Starting Mounjaro (tirzepatide) for the first time requires careful preparation and understanding of what to expect during your initial weeks of treatment. This once-weekly injectable medication is licensed in the UK for managing type 2 diabetes and weight management in adults with obesity or overweight with weight-related comorbidities. As a dual GIP and GLP-1 receptor agonist, Mounjaro works by enhancing insulin secretion, suppressing appetite, and slowing gastric emptying. Your healthcare provider will typically begin treatment with a low starting dose of 2.5 mg weekly, gradually increasing every four weeks to minimise side effects whilst optimising therapeutic benefit. Understanding proper injection technique, recognising common side effects, and maintaining regular monitoring are essential for safe and effective treatment outcomes.

What to Expect When Starting Mounjaro

Mounjaro (tirzepatide) is a once-weekly injectable medication licensed in the UK for the treatment of type 2 diabetes mellitus. It is also licensed for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. As a dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist, Mounjaro works by enhancing insulin secretion when blood glucose levels are elevated, suppressing glucagon release, slowing gastric emptying, and reducing appetite.

When you first begin treatment with Mounjaro, your GP or specialist will typically prescribe the lowest starting dose—2.5 mg once weekly—to allow your body to adjust gradually to the medication. This initial dose is primarily intended to minimise gastrointestinal side effects rather than to achieve maximum therapeutic benefit. You should anticipate a dose escalation schedule over the following weeks or months, with increases occurring every four weeks as tolerated, moving through 5 mg, 7.5 mg, 10 mg, 12.5 mg, and potentially up to the maximum maintenance dose of 15 mg weekly.

During your first few weeks on Mounjaro, it is normal to experience some adjustment symptoms, particularly related to your digestive system. Many patients notice a reduced appetite and earlier feelings of fullness during meals. If you are using Mounjaro for diabetes management, you may begin to see improvements in your blood glucose readings within the first few weeks, though optimal glycaemic control typically develops over several months. For those prescribed Mounjaro for weight management, weight loss usually becomes apparent after the first month, though the timing and extent of weight loss varies between individuals. It is essential to maintain realistic expectations and understand that Mounjaro works best when combined with a balanced diet and regular physical activity, as recommended by NICE guidance for both diabetes and obesity management.

Common Side Effects in Your First Weeks

Gastrointestinal symptoms are the most frequently reported side effects when starting Mounjaro, affecting a significant proportion of patients during the initial weeks of treatment. The most common adverse effects include nausea, diarrhoea, vomiting, constipation, abdominal pain, and decreased appetite. These symptoms typically emerge within the first few days to weeks of starting treatment or following dose increases, and they often diminish in intensity as your body adapts to the medication. The gradual dose escalation protocol is specifically designed to reduce the severity of these gastrointestinal effects.

To manage nausea and other digestive symptoms effectively, consider the following practical strategies:

• Eat smaller, more frequent meals rather than large portions

• Avoid high-fat, greasy, or heavily spiced foods that may exacerbate nausea

• Stay well hydrated, particularly if experiencing vomiting or diarrhoea

• Eat slowly and stop when you feel comfortably satisfied

• Avoid lying down immediately after eating

Some patients may experience injection site reactions, including redness, itching, or mild discomfort at the injection site. These reactions are generally mild and resolve spontaneously. Rotating injection sites between the abdomen, thigh, and upper arm can help minimise local reactions.

Other potential side effects during the initial treatment period include fatigue, dizziness, and headache. Whilst these are usually mild and transient, you should monitor how you feel, particularly when driving or operating machinery. Hypoglycaemia (low blood sugar) is uncommon with Mounjaro when used alone, but the risk increases if you are also taking insulin or sulfonylureas. Your insulin or sulfonylurea dose may need to be reduced when starting Mounjaro. Symptoms of hypoglycaemia include trembling, sweating, confusion, palpitations, and hunger. If you experience hypoglycaemia, take a fast-acting carbohydrate such as glucose tablets or fruit juice.

There is also a risk of gallbladder problems (gallstones, inflammation) with medications like Mounjaro, particularly during weight loss.

When to seek medical advice: Contact your GP or diabetes specialist nurse if you experience persistent vomiting that prevents you from keeping down fluids, severe abdominal pain (particularly if radiating to the back, which may indicate pancreatitis), persistent right-sided abdominal pain with fever or yellowing of the skin/eyes (possible gallbladder problems), signs of dehydration, or any symptoms that significantly interfere with your daily activities. If you develop symptoms of a severe allergic reaction—such as difficulty breathing, facial swelling, or widespread rash—seek emergency medical attention immediately. If you suspect any side effects, you can report them to the MHRA Yellow Card Scheme at yellowcard.mhra.gov.uk or via the Yellow Card app.

Monitoring Your Progress on Mounjaro

Regular monitoring is essential when starting Mounjaro to assess both therapeutic effectiveness and safety. Your healthcare team will establish a monitoring schedule tailored to your individual circumstances, treatment indication, and any concurrent medical conditions.

For patients using Mounjaro for type 2 diabetes management, monitoring typically includes:

• Blood glucose levels: You may be advised to check your blood glucose more frequently during the initial weeks, particularly if you are taking other glucose-lowering medications. Your GP or diabetes team will discuss target ranges appropriate for you.

• HbA1c measurements: This test, which reflects average blood glucose control over the preceding 2–3 months, is usually checked every 3–6 months to assess longer-term glycaemic control.

• Weight monitoring: Regular weight measurements help evaluate treatment response, as weight loss often accompanies improved glucose control.

• Blood pressure: As weight loss and improved metabolic health can affect blood pressure, periodic monitoring is recommended.

• Diabetic eye screening: Rapid improvements in blood glucose can sometimes temporarily worsen existing diabetic retinopathy. Ensure you attend your regular diabetic eye screening appointments and report any new or worsening visual symptoms promptly.

For those prescribed Mounjaro for weight management, monitoring focuses on:

• Body weight: Weigh yourself regularly (e.g., weekly or fortnightly) at the same time of day, using the same scales. Treatment response will be reviewed according to NICE guidance or local specialist weight management service criteria, with continuation typically dependent upon achieving at least 5% weight loss from baseline at specified review points.

• Waist circumference: This measurement provides additional information about changes in abdominal adiposity.

• Metabolic parameters: Periodic assessment of blood glucose, lipid profile, and blood pressure helps evaluate improvements in cardiovascular risk factors.

Regardless of indication, your healthcare provider may recommend baseline and periodic monitoring of:

• Renal function: Particularly important if you have existing kidney disease or experience significant gastrointestinal side effects that could lead to dehydration

• Lipase and amylase levels: If there is clinical suspicion of pancreatitis

While routine thyroid function monitoring is not recommended in the UK product information, you should report any symptoms such as a lump in the neck, persistent hoarseness, or difficulty swallowing to your GP.

Self-monitoring at home is equally important. Keep a record of any side effects, changes in appetite, energy levels, and overall wellbeing. Note any patterns—for example, whether symptoms are worse on injection days or improve over time. This information is valuable for your healthcare team when making decisions about dose adjustments or additional support measures. If you are monitoring blood glucose at home, maintain a log to share with your diabetes team at follow-up appointments.

How to Use Mounjaro for the First Time

Mounjaro is administered as a subcutaneous injection once weekly, on the same day each week, at any time of day, with or without food. The medication comes in a pre-filled, single-dose pen that is designed for self-administration. Your GP, practice nurse, or diabetes specialist nurse should provide comprehensive training on injection technique before you administer your first dose.

Step-by-step injection guidance:

1. Preparation: Remove the Mounjaro pen from the refrigerator approximately 30 minutes before injection to allow it to reach room temperature, which can make the injection more comfortable. Wash your hands thoroughly with soap and water.

2. Inspect the pen: Check the medication through the viewing window—it should be clear and colourless to slightly yellow. Do not use if the solution is cloudy, discoloured, or contains particles. Verify the dose on the pen label matches your prescribed dose.

3. Choose and prepare the injection site: Mounjaro can be injected into the abdomen (avoiding the area around the navel), front of the thigh, or upper arm. Note that upper arm injections generally require assistance from another person. Clean the chosen site with an alcohol wipe and allow it to dry completely. Rotate injection sites each week to reduce the risk of lipodystrophy (changes in fat tissue under the skin).

4. Administer the injection: Remove the pen cap. Place the pen firmly against your skin at a 90-degree angle. Press and hold the injection button—you will hear a click. Keep holding the button and the pen against your skin until you hear a second click, which indicates the full dose has been delivered. This usually takes about 5–10 seconds. The viewing window will turn grey when the injection is complete.

5. After injection: Remove the pen from your skin and dispose of it safely in a sharps container—never reuse pens or share them with others. Do not recap the pen. A small amount of bleeding or bruising at the injection site is normal; apply gentle pressure with a clean tissue if needed.

Storage and handling: Store unopened Mounjaro pens in the refrigerator (2°C to 8°C) in the original carton to protect from light. Do not freeze. Unopened pens may be kept at room temperature (not exceeding 30°C) for up to 21 days. Keep the pen away from direct heat and sunlight.

What if you miss a dose? If you miss your scheduled dose and it has been less than 4 days since you should have taken it, administer the missed dose as soon as you remember, then continue with your regular weekly schedule. If more than 4 days have passed, skip the missed dose and take your next dose on your regularly scheduled day. Never take two doses within 3 days of each other. If you are unsure what to do, contact your GP, diabetes specialist nurse or pharmacist for advice.

Important safety considerations: Do not share your Mounjaro pen with others, even if the needle has been changed, as this poses a risk of infection transmission. If you are pregnant, planning pregnancy, or breastfeeding, discuss this with your GP or specialist, as Mounjaro is not recommended during pregnancy.

Mounjaro can affect the absorption of oral medications due to delayed gastric emptying. This includes oral contraceptives—if you use oral contraceptive pills, you should use an additional non-oral contraceptive method for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. Inform your healthcare team about all medications you are taking.

Frequently Asked Questions