affect kidney function. Most adults should aim for six to eight drinks a day, more when unwell, hot or active.

Why dehydration can happen on GLP-1 medicines

Gastrointestinal side effects

Digestive side effects are the most common problems reported with GLP-1 medicines. The Wegovy product information, based on pooled trial data for semaglutide 2.4 mg, lists nausea in about 44% of people and diarrhoea in about 30%. In the SURMOUNT-1 trial of tirzepatide for weight management, diarrhoea was reported by roughly a fifth of participants on active treatment. Vomiting is also listed as very common in the weight-management setting for Mounjaro.

Vomiting and diarrhoea both lose water and salts (electrolytes) from the body. Even a day or two of loose stools or being sick can leave you depleted, especially if you are not eating and drinking much at the same time. For practical advice on these symptoms, see our guides to diarrhoea on GLP-1 medication and managing nausea.

Drinking less because you feel full

GLP-1 medicines slow stomach emptying and reduce appetite. Many people find that drinks, like food, make them feel uncomfortably full, so they drink less without realising. Some people also report feeling less thirsty than usual on treatment. And because we get a meaningful share of our water from food (soups, fruit, vegetables, yoghurt), eating much less can reduce total water intake further.

Less food, less water from food

The European Food Safety Authority estimates that around a fifth of total water intake typically comes from food rather than drinks. When you eat significantly less on GLP-1 treatment, that source shrinks too, which is one reason to pay a bit more attention to drinks.

Other contributing factors

Dehydration risk is higher if you:

• are older, as the sense of thirst tends to diminish with age
• take diuretics ("water tablets") or certain blood pressure medicines
• have diabetes, particularly if blood sugar is running high
• drink alcohol, which can increase urine output
• exercise hard, or are in hot weather or a hot workplace
• have an illness causing fever

Why it matters: kidneys and more

The most serious concern with dehydration on GLP-1 treatment is the effect on the kidneys. The Mounjaro product information states that gastrointestinal side effects, including nausea, vomiting and diarrhoea, may lead to dehydration, which could lead to a deterioration in renal function, including acute renal failure. The Wegovy product information similarly advises that nausea, vomiting and diarrhoea may cause dehydration and that this should be considered in people with impaired kidney function.

Acute kidney injury is uncommon, but it is more likely in people who already have reduced kidney function, are older, or are taking medicines that affect the kidneys, such as some blood pressure tablets, diuretics or anti-inflammatory painkillers.

Dehydration can also contribute to:

• headaches and tiredness, which are sometimes blamed on the medicine itself
• dizziness, particularly when standing up quickly
• constipation, which is already common on GLP-1 treatment (see our guide to constipation on GLP-1 medication)
• worsening nausea, creating a cycle where feeling sick makes it harder to drink
• in some people, an increased risk of kidney stones over time

How much should you drink?

General UK guidance

The NHS Eatwell Guide recommends six to eight cups or glasses of fluid a day for most adults. The EFSA reference values for total water intake (from both food and drink) are around 2.0 litres a day for women and 2.5 litres for men, with drinks typically supplying most of this.

No specific fluid target has been set for people on GLP-1 medicines. The 2025 joint advisory on nutritional priorities for GLP-1 therapy emphasised drinking adequate fluids and making efforts to prevent dehydration, particularly when gastrointestinal side effects occur.

When you need more

You are likely to need more than usual if:

• you are vomiting or have diarrhoea
• the weather is hot, or you are in a hot environment
• you are exercising, especially for longer sessions
• you have a fever
• you are breastfeeding (though GLP-1 medicines are not generally used during breastfeeding; discuss with your prescriber)

When you may need less

Some people need to restrict fluids for medical reasons, for example with heart failure or advanced kidney disease. If your doctor has given you a fluid limit, keep to it and ask them how to manage GLP-1 side effects safely within it.

Using urine colour as a guide

A simple practical check is the colour of your urine. Pale straw or light yellow usually suggests you are drinking enough. Dark yellow, amber or strong-smelling urine suggests you need more fluid. Some vitamins (particularly B vitamins) and foods such as beetroot can change urine colour, so take this into account.

Practical ways to drink enough when you feel full

Sip steadily through the day

Large drinks at once can make you feel full or nauseous. Small, frequent sips are usually easier. Keeping a bottle or glass in sight, at your desk, in the car or by your chair, acts as a reminder. Some people use phone alarms or a marked water bottle.

Drink between meals

Drinking a lot with meals can add to fullness and make it harder to eat enough protein. Try to take most of your fluid between meals, with small sips during meals if needed.

Vary what you drink

Water is ideal, but it is not the only option. The NHS says that lower-fat milk, sugar-free drinks, tea and coffee all count. Options to try include:

Eat water-rich foods

Foods with a high water content can contribute meaningfully, especially when you are eating less. Examples include cucumber, melon, watermelon, oranges, strawberries, tomatoes, courgettes, lettuce, soups, stews, yoghurt and jelly.

Try temperature and texture

Some people find cold drinks, ice lollies or ice chips easier when nauseous. Others prefer warm drinks. Sucking ice or a sugar-free ice lolly can be a gentle way to take in fluid when larger sips are difficult.

Link drinking to routines

Having a glass of water when you wake up, with each medication dose, before and after a walk, and mid-afternoon builds a habit without needing to think about it.

A sample day of drinking on GLP-1 treatment

This example shows one way of spreading fluids across a day for someone who feels full easily. It adds up to around eight drinks, plus water from food. Adjust it to your own routine and needs.

Common hydration myths

"You need to drink eight large glasses of water a day on top of everything else"

The NHS recommendation of six to eight drinks a day includes all non-alcoholic drinks, including tea, coffee and milk. Food also contributes. You do not need to force large volumes of water.

"Tea and coffee dehydrate you"

At normal intakes, tea and coffee contribute to fluid intake. The caffeine effect on urine output does not outweigh the fluid in the drink. However, they may worsen reflux or loose stools in some people.

"If you're not thirsty, you're fine"

Thirst is a useful signal, but it can be less reliable in older adults and may be dulled in some people on GLP-1 treatment. Urine colour and frequency are more reliable checks.

"More water is always better"

Drinking extremely large volumes of water in a short time can be harmful, by diluting the salt in your blood. This is rare, but it is another reason to sip steadily rather than force large amounts. People with heart failure or kidney disease may need to limit fluids on medical advice.

Electrolytes: do you need them?

Electrolytes are salts and minerals, such as sodium, potassium and chloride, that help regulate fluid balance and muscle and nerve function. Most people get enough from a normal diet. But when you lose fluid through vomiting or diarrhoea, you lose electrolytes too.

Oral rehydration solution (ORS) sachets, available from pharmacies, are designed to replace both water and salts in the right balance. They are a sensible option during episodes of vomiting or diarrhoea. A pharmacist can advise on suitable products.

Sports drinks contain some electrolytes but often a lot of sugar, and they are not designed for rehydration during illness. Many "electrolyte" tablets and powders sold for general use vary widely in content.

If you have high blood pressure, heart failure or kidney disease, ask a pharmacist or your GP before using electrolyte products, as the salt or potassium content may not be suitable for you.

Hydration, exercise and hot weather

Physical activity is encouraged on GLP-1 treatment, and is part of the licensed use of these medicines alongside diet. But exercise increases fluid loss through sweat. To stay safe:

• drink before, during and after exercise, especially in warm weather
• start gently if you have had recent vomiting or diarrhoea, or feel light-headed
• stop and rest if you feel dizzy, faint or unusually breathless

In hot weather, carry water with you, avoid the hottest part of the day for exercise, and be alert to early signs of heat exhaustion. Our guide to exercise on GLP-1 medication has more on staying safe while active.

Special considerations

Older adults

Thirst tends to diminish with age, and older people are more likely to take medicines that affect fluid balance or kidney function. Our guide to GLP-1 medicines over 65 covers this in more detail.

People with diabetes

High blood sugar increases urine output and can worsen dehydration. If you have diabetes and become unwell with vomiting or diarrhoea, follow your diabetes team's "sick day rules", which may include temporarily adjusting certain medicines. Do not do this without guidance. See our guide to GLP-1 medicines and type 2 diabetes.

People taking oral contraceptives

Vomiting and severe diarrhoea can reduce the absorption of the oral contraceptive pill. In addition, the Mounjaro product information advises switching to a non-oral method, or adding a barrier method, for four weeks after starting and after each dose increase. Our guide to GLP-1 medicines and contraception explains this.

Tablet forms of GLP-1 medicines

Oral semaglutide has specific instructions about taking the tablet on an empty stomach with only a small amount of water, then waiting before eating or drinking anything else. Follow the patient information leaflet for your product. Outside that window, normal fluid advice applies. Our oral GLP-1 hub covers tablet treatments in more detail.

Hydration and other side effects

Getting fluids right can help with several other common problems:

• Constipation: adequate fluid helps fibre work properly. Increasing fibre without increasing fluid can make constipation worse.
• Headaches and fatigue: sometimes linked to dehydration and low food intake rather than the medicine directly.
• Nausea: a cycle can develop where feeling sick stops you drinking, and dehydration worsens nausea. Small sips, little and often, can help break it.
• Dizziness: low blood pressure-related events are listed as common in the Mounjaro product information for weight management, and dehydration can contribute.

For a full overview of side effects, see our Mounjaro side effects guide and the Mounjaro side effects page. For background on the medicines, start at our weight loss injections hub.

How providers can help

Good prescribers will explain how to manage side effects and when to get in touch, and may adjust the pace of dose increases if side effects are troublesome. If you are comparing services, it is worth asking how they support people with side effects, what their response times are, and whether you can speak to a clinician directly. Our guide on how to choose an online pharmacy for weight loss and our provider comparison can help.

When to speak to a professional

Speak to your GP, prescriber or pharmacist if:

• you have had vomiting or diarrhoea for more than a day or two, even if you are keeping some fluids down
• you are regularly struggling to drink more than a few glasses of fluid a day
• you have persistent headaches, dizziness or tiredness
• you have kidney disease, heart failure or diabetes and develop vomiting or diarrhoea
• you take diuretics, blood pressure medicines or metformin and become unwell

Seek urgent medical help (call NHS 111, or 999 in an emergency) if you:

• cannot keep any fluids down
• have not passed urine all day, or are passing only very small amounts of dark urine
• feel confused, very drowsy or disorientated
• have a fast heartbeat, or feel faint when standing
• have severe abdominal pain, particularly if it spreads to your back, with or without vomiting (a possible sign of pancreatitis)
• notice blood in your vomit or stools

Decisions about your GLP-1 treatment, including whether to pause or adjust the dose during illness, belong to your prescriber.