You've opened your Mounjaro pen — here's exactly how to store it in the fridge

Step 1, after the first injection, return the pen to the fridge

After you give your first weekly dose, replace the inner needle cap, then the outer cap, and put the pen back in the fridge at 2–8°C. Keep it away from the freezer compartment and away from direct contact with the back wall of the fridge, where temperatures can drop below zero. The fridge door shelf is a reasonable spot for most household fridges, since it stays a little more stable in temperature than the shelves nearest the cooling element, though the exact location matters less than keeping it within the 2–8°C range.

Do not store the pen with the needle attached. Needles should be removed and disposed of in a sharps container immediately after each injection; attaching a new one for each dose keeps the pen sterile and prevents air entering the cartridge.

The NHS medicines guidance for tirzepatide confirms refrigerated storage as the standard requirement between uses, and if you want a detailed answer to whether you keep Mounjaro in the fridge once opened, we cover that fully on its own page. If you are unsure whether your fridge is maintaining the right temperature, a simple fridge thermometer costs a few pounds and removes the guesswork.

Step 2, when the fridge isn't an option, use the room-temperature window carefully

There will be times when keeping the pen refrigerated is impractical: a weekend away, a long commute, time at the gym. Mounjaro can be stored at room temperature for a limited period, but the specific number of days is something we always direct readers to check in the Patient Information Leaflet that came with their pen, because this window applies to the opened pen and may differ from guidance for an unopened pen stored outside the fridge. The leaflet is the authoritative source, and Eli Lilly may update it; treating a recalled figure from a website as definitive is a risk not worth taking.

What is consistent across all guidance: once the room-temperature window has elapsed, the pen should not be used and must not be returned to the fridge to be 'reset'. Discard it safely. Room temperature in this context means below 30°C, not a warm car, not a sunny windowsill.

If you are planning travel and want to know more about keeping Mounjaro stable on the move, our broader Mounjaro fridge and storage guidance covers those scenarios in more detail.

Step 3, before each dose, check the pen

Regardless of where the pen has been stored, take a moment before each weekly injection to inspect it. The solution should be clear and colourless or very slightly yellow. Do not use the pen if the liquid looks cloudy, contains visible particles, or has changed colour. Do not use it if the pen looks damaged or if you suspect it has been frozen at any point, ice crystal formation can degrade the medicine even if the pen appears normal once thawed.

If anything looks wrong, contact your prescriber or pharmacist before injecting. Our clinical team is available through the support page seven days a week for exactly these questions. This is also the right moment to check your injection-site rotation, abdomen, thigh and upper arm are the licensed sites, and rotating each week reduces local reactions. Clean the site with an alcohol swab and allow it to dry before injecting.

For a full overview of injection technique and what to expect from each dose, the Mounjaro injection guide on this site walks through the process clearly.

Why storage conditions matter for a biological medicine like Mounjaro

Tirzepatide is a peptide (a chain of amino acids) and like all peptides it is sensitive to temperature extremes. Exposure to heat or freezing temperatures can cause the protein structure to break down in ways that are not always visible to the naked eye. A pen that has been left in a warm car boot may look perfectly normal; the medicine inside may no longer be effective, or may cause unexpected reactions.

This is one of the reasons that counterfeit or unregulated supply is genuinely dangerous. Pens that have not travelled through the licensed UK cold chain may have been stored incorrectly at any point between manufacture and delivery, and if you are wondering how long you can keep Mounjaro in the fridge unopened, that page sets out the full guidance. The MHRA's Yellow Card scheme is where patients can report suspect pens or unexpected reactions, worth bookmarking.

Questions about cost and what a full course of treatment involves are answered on the Mounjaro cost page, including what's included in a private prescription from a regulated pharmacy.

Mounjaro is a prescription-only medicine licensed for weight management in eligible adults. If you are considering starting treatment or have questions about whether it is right for you, speaking with a prescriber is the right first step. Check your eligibility with our clinical team, the consultation is free, and every assessment is carried out by a GPhC-registered prescriber on the same day.