How to set up a Mounjaro pen before your first injection

Your pen is in the box, here's what to do before you touch it

Picture this: your delivery arrived this morning in that plain DPD package, you've put the pen in the fridge, and injection day has come around. The instinct is to pull everything out at once, but the order matters. Start by washing your hands thoroughly. Then take the pen out and leave it on a clean surface for roughly 30 minutes, injecting a cold liquid is noticeably more uncomfortable, and a few minutes at room temperature makes a real difference.

While you wait, gather what you need: one fresh needle from the foil pouch (compatible pen needles are a 4mm or 5mm size in most cases, confirm with your Patient Information Leaflet), an alcohol swab for cleaning the injection site, and a sharps bin within reach for the used needle. Never recap a used needle; it goes straight into the sharps bin after the injection.

Check the pen's label to confirm it is the strength your prescriber has prescribed. Look through the clear viewing window: the liquid should appear clear and colourless to slightly yellow. If it looks cloudy, discoloured or contains particles, do not use it. Contact our team via the support page or call your prescriber.

Attaching the needle and priming, the step most people skip by mistake

Pull the paper tab off a new needle. Hold the pen with the cap pointing down, peel the foil from the needle, and screw it straight onto the pen tip until it's snug. Don't overtighten, firm is enough. Once the needle is on, pull off the outer needle cap (keep it, you'll use it to remove the needle safely afterwards) and then the inner needle cap. The inner cap goes straight in the bin; it is not reused.

Priming is only needed for the first injection from each new pen. To prime: turn the dose dial until the dose indicator shows the dose arrow, then point the pen needle-up and tap the barrel gently so any air rises. Press the injection button and watch for a small stream or drop of liquid to appear at the needle tip. If nothing appears, repeat once. This step confirms the pen is working and the needle is clear. For more on the mechanics of the device itself, the Mounjaro pen guide covers the parts of the KwikPen in detail.

After priming, dial your prescribed dose, you will hear a click and see the dose in the window. If you want a full visual walkthrough of every dial and button before you get started, our guide on how to use the Mounjaro pen takes you through each step from cap to sharps bin. The dose window is a useful double-check: if the number shown does not match your prescribed dose, do not inject. Redial carefully.

Injecting, then finishing safely

Clean the injection site with an alcohol swab and let it dry for a few seconds. The licensed sites are the abdomen, the front of the thigh, or the upper arm; rotate between them each week to give tissue time to recover. Some patients ask about injecting into stretch marks, there is specific guidance on that separately. A full walkthrough of injection technique is on the injection technique page.

Press the pen firmly against the skin, hold it at a 90-degree angle, and press the injection button. Hold it there for ten full seconds before removing, that count matters for complete delivery. The dose window will show zero when the full dose has been given. If you'd like a step-by-step companion to keep handy on injection day, our page on how to use your Mounjaro pen covers the full process from attaching the needle through to safe disposal. Remove the pen, then place the outer needle cap back over the needle without touching the needle tip, unscrew it, and drop the needle into your sharps bin.

Replace the pen cap and return the pen to the fridge. Record the date of your injection, the same day each week is easiest to keep track of, though if life gets in the way (a holiday, a busy Monday), the FAQs cover what to do if your schedule shifts. The NHS tirzepatide medicines page has patient-level guidance on storage and missed doses that's worth bookmarking.

Side effects to know about, and when to get medical help

The most common effects in the weeks after starting or moving up a dose are gastrointestinal: nausea, loose stools, constipation, indigestion. These tend to ease as your body adjusts, usually within one to two weeks of a dose change. Eating smaller meals, avoiding very fatty or rich food, and staying well hydrated all help. Our Mounjaro injection overview has more on managing the early weeks.

Seek urgent medical attention if you develop severe, persistent stomach pain that spreads to your back, with or without vomiting. This can be a sign of acute pancreatitis, which the MHRA highlighted as an infrequent but serious risk in a 2026 Drug Safety Update for GLP-1 medicines, the Yellow Card scheme is where you report any suspected side effect. Other urgent signs include symptoms of a serious allergic reaction, significant injection-site swelling, or signs of severe dehydration. If you're ever uncertain, contact a healthcare professional the same day rather than waiting for your next review.

If you're still working out whether Mounjaro suits your situation, our prescribers review every consultation personally. You can speak to our prescribers through a free consultation, there's no obligation, and the clinical review happens the same day.