Mounjaro (tirzepatide) is a once-weekly injectable medication licensed in the UK for type 2 diabetes and weight management in adults with obesity or weight-related comorbidities. As a dual GIP and GLP-1 receptor agonist, it enhances insulin secretion, suppresses glucagon, slows gastric emptying, and reduces appetite. Supplied as a pre-filled, single-dose auto-injector, Mounjaro is administered subcutaneously into the abdomen, thigh, or upper arm. Correct injection technique is essential for safe, effective treatment and to minimise injection site complications. This guide explains how to use Mounjaro properly, covering preparation, administration steps, common mistakes, and important safety checks before your first injection.

What Is Mounjaro and How Is It Used?

Mounjaro (tirzepatide) is a once-weekly injectable medication licensed in the UK for the treatment of type 2 diabetes mellitus and, more recently, for weight management in adults with obesity or overweight with weight-related comorbidities. It is a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist, which works by enhancing insulin secretion, suppressing glucagon release, slowing gastric emptying, and reducing appetite.

In the UK, Mounjaro is supplied as a single-dose, pre-filled auto-injector designed for subcutaneous injection into the abdomen, thigh, or upper arm. Each auto-injector contains one fixed dose (2.5 mg, 5 mg, 7.5 mg, 10 mg, 12.5 mg, or 15 mg) and should be discarded after a single use.

It is important to understand that Mounjaro auto-injectors do not require priming before use. The device has a hidden needle and is designed to deliver the full dose with a simple injection process. Unlike some other injectable medications, there is no need to attach a needle, remove air bubbles, or select a dose.

Following the correct administration technique helps ensure safe, effective treatment and reduces the risk of injection site complications. Always read the Patient Information Leaflet and Instructions for Use that come with your medication, and consult your healthcare professional if you have any questions about using Mounjaro.

Step-by-Step Guide: How to Use Your Mounjaro Pen

Using your Mounjaro auto-injector is straightforward when you follow the manufacturer's instructions carefully. Begin by washing your hands thoroughly with soap and water to reduce infection risk. Remove the Mounjaro pen from the refrigerator and allow it to reach room temperature for approximately 30 minutes—injecting cold medication can cause discomfort at the injection site.

Step 1: Check the pen. Inspect the medication through the viewing window. The solution should be clear and colourless to slightly yellow. Do not use the pen if the liquid appears cloudy, discoloured, or contains particles, or if the pen is damaged.

Step 2: Choose an injection site. Suitable areas include your abdomen (except for a 5 cm area around your navel), thigh, or upper arm. Rotate injection sites each week to reduce the risk of lipohypertrophy (fatty lumps under the skin). Clean the chosen area with an alcohol wipe and allow it to dry.

Step 3: Prepare the pen. Remove the grey base cap from the auto-injector. Do not put the base cap back on as this could damage the needle. The pen must be used within 5 minutes of removing the cap.

Step 4: Position and inject. Place the clear base flat against your skin at the injection site. Turn the lock ring to unlock the pen. Press and hold the injection button until you hear a second click and/or see the indicator window show the injection is complete. This typically takes about 10 seconds. Keep holding the button down and count slowly to 5 before removing the pen from your skin.

Step 5: Dispose of the pen. Place the used auto-injector in a sharps bin immediately after use. Never reuse a Mounjaro pen, even if there appears to be medication left inside.

Common Mistakes When Using Mounjaro

Several common errors can occur when using Mounjaro auto-injectors, potentially affecting the safety and efficacy of your treatment. Not allowing the pen to reach room temperature is a frequent mistake. Cold medication can cause injection site pain, stinging, or discomfort. Always allow the pen to reach room temperature naturally—never use external heat sources such as microwaves or hot water, as this can damage the medication.

Not removing the base cap before injection or not unlocking the pen will prevent the injection from working. Similarly, not pressing the pen firmly against the skin or not holding the injection button long enough (until you hear the second click or see the completion indicator) can result in an incomplete dose.

Injecting through clothing is not recommended as it may prevent proper placement and increase the risk of infection. Always inject into clean, exposed skin.

Not rotating injection sites can lead to lipohypertrophy, which may affect medication absorption. Use a different site for each weekly injection, keeping a record of previous sites if needed.

Storing the medication incorrectly can reduce its effectiveness. Unopened pens should be refrigerated (2-8°C), while pens removed from the refrigerator can be kept at room temperature (not above 30°C) for up to 21 days.

Finally, attempting to reuse the auto-injector is a serious error. Mounjaro pens are single-use only and should be disposed of in a sharps container after injection, even if you think there is medication remaining. Always follow the manufacturer's instructions precisely and consult your healthcare professional if you are unsure about any step.

What to Do If Your Mounjaro Injection Doesn't Complete

If your Mounjaro injection doesn't start or complete properly, do not panic—there are several troubleshooting steps you can take. First, check that you've prepared the pen correctly. Ensure you have removed the grey base cap and turned the lock ring to the unlock position.

Ensure proper placement of the auto-injector. The clear base should be flat against your skin at the injection site. If the base is not in full contact with the skin, the injection may not activate or complete. Try repositioning the pen firmly against your skin.

Press and hold the injection button until you hear the second click and/or see the indicator window show the injection is complete. This typically takes about 10 seconds. If you release the button too early, the full dose may not be delivered.

Inspect the medication through the viewing window before use. If the solution appears cloudy, discoloured, or contains particles, do not use the pen. Similarly, if the pen has been frozen, dropped, or damaged, it should be discarded safely in a sharps container.

If you've followed all steps correctly and the injection still doesn't complete, do not attempt a second injection with the same pen. The pen may be faulty. Contact your pharmacy or healthcare provider for a replacement. Do not attempt to dismantle the pen or force the mechanism, as this can cause injury.

Keep a record of any faulty pens, including the batch number and expiry date (found on the pen label), and report the issue to your pharmacist. In the UK, you can report defective medicines to the Medicines and Healthcare products Regulatory Agency (MHRA) via the Yellow Card scheme at yellowcard.mhra.gov.uk or using the Yellow Card app. If you miss a dose due to a faulty pen, consult your healthcare professional for advice on when to administer your next injection—do not double your dose to compensate.

Safety Checks Before Your First Mounjaro Injection

Before administering your first Mounjaro injection, several essential safety checks must be completed to ensure safe and effective treatment. First, verify your prescription. Confirm that the dose prescribed by your doctor matches the dose indicated on your pen. Mounjaro is typically initiated at a lower dose (2.5 mg once weekly) and gradually increased every four weeks to minimise gastrointestinal side effects such as nausea, vomiting, and diarrhoea. Never adjust your dose without medical advice.

Check the expiry date on the pen label and ensure the medication has been stored correctly. Unopened Mounjaro pens should be refrigerated at 2–8°C and protected from light. Once removed from the refrigerator, the pen can be kept at room temperature (not above 30°C) for up to 21 days. Do not use the pen if it has been frozen or exposed to excessive heat.

Inspect the medication visually. The solution should be clear and colourless to slightly yellow. Discard the pen if the liquid is cloudy, discoloured, or contains particles, or if the pen appears damaged.

Review potential side effects and precautions with your healthcare provider. Common adverse effects include gastrointestinal symptoms, which usually improve over time. Serious but rare risks include pancreatitis and hypoglycaemia (especially if used with insulin or sulfonylureas). Rapid improvement in blood glucose control may be associated with temporary worsening of diabetic retinopathy, particularly in patients using insulin who have pre-existing retinopathy.

Mounjaro is not recommended during pregnancy or breastfeeding. If you have a personal or family history of thyroid cancer, discuss this with your doctor before starting treatment.

Know when to seek help. Contact your GP or call NHS 111 if you experience severe abdominal pain, persistent vomiting, or signs of pancreatitis. Call 999 if you experience symptoms of a serious allergic reaction (rash, swelling, difficulty breathing). Report any suspected side effects via the Yellow Card scheme at yellowcard.mhra.gov.uk. If you have any concerns about injection technique, ask your practice nurse or diabetes specialist for a demonstration. Proper training and adherence to safety checks optimise treatment outcomes and minimise risks.

Frequently Asked Questions