Feeling less hungry on Mounjaro or Wegovy is the whole point. But eating less often means drinking less too. Add side effects like vomiting or diarrhoea, and dehydration on Mounjaro or Wegovy can creep up faster than you'd expect.
That matters even more once you start exercising. This part of our series on exercising on GLP-1 medication covers why your dehydration risk goes up, how much to drink, whether you need electrolytes and which warning signs you should act on.
Staying on top of fluids is one of the easiest, and most important, things to prioritise, especially on days you train or feel unwell.
Does Mounjaro cause dehydration?
Not directly. Neither Mounjaro nor Wegovy make you lose more fluids from your body on its own. The risk comes from the knock-on effects of how they work.
Both medicines slow down how quickly your stomach empties, so you feel full for longer.1,2 That's great for appetite. But it can also make a big glass of water feel like too much, so it's easy to drink less without noticing.
Eating less plays a part too. Around a fifth of the water most adults take in each day comes from food rather than drinks.3 Smaller meals mean less of that.
Then there are the side effects. Nausea, vomiting and diarrhoea are all very common on both medicines, particularly while your dose is being increased.1,2 The official prescribing information for each one warns that losing fluid this way can lead to dehydration.1,2
The medicine isn't the problem on its own. It's the mix of drinking less, eating less and losing fluid through side effects that adds up.
Some people need to be a little more careful. Older adults, people with diabetes and anyone taking water tablets (diuretics) dehydrate more easily.4 Mounjaro's prescribing information also singles out older people as more vulnerable to the complications of fluid loss.1
How much water should you drink on Mounjaro or Wegovy?
There's no special amount. Neither medicine's prescribing information sets a daily fluid target, so general UK guidance is the sensible place to start.
The Eatwell Guide recommends 6 to 8 cups or glasses of fluid a day. Water, lower-fat milk, sugar-free drinks, tea and coffee all count.5
European guidance puts total water needs at about 2 litres a day for women and 2.5 litres for men, from food and drink combined.3 With food usually covering around 20% of that, it works out at roughly 1.6 litres from drinks for women and 2 litres for men.
On a GLP-1, you're eating less. So your drinks may need to cover a bit more than usual. Treat those figures as a floor rather than a ceiling, and drink more when you're:
- exercising or sweating
- out in hot weather
- being sick or have diarrhoea
Not sure if you're getting enough? Check the colour of your pee. Aim for pale yellow through the day.4
Pee colour is a better day-to-day guide than any fixed number. If it's darker than pale straw, top up.
How to stay hydrated when you don't feel like drinking
If big drinks leave you feeling uncomfortably full, little and often tends to work better. A few things that help:
- Sip, don't gulp - keep a bottle with you and take small mouthfuls through the day rather than downing a pint in one go.
- Drink between meals - spacing drinks away from mealtimes can stop your stomach feeling overloaded.
- Eat your fluids - soup, yoghurt, melon and other watery foods all add to your total.4
- Make water easier to like - sparkling water, sugar-free squash or a slice of lemon are all fine swaps.5
- Go easy on alcohol - it dehydrates you, and is best avoided altogether when you're feeling sick.4
Constipation is also very common on both medicines.1,2 Drinking plenty of water and other fluids is one of the first things the NHS suggests for it, so these habits pull double duty.6
Why hydration matters more when you exercise
Sweating is one of the main ways you lose fluid. The NHS lists sweating a lot after exercise as a common cause of dehydration.4 On a GLP-1, you might be starting that session a little low already.
Even mild dehydration can make a workout feel harder. Tiredness, headaches and dizziness are classic signs.4 Tiredness and dizziness are also listed side effects of Mounjaro, which can make it tricky to tell which is which.1
A simple routine helps:
- Before - start topped up, especially if you've eaten very little that day.
- During - sip water through longer or sweatier sessions instead of waiting until the end.
- After - water is the best way to replace what you've lost through sweat, so keep drinking until your pee is pale again.5
If you take insulin or a sulfonylurea for diabetes, feeling shaky or dizzy mid-workout could also be low blood sugar, so check your levels.1
Do not train through dizziness or light-headedness. Sit down, sip some water and only carry on once it has fully passed.
Just getting going with walking on a GLP-1 or our 4-week beginner workout plan? Building fluids into your routine from day one is far easier than playing catch-up. For more on planning sessions around nausea, tiredness and injection day, read our guide to managing GLP-1 side effects while exercising.
Do you need electrolytes on Mounjaro?
Electrolytes are minerals such as sodium and potassium that help your body keep its fluid balance steady. Most people get plenty from everyday food.
Mounjaro's prescribing information does mention taking care to avoid electrolyte disturbances alongside fluid loss.1 For most people, though, that doesn't mean buying supplements. It means eating regular meals, even small ones, and replacing fluids properly.
Gentle, food-first sources include:
- soups and broths, which provide sodium
- milk and yoghurt
- bananas and potatoes, which provide potassium
Where electrolytes really matter is after sickness or diarrhoea. You lose salts and minerals as well as water, and a pharmacist can recommend an oral rehydration solution to replace them.4
For an everyday workout, water is usually enough.5 Many sports drinks are high in sugar, so check the label if you use them.
Have a kidney or heart condition, or take medicine for blood pressure? Speak to a pharmacist or GP before using electrolyte products regularly.
Ordinary meals cover most people's electrolyte needs. Rehydration sachets come into their own after a bout of sickness or diarrhoea.
Can dehydration on Mounjaro affect your kidneys?
It's a common worry, and dehydration is the main link. Both medicines' prescribing information warns that fluid lost through vomiting and diarrhoea can lead to worsening kidney function.1,2
Wegovy's prescribing information describes this as rare.2 Mounjaro's includes acute kidney failure in the same warning.1 It's the reason the official advice puts so much weight on avoiding fluid loss in the first place.
Kidney stones aren't listed as a side effect in Mounjaro's prescribing information.1 But not drinking enough fluid is a well-known cause of kidney stones in anyone.7 That's one more reason to keep your pee pale.
Gallstones are a separate issue, and they are listed as a common side effect of Mounjaro.1 So any new pain in your tummy, side or back that does not go away is worth getting checked rather than guessed at.
Already have a kidney condition? Make sure your prescriber knows, as they'll want to keep a closer eye on you.2
Keeping fluids up when you're unwell is the single most useful thing you can do to protect your kidneys on either medicine.
When should you seek medical advice?
Mild dehydration usually settles once you drink more. Speak to your prescriber, a pharmacist or NHS 111 if:
- you're being sick or have diarrhoea and cannot keep fluids down
- you feel dizzy when you stand up and it does not go away
- your pee stays dark yellow, or you're peeing much less than normal
- you feel unusually tired, or you're breathing quickly or have a fast heartbeat
- you have pain in your side or back that does not go away
Call 999 or go to A&E if someone with signs of dehydration becomes confused, very sleepy or hard to wake, has cold, pale or blotchy skin, or is struggling to breathe. These can be signs of shock.4
Severe stomach pain that will not go away, with or without vomiting, also needs urgent medical attention.1
Being unable to keep fluids down is the point to contact your prescriber. Do not wait for it to pass on its own.
Frequently asked questions
Does Mounjaro make you thirsty?
Thirst isn’t listed as a side effect in Mounjaro’s prescribing information. Feeling thirsty is usually a sign you need more fluids, especially after being sick, having diarrhoea or exercising. Constant thirst alongside peeing much more than usual can be a sign of high blood sugar, so speak to your GP or prescriber if that sounds familiar.
Can dehydration on Mounjaro cause headaches?
Yes. Headaches are one of the common signs of dehydration, and they’re more likely if you’ve been drinking less or losing fluid through side effects. Try sipping water steadily over the next few hours and see if it eases. If a headache keeps coming back or does not improve once you’re drinking normally, speak to a pharmacist or your prescriber.
What are the best electrolytes to take on Mounjaro?
There isn’t a single best product. For everyday life and normal exercise, regular meals and water usually cover what you need. If you’ve been sick or had diarrhoea, an oral rehydration solution from a pharmacy is designed to replace the salts and minerals you’ve lost. Ask a pharmacist first if you have a kidney or heart condition.
Do tea and coffee count towards your water intake?
Yes. UK guidance counts tea and coffee towards the 6 to 8 cups or glasses of fluid recommended each day, along with water, lower-fat milk and sugar-free drinks. Water is still the best way to replace fluid lost through sweat. If you’re feeling sick or already dehydrated, stick to water and avoid caffeinated drinks until you feel better.
When is the risk of dehydration highest on Mounjaro?
The risk tends to be highest in the first weeks of treatment and after each dose increase. That’s when nausea, vomiting and diarrhoea are most likely, before they usually ease over time. Hot weather, hard exercise or a stomach bug on top of your usual side effects can also tip the balance, so drink a little extra at these times.
Can you take Mounjaro if you have kidney disease?
Possibly, but it’s a decision for your prescriber. Mounjaro doesn’t need a dose change for kidney problems, but experience in people with severe kidney disease is limited, so extra caution is advised. Always tell your prescriber about any kidney condition. Staying well hydrated matters even more, as fluid lost through side effects can affect kidney function.
References
- Eli Lilly and Company Limited. Mounjaro KwikPen 2.5mg solution for injection in pre-filled pen: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- Novo Nordisk Limited. Wegovy 0.25 mg, FlexTouch solution for injection in pre-filled pen: Summary of Product Characteristics. Electronic Medicines Compendium; 2026. medicines.org.uk/emc
- EFSA Panel on Dietetic Products, Nutrition, and Allergies. Scientific Opinion on Dietary reference values for water. EFSA Journal. 2010;8(3):1459. doi:10.2903/j.efsa.2010.1459
- NHS. Dehydration. NHS website; 2026. nhs.uk
- NHS. Water, drinks and hydration. NHS website; 2026. nhs.uk
- NHS. Constipation. NHS website; 2026. nhs.uk
- NHS. Kidney stones. NHS website; 2026. nhs.uk
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.