Independent UK medicine guidance
Mounjaro electrolytes and sick-day fluids: what to drink when you are vomiting or have diarrhoea
The Mounjaro (tirzepatide) leaflet asks you to drink plenty of fluids if you have nausea, vomiting or diarrhoea, because dehydration can affect your kidneys. Water alone is not always enough: vomiting and diarrhoea also lose salts, and drinking a lot of plain water while losing salts can leave you feeling worse. This guide explains what electrolytes are, when a rehydration solution is worth using, and what the warning signs of dehydration are.
Key facts
- The leaflet warns that vomiting and diarrhoea can cause dehydration, which may lead to kidney problems, and tells you to drink plenty of fluids.[1]
- Electrolytes are the salts, mainly sodium and potassium, lost in vomit, diarrhoea and sweat. They keep fluid where it should be and muscles and nerves working.
- The NHS advises oral rehydration solutions from pharmacies for people at risk of dehydration from vomiting or diarrhoea.[2] They contain the right balance of salts and glucose; sports drinks and squash do not.
- Signs of dehydration include thirst, dark strong-smelling urine, passing urine less often, dizziness, tiredness and a dry mouth.[2]
- Red flag: unable to keep fluids down, very little or no urine, confusion, or feeling faint means NHS 111 or 999. Our sick-day rules cover the full picture.
Why plain water is not the whole answer on a bad day
Fluid moves in and out of cells according to salt balance. Vomiting and diarrhoea lose both water and salts. Replace only the water and the body cannot hold on to it well; some people feel washed out, get cramps or headaches, or simply keep feeling thirsty. An oral rehydration solution replaces water, sodium, potassium and a little glucose in the proportions that the gut absorbs best, which is why the NHS recommends it over water for people losing fluid.[2]
On ordinary days on Mounjaro, water, tea, milk and squash are fine. This page is about the days when you are being sick or have diarrhoea, or when you are sweating a lot in hot weather while eating very little.
What to drink
| Situation | Drink | Notes |
|---|---|---|
| Normal day, mild nausea | Water, diluted squash, tea, milk, clear soup | Sip through the day; more on hot days |
| Vomiting or diarrhoea, able to keep fluids down | Oral rehydration solution from a pharmacy, made up exactly as the pack says, in small frequent sips | Ask the pharmacist which product suits you if you have diabetes, kidney disease or take diuretics |
| Nothing else stays down | Ice chips or tiny sips of rehydration solution every few minutes | If this fails for several hours, contact NHS 111 |
| Hot weather, sweating, eating very little | Water plus salty food such as soup or crackers, or a rehydration sachet | Appetite for salt drops with appetite for food |
Fizzy sugary drinks and sports drinks are often suggested but have the wrong balance for rehydration; the NHS does not recommend them for this purpose.[2] Alcohol makes dehydration worse. Coffee is not the problem people think, but it is unlikely to appeal on a sick day.
Rehydration solutions: what to know
- They are sold as sachets to dissolve in a set volume of water. The volume matters; too little water makes the solution too concentrated and can worsen things.
- Follow the pack for how much to drink and when. This page does not give dosing.
- People with type 2 diabetes, kidney disease or heart failure, and anyone on diuretics or blood pressure medicines, should ask a pharmacist first; salt and potassium content matters for them.
- They are a short-term measure for illness, not a daily supplement.
- Electrolyte powders and tablets marketed for fitness are not the same product; some are mostly flavouring, others are very high in sodium. Check the label, and prefer a pharmacy rehydration product when unwell.
Recognising dehydration
The NHS lists thirst, dark yellow strong-smelling urine, passing urine fewer than four times a day, dizziness or light-headedness, tiredness, dry mouth, lips and eyes, and feeling confused or unusually drowsy.[2] On Mounjaro, dizziness is also a listed side effect in its own right, so the two can overlap; if in doubt, drink and see whether it improves. See dizziness and low blood pressure and Mounjaro and the kidneys.
When to get help
Call 999 for confusion, fainting, a very fast or weak pulse, severe persistent stomach pain, or vomiting blood.
Contact NHS 111 or your prescriber the same day if you cannot keep fluids down for several hours, have passed very little urine, have diarrhoea or vomiting lasting more than two days, or have signs of dehydration that are not improving with fluids. Tell them you are on Mounjaro. Your prescriber may advise holding the next dose until you are well; that is their call.
Common questions
Should I take electrolytes on Mounjaro?
On ordinary days, no; water and normal food cover it. When you are vomiting, have diarrhoea or are sweating heavily while eating little, an oral rehydration solution from a pharmacy replaces lost salts better than water alone.
Can I use a sports drink instead of a rehydration sachet?
Sports drinks have the wrong balance of sugar and salt for rehydration during illness and the NHS does not recommend them for it. A pharmacy rehydration product is the right tool.
How much should I drink on Mounjaro?
There is no single figure. The practical test is pale urine several times a day. Drink more on hot days and on days with vomiting or diarrhoea. Our hydration guide has a routine that works around a reduced appetite.
Should I skip my Mounjaro dose if I am dehydrated?
Do not decide that alone. Tell your prescriber; they may advise holding the dose until you are keeping fluids down. The leaflet’s missed-dose rule then tells you when to take the next one.
Related guides
About this page. Monj is an independent information and price comparison service run by Medstack Ltd. It is not a pharmacy and does not prescribe, dispense or sell medicines, and it does not recommend or dose any product. This page provides general information, not diagnosis or individual medical advice. It has not been clinically reviewed; a named clinical reviewer will be added when one has approved it. Read our editorial and review policy and independence statement.
References
- Eli Lilly. Mounjaro KwikPen: Package leaflet, information for the patient (UK), revised August 2026, section 2. electronic medicines compendium. medicines.org.uk/emc/product/15481/pil. Accessed 6 September 2026.
- NHS. Dehydration. nhs.uk/conditions/dehydration. Accessed 6 September 2026.
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