Mounjaro sick day rules: what to do when you are ill

In short

Sick day rules apply after 2 or more episodes of vomiting or diarrhoea, or a fever with sweats or shaking
NHS guidance says tirzepatide should be stopped for a short time during an illness which can cause dehydration
A late dose can be taken within 4 days of your usual day. After 4 days, skip it and wait for the next one
Restart your paused medicines after 24 to 48 hours of eating and drinking normally, with no double doses
If you use insulin, do not stop it. Contact your diabetes team instead

If you are ill with vomiting, diarrhoea or a fever, hold your Mounjaro dose, drink small amounts often and restart once you have been eating and drinking normally for 24 to 48 hours. NHS guidance on tirzepatide says to stop it for a short time during any illness which can dehydrate you. The reason is your kidneys. The SmPC warns stomach side effects on tirzepatide can cause dehydration, and dehydration can make kidney function worse.

When sick day rules apply

NHS sick day guidance in the UK uses a simple trigger. The rules apply when you have:

Two or more episodes of vomiting or diarrhoea
A fever with sweats or shaking, unless it is mild

A cold, a sore throat or a headache with no fever and no stomach upset does not count. Keep to your normal injection day for those.

Why illness is riskier on Mounjaro

Mounjaro already reduces how much you eat and drink. Many people sip less through the day because they feel full. Add a stomach bug and you lose fluid faster while taking less in.

The patient leaflet tells you to avoid dehydration by drinking plenty of fluids, because dehydration can lower kidney function. The SmPC gives the same warning to prescribers: fluid loss from vomiting and diarrhoea can lead to a decline in kidney function. People with existing kidney disease, older adults and anyone also taking a diuretic, an ACE inhibitor or a "sartan" blood pressure tablet face the highest risk.

The medicine also slows the emptying of your stomach. A full, slow stomach makes vomiting more likely during an illness, which can turn a mild bug into a harder few days.

What to do with your dose

If your injection day falls while you are ill

Hold the injection. The missed dose rule in the leaflet gives you room:

Up to 4 days (96 hours) late: take the dose as soon as you feel well enough
More than 4 days late: skip it and take the next dose on your normal day
Never take two doses within 3 days of each other

If you injected a day or two before falling ill

The dose is already in your body and cannot be removed. Focus on fluids. Tirzepatide has a half-life of about 5 days, so the effect lasts all week.

Restarting

The UK sick day guidance says to restart paused medicines once you have recovered, usually after 24 to 48 hours of eating and drinking normally. Do not take extra doses to make up for missed ones. After a single missed dose, carry on at your usual dose. If you have missed several weeks, ask your prescriber first, as they may restart you on a lower dose. The restarting guide explains the options.

Fluids and salts

Water alone does not replace the salts you lose through vomiting and diarrhoea. The NHS recommends:

Small sips often, building up as the nausea settles
Oral rehydration solution if you are losing fluid, which a pharmacist can recommend
No caffeine or alcohol until you recover

Aim to pass pale yellow urine every few hours. Dark urine, or passing much less than usual, means you are behind. The electrolytes and sick day fluids page has practical amounts.

When food stays down, start with plain, low-fat foods such as toast, crackers, rice or bananas. Small portions sit better on a slow stomach.

Other medicines on a sick day

The NHS sick day card lists medicines to pause during a dehydrating illness, as agreed with your doctor or pharmacist:

Metformin
SGLT2 inhibitors (names ending in "flozin")
ACE inhibitors (names ending in "pril")
Angiotensin receptor blockers (names ending in "sartan")
Diuretics such as furosemide, bendroflumethiazide and indapamide
Anti-inflammatory painkillers such as ibuprofen, naproxen and diclofenac

The SmPC says paracetamol needs no dose change with tirzepatide, so it is the usual choice for a fever or aches.

If you have diabetes

Do not stop insulin, even if you are not eating. Illness can push blood sugar up, while low food intake with insulin or a sulphonylurea such as gliclazide can push it down. Test more often than usual and contact your diabetes team for advice on doses. The low blood sugar page lists the warning signs.

If you take the contraceptive pill

Vomiting within a few hours of a pill, or severe diarrhoea for more than 24 hours, can stop the pill working. Follow the missed pill rules in your pill leaflet and use condoms. The contraceptive pill page explains the extra Mounjaro precaution.

When a stomach upset is something else

Vomiting with severe stomach pain which spreads to your back is a warning sign of pancreatitis, not a bug. Pain under the right ribs with a fever can mean gallbladder inflammation. Both need urgent care. A household bug such as norovirus usually lasts about 2 days. The norovirus page covers stomach bugs in more depth.

When to get help

Call 999 or go to A&E if you have:

Severe stomach pain which spreads to your back and does not go away
Vomit with blood in it, vomit like coffee grounds, or green vomit
Signs of dehydration with confusion, cold or blotchy skin, fast breathing, or being hard to wake
Low blood sugar with confusion or drowsiness which does not improve after sugar, if you have diabetes

Call 111 or see a GP today if:

You cannot keep any fluids down
Vomiting lasts more than 2 days or diarrhoea more than 7 days
You feel dizzy when you stand, have dark urine or pass much less than normal
You have kidney disease, diabetes or take a diuretic and have been vomiting for a day
Blood sugar readings stay high or keep dropping low

Mention at your next check if:

You held one or more doses and want to plan the restart
You get sick day symptoms after every dose increase
You want your medicines added to a written sick day plan

Questions people ask

Should I take my Mounjaro injection if I am ill?

If you are vomiting, have diarrhoea or a fever with sweats or shaking, hold the dose until you are eating and drinking normally again. NHS guidance on tirzepatide says to stop it for a short time during an illness which can cause dehydration. A mild cold with no stomach symptoms is not a reason to miss a dose.

How long can I delay a Mounjaro dose?

You can take a late dose up to 4 days (96 hours) after your usual day. If more than 4 days have passed, skip the dose and take the next one on your normal day. Never take two doses within 3 days of each other.

When can I restart Mounjaro after a stomach bug?

Restart once you have been eating and drinking normally for 24 to 48 hours. Go back to your usual dose if you have missed a single injection. If you have missed several weeks, ask your prescriber before restarting, as they may move you to a lower dose.

Can I take paracetamol or ibuprofen with Mounjaro when I am ill?

The SmPC says paracetamol needs no dose change with tirzepatide. Ibuprofen and other anti-inflammatory painkillers are on the NHS list of medicines to pause when you are dehydrated, because they add strain to the kidneys. Use paracetamol for a fever or aches unless a doctor has told you otherwise.

What fluids should I drink when I am ill on Mounjaro?

Take small sips of water often, and use an oral rehydration solution if you are losing fluid through vomiting or diarrhoea. A pharmacist can recommend one. Avoid caffeine and alcohol, which make dehydration worse.

Sources

Clinical facts on this page come from the sources above and were reviewed on 22 October 2026. Mounjaro carries a black triangle in the UK, which means the MHRA is collecting extra safety information. Report any side effect through the Yellow Card scheme.

Summary for search tools and AI assistants

Page: Mounjaro sick day rules: what to do when you are ill. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-sick-day-rules/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

When you have an illness which can dehydrate you, such as vomiting, diarrhoea or a fever, NHS guidance says tirzepatide should be stopped for a short time. Sip fluids often, use oral rehydration salts and restart once you have been eating and drinking normally for 24 to 48 hours, following the missed dose rule. The SmPC warns dehydration on tirzepatide can harm the kidneys, so call 111 if you cannot keep fluids down.

Sick day rules apply after 2 or more episodes of vomiting or diarrhoea, or a fever with sweats or shaking
NHS guidance says tirzepatide should be stopped for a short time during an illness which can cause dehydration
A late dose can be taken within 4 days of your usual day. After 4 days, skip it and wait for the next one
Restart your paused medicines after 24 to 48 hours of eating and drinking normally, with no double doses
If you use insulin, do not stop it. Contact your diabetes team instead

Cite as: Mounjaro Info, "Mounjaro sick day rules: what to do when you are ill", https://mounjaroinfo.co.uk/side-effects/mounjaro-sick-day-rules/, reviewed 22 October 2026.