The UK's most complete GLP-1 guide
Every Question Answered. Every Side Effect Explained.
- How GLP-1 medications work
- Mounjaro vs Wegovy compared
- Injection instructions
- Side effects & solutions
- What to eat on treatment
- FAQs answered
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Mounjaro Lump At Injection Site — UK Health Information
Lumps, swelling, or hardness at the injection site are among the most common side effects of Mounjaro (tirzepatide), the weekly weight loss injection used in UK weight management. Most injection site reactions are mild and resolve within a few days, but understanding what's normal and when to contact your GP is important. This guide explains what causes these lumps, when they're expected, and how they compare to similar reactions with Wegovy, the alternative GLP-1 weight loss injection available in the UK.
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What Is BMI and Why Does It Matter?
Body Mass Index (BMI) is the most widely used screening tool for assessing whether your weight falls within a healthy range. Used by the NHS, GPs, and weight management services across the UK, BMI provides a quick snapshot of your weight relative to your height. The formula is simple: your weight in kilograms divided by your height in metres squared (kg/m²).
While BMI isn't a perfect measure — it doesn't account for muscle mass, bone density, or fat distribution — it remains the standard first step in identifying potential weight-related health risks including type 2 diabetes, cardiovascular disease, high blood pressure, and certain cancers.
The NHS also recommends adjusted thresholds for people of South Asian, Chinese, or Black African heritage — a BMI of 23 or above is classified as overweight, and 27.5 as obese. Our calculator includes this adjustment as a one-click toggle.
Injection Site Reactions: Mounjaro vs Wegovy
Mounjaro injection site lumps occur because the KwikPen delivers medication into the subcutaneous fat layer beneath your skin. A small amount of inflammation or localised swelling is a normal immune response. According to clinical trial data (SURMOUNT-1), injection site reactions are classified as common side effects — affecting up to 1 in 10 people using Mounjaro.
What's typically normal:
- Small lump or firm area at injection site lasting 1–7 days
- Mild redness or bruising around the needle entry point
- Slight tenderness or soreness when touched
- Slight warmth in the area
Wegovy comparison: Wegovy (semaglutide) users report similar injection site reactions in clinical trials. Both are once-weekly injections and both can cause localised lumps. The frequency and severity are comparable between the two medications. Individual variation is significant — some people experience no lump at all, whilst others notice a small firm nodule for several days.
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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When To Contact Your GP — Serious Signs
Most Mounjaro injection site lumps resolve without treatment, but contact your GP urgently if you experience:
- Lump that grows larger after several days (not improving)
- Signs of infection: increasing redness, warmth, pus, or discharge
- Severe pain or swelling that limits arm movement
- Lump accompanied by fever or feeling unwell
- Allergic reaction signs: swelling of face, lips, tongue, or throat
Keep a record of injection sites using our free progress tracker — rotating your injection site each week (stomach, thigh, or upper arm) reduces the risk of repeated lumps in the same location. Always use a new needle and inject into different areas as instructed by your prescriber.
- Dual GIP/GLP-1 receptor agonist
- Up to 22.5% body weight loss (SURMOUNT-1)
- Weekly injection, dose escalation over 20+ weeks
- Available via private prescription in the UK
- Doses: 2.5mg → 5mg → 7.5mg → 10mg → 12.5mg → 15mg
- GLP-1 receptor agonist
- Up to 15–17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg → 0.5mg → 1mg → 1.7mg → 2.4mg
Mounjaro vs Wegovy: Head-to-Head
A side-by-side comparison of the two most popular GLP-1 weight loss medications available in the UK.
| Feature | Mounjaro | Wegovy |
|---|---|---|
| Drug class | Dual GIP/GLP-1 agonist | GLP-1 agonist |
| Active ingredient | Tirzepatide | Semaglutide |
| Avg. weight loss | 15–22.5% | 12–17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20–32 weeks | 16–20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | £150–£250/month | £150–£300/month |
| Common side effects | Nausea, diarrhoea, constipation | Nausea, vomiting, diarrhoea |
Data sourced from SURMOUNT-1 and STEP 1 clinical trials. Costs are approximate and vary by provider. Always consult a prescriber.
Am I Eligible for Weight Loss Medication?
In the UK, eligibility for GLP-1 weight loss medications like Mounjaro and Wegovy is based on your BMI and overall health profile.
- BMI of 30 or above (obese)
- Age 18 or over
- No contraindicated conditions
- Committed to lifestyle changes
- BMI of 27 or above
- Plus type 2 diabetes
- Or high blood pressure
- Or obstructive sleep apnoea
Use our BMI calculator to check whether your BMI falls within the eligible range.
How We Built These Tools
Every calculator and guide on myweightcheck is built with accuracy and transparency in mind. Our BMI calculator uses the standard WHO formula with NHS-aligned category thresholds. Our GLP-1 projections are modelled on published clinical trial data from SURMOUNT and STEP studies.
All data stays on your device. We don't require sign-ups, and we don't store your health information on any server. Your progress tracker saves data locally in your browser using localStorage, so only you can see it.
We're based in the UK and our tools are designed specifically for UK users — supporting stone & lbs by default, referencing NHS guidelines, using NICE-approved medication data, and linking to GPhC-registered pharmacies.
Frequently Asked Questions
Common questions about BMI, weight loss medications, and our free UK tools.
Is a lump after Mounjaro injection normal?
Yes, small lumps at the injection site are common and affect up to 1 in 10 people using Mounjaro. Most resolve within 1–7 days without treatment. This is a normal local inflammatory response to the injection. Contact your GP if the lump grows larger, shows signs of infection, or causes severe pain.
How long does a Mounjaro injection site lump last?
Typically 1–7 days. Some people experience mild firmness for just a few hours, whilst others notice a small lump for a week. Applying a cold compress for 10–15 minutes can help reduce swelling. Ensure you're rotating injection sites each week to prevent repeated irritation in the same area.
Does Wegovy cause injection site lumps like Mounjaro?
Yes, Wegovy (semaglutide) causes similar injection site reactions, including lumps and swelling. Both are GLP-1 receptor agonist medications delivered by weekly injection, and both can cause localised inflammation. The frequency and severity are comparable, though individual experiences vary significantly between users.
Can I prevent lumps when using Mounjaro?
Rotate your injection site each week between your stomach, upper thigh, and upper arm. Use a fresh needle each time. Allow the skin to dry completely after cleaning with an alcohol wipe. Ensure the medication is at room temperature before injecting. Avoid injecting into areas with existing bruises or lumps.
Should I be worried about a hard lump after a Mounjaro injection?
A firm area or small lump is usually nothing to worry about — it's a normal local reaction. However, seek medical advice if the lump is painful, growing, warm to touch, or accompanied by redness, pus, or signs of infection. Your GP can assess whether it requires treatment or if it's a straightforward injection site reaction. ```
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