Weight Loss Medication Safety During Heatwaves (UK Guide)

Last updated: 18 September 2026

Hot weather does not automatically mean you need to stop treatment. However, heat can make dehydration more likely, especially if you have nausea, vomiting or diarrhoea. Follow the Instructions for Use supplied with your own medicine and ask a pharmacist or prescriber if you are unsure.

Can you take weight-loss injections in hot weather?

Yes. Mounjaro and Wegovy can usually be used during hot weather as prescribed. Take extra care with hydration, avoid overheating and keep pens within the storage limits in their patient leaflet. Do not change your dose or injection day because of the weather unless your prescriber tells you to.

Why heat can be harder when you are taking treatment

Medicines such as Mounjaro can cause nausea, vomiting or diarrhoea. In hot conditions, losing fluid through these side effects or sweating can make dehydration more likely. A reduced appetite can also make it easier to forget to drink regularly.

Pay attention to thirst, dizziness, unusual tiredness and how often you are passing urine. Drink regularly throughout the day, take breaks from the heat, and avoid strenuous activity during the hottest part of the day. If side effects are troublesome, see our guide to weight-loss injection side effects and how to manage them.

Keep your pen stored correctly

Always follow the leaflet for the exact pen you have, as storage instructions differ between medicines and pen versions. For Mounjaro, unused pens should be stored in a refrigerator at 2–8°C and must not be frozen. After first use, the Mounjaro patient leaflet permits storage for up to 30 days at no more than 30°C. Protect the pen from light.

• Do not leave a pen in a car, conservatory, direct sunlight or beside a heat source.
• For travel, use an insulated bag or cool pouch if needed, but do not let the pen touch ice or a frozen gel pack directly.
• If you think a pen has been overheated, frozen or stored outside its instructions, do not guess. Contact your pharmacist for advice before using it.

When to get medical advice

Contact NHS 111, your prescriber or pharmacist if you cannot keep fluids down, have ongoing vomiting or diarrhoea, feel faint, or are worried about pen storage. Call 999 if someone is confused, collapses, has a seizure, severe breathing difficulty, chest pain, or other signs of heatstroke or a serious medical emergency.

Practical heatwave checklist

• Check heat-health alerts and plan cooler parts of the day for activity.
• Keep a drink close by and take regular sips, particularly if your appetite is reduced.
• Store your medicine according to its own leaflet before travelling.
• Continue prescribed medicines unless a healthcare professional advises otherwise.

This article is general information, not individual medical advice.

Sources

• MHRA: taking medication in summer.
• MHRA: heat, travel and medicines.
• Mounjaro Patient Information Leaflet.

Last updated: 17 September 2026.