Mounjaro in hot weather: storage, dehydration and dizziness

In short

Unused pens: fridge at 2C to 8C, never frozen
In-use pen: out of the fridge at no more than 30C for up to 30 days
Never leave a pen in a parked car, a glovebox or in direct sun
Dizziness and low blood pressure are common side effects in the weight management trials (SmPC section 4.8)
Heat exhaustion should improve within 30 minutes of cooling down. If not, call 999

In hot weather, keep unused Mounjaro pens in the fridge and your in-use pen below 30C. Drink small amounts often through the day, because Mounjaro reduces thirst and appetite while heat makes you lose fluid through sweat. Watch for dizziness on standing, dark urine and headache. If you feel hot, sick and faint, cool down straight away. Call 999 if you are no better after 30 minutes.

Storing pens in the heat

The patient leaflet sets the limits:

Pen	Rule
Unused	Fridge at 2C to 8C until you start it. Do not freeze
In use	Out of the fridge at no more than 30C for up to 30 days from first use
After 30 days	Throw away, even if doses remain
Frozen	Throw away, even after thawing

The instructions for use add a further point: keep the pen away from heat. UK homes rarely pass 30C indoors, but during a heatwave a south-facing room, a windowsill or a conservatory can. A parked car can become far hotter within minutes.

Practical storage steps

On the hottest days, the fridge is the simplest way to keep an in-use pen under the 30C ceiling.
Keep it away from windows, radiators, ovens and the top of the fridge.
Never leave a pen in a parked car, a glovebox or a bag in the sun.
Use an insulated pouch or cool bag when you go out, with an ice pack wrapped in a cloth so the pen does not freeze.
Write the date you started each pen on the box so you know when the 30 days run out.

Check the liquid before every injection. It should be clear and colourless to slightly yellow. Do not use a pen with cloudy, discoloured liquid or bits in it. The leaflet does not say what to do after a pen has been above 30C, so ask a pharmacist before you use one.

If you are going abroad, the travelling abroad guide covers flights, cool bags and hotel fridges.

Why dehydration is more likely on Mounjaro

Mounjaro makes you feel full and less hungry. Many people find they drink less as well, because thirst is quieter and the stomach feels full. A lot of daily fluid also comes from food, so smaller meals mean less fluid.

Add sweating in the heat and the gap widens. The leaflet tells you to avoid dehydration by drinking plenty of fluids, because dehydration can lower kidney function. The SmPC gives prescribers the same warning about fluid loss from vomiting and diarrhoea. The kidneys and dehydration page explains the risk in detail.

The NHS lists these signs of dehydration:

Thirst
Headache or light-headedness
Dark yellow, strong-smelling urine
Passing urine less often
Dizziness and tiredness
Dry mouth, lips and tongue

Drinking enough in the heat

Sip water through the day. A glass with each meal and one between meals is a good base.
Drink more if you are sweating, active or outdoors.
Use urine colour as your guide. Pale yellow is the target.
If you sweat heavily, replace salts as well as water with an oral rehydration drink. The electrolytes page has more detail.
Eat foods with a high water content, such as melon, cucumber, soup and yoghurt.
Keep alcohol low. It adds to fluid loss, and the NHS says alcohol can make Mounjaro nausea worse. See the alcohol guide.

Large gulps can bring on nausea when your stomach empties slowly, so small, regular amounts work better. The hydration page has more tips.

Dizziness and low blood pressure in the heat

The SmPC lists dizziness and events related to low blood pressure as common side effects in the weight management trials. It reports 94% of the low blood pressure events as mild to moderate.

Heat widens blood vessels in the skin to help you cool down, which lowers blood pressure further. Dehydration lowers it again. Together they can make you feel faint when you stand up, especially after a hot bath or shower or in the afternoon.

To reduce dizziness:

Stand up slowly, and sit on the edge of the bed for a moment first
Keep drinking through the day
Avoid long, hot baths during a heatwave
Sit or lie down at once if you feel faint

If you take blood pressure tablets and have lost a lot of weight, ask your GP to check your blood pressure. Your dose may need adjusting. The dizziness page covers this in more detail.

Heat exhaustion and heatstroke

The NHS lists the signs of heat exhaustion as tiredness, dizziness, headache, feeling or being sick, heavy sweating with pale clammy skin, cramps, a high temperature, strong thirst and irritability. Several of these overlap with Mounjaro side effects, so treat them as heat first when the weather is hot.

The NHS steps for cooling down:

Move somewhere cool.
Take off extra clothing.
Drink plenty of water, or an isotonic drink or oral rehydration solution.
Cool the skin with cool water and fanning. Cold packs wrapped in cloth can go under the armpits or on the neck.

You should start to feel better within 30 minutes. If not, call 999. Heatstroke is a medical emergency.

To prevent heat illness, the NHS advises avoiding the sun between 11am and 3pm, wearing loose, light-coloured clothes, limiting alcohol and hard exercise, and keeping your home cool with curtains closed in the day.

When to get help

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

No improvement after 30 minutes of cooling down and drinking
A very high temperature with hot skin which has stopped sweating
Confusion, a fit or loss of consciousness
Fast breathing, cold or blotchy skin and signs of dehydration

Call 111 or see a GP today if:

You feel dizzy each time you stand and it does not go away
Your urine is dark or you are passing much less than normal
You cannot keep fluids down
You feel unusually tired, or have a fast heartbeat, after a day in the heat

Mention at your next check if:

You often feel faint on standing in warm weather
You take blood pressure tablets and have lost a lot of weight
You are planning a trip somewhere hot and want advice on storing pens

Questions people ask

How hot is too hot for a Mounjaro pen?

The leaflet allows an in-use pen to stay out of the fridge at no more than 30C for up to 30 days. Unused pens should stay in the fridge at 2C to 8C. If a pen has been above 30C, for example in a hot car, ask a pharmacist before you use it.

Can I keep my Mounjaro pen in the fridge during a heatwave?

The leaflet sets a ceiling of 30C for an in-use pen, and the fridge keeps it well below this. The 30 day limit from first use still applies, wherever you keep it. Check with your pharmacist if you are unsure.

Does hot weather make Mounjaro side effects worse?

Heat itself does not change how tirzepatide works. Heat makes you lose fluid through sweat, while Mounjaro reduces how much you eat and drink, so dehydration, dizziness and tiredness become more likely. Heat exhaustion also causes nausea, which can feel like a Mounjaro side effect.

How much should I drink on Mounjaro in hot weather?

Drink enough to keep your urine pale yellow, and more if you are sweating or active. Sip through the day instead of drinking a lot at once, as a full stomach on Mounjaro can bring on nausea. Add an oral rehydration drink if you have been sweating heavily.

How do I keep Mounjaro cool on holiday?

Carry pens in a cool bag or insulated pouch, keep them out of direct sun and put them in the hotel fridge on arrival. Do not let a pen touch an ice pack directly, as freezing ruins it. Take your pens in hand luggage when you fly.

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 in hot weather: storage, dehydration and dizziness. Site: Mounjaro Info (https://mounjaroinfo.co.uk/side-effects/mounjaro-in-hot-weather/). Reviewed 22 October 2026. Topic: Mounjaro (tirzepatide), UK.

Unused Mounjaro pens belong in the fridge at 2C to 8C. Once started, a pen can stay out of the fridge at no more than 30C for up to 30 days, so in a heatwave keep it in the coolest room or a cool bag. Mounjaro lowers thirst and food intake, and the SmPC warns dehydration can harm the kidneys, so drink regularly in the heat and watch for dizziness, dark urine and signs of heat exhaustion.

Unused pens: fridge at 2C to 8C, never frozen
In-use pen: out of the fridge at no more than 30C for up to 30 days
Never leave a pen in a parked car, a glovebox or in direct sun
Dizziness and low blood pressure are common side effects in the weight management trials (SmPC section 4.8)
Heat exhaustion should improve within 30 minutes of cooling down. If not, call 999

Cite as: Mounjaro Info, "Mounjaro in hot weather: storage, dehydration and dizziness", https://mounjaroinfo.co.uk/side-effects/mounjaro-in-hot-weather/, reviewed 22 October 2026.