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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Lump After Injecting Mounjaro — What You Need to Know
A lump, swelling, or bruising at your Mounjaro injection site is a common and usually harmless reaction. Most lumps resolve naturally within a few days to a week. This guide explains what happens when you inject Mounjaro, when a lump is normal, and when you should contact your GP or prescriber. If you're taking tirzepatide as part of your weight management journey, understanding expected injection site reactions helps you use your KwikPen safely and confidently.
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Enter your height and weight below. Supports both imperial (stone & lbs) and metric (kg). Results include NHS category, healthy weight range, and ethnicity adjustments.
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.
What Causes a Lump After Mounjaro Injection?
A lump or swelling after a Mounjaro injection happens because the liquid medication is injected into the fatty tissue beneath your skin (subcutaneous injection). The body's natural inflammatory response creates a small area of swelling as it absorbs the medicine. This is completely normal and expected with tirzepatide injections.
- Subcutaneous injection: Mounjaro is injected into stomach, thigh, or upper arm tissue — not muscle or vein
- Fluid volume: Each KwikPen dose contains medication solution that temporarily creates a small raised area
- Individual variation: Some people develop more noticeable lumps than others depending on skin thickness, hydration, and injection technique
- Normal healing: The body absorbs the injected liquid over several hours to days, and the lump naturally resolves
Most injection site lumps are completely normal. However, knowing the difference between a routine lump and a sign you need medical attention is important.
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Three simple steps to understand your weight and take action.
Enter your height and weight in our free calculator. Get your BMI, NHS category, healthy weight range, and ethnicity-adjusted thresholds instantly.
If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
Use our progress tracker to log your weight weekly, visualise your journey with charts, and stay motivated — all stored privately in your browser.
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When to Contact Your GP — Red Flag Signs
Most Mounjaro lumps are harmless, but contact your GP or prescriber if you notice:
- Severe pain at the injection site that doesn't improve within a few hours
- Signs of infection: increasing redness, warmth, pus, or fever
- Large or spreading swelling that grows larger after 24 hours rather than improving
- Numbness or tingling around the injection area
- A hard lump that doesn't soften after 2 weeks or becomes increasingly painful
These signs may indicate infection, allergic reaction, or incorrect injection technique. Your GP can assess and advise on next steps. Standard injection site reactions (mild bruising, small lumps, slight tenderness) do not require medical attention.
- 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.
How long does a lump last after a Mounjaro injection?
Most lumps resolve within 24–48 hours as your body absorbs the injected medication. Some mild swelling may persist for up to a week. If a lump is still present and growing after 2 weeks, contact your prescriber for assessment.
Can I inject Mounjaro in the same spot every week?
No — always rotate your injection site each week to prevent lumps and tissue damage. Rotate between stomach, upper thigh, and upper arm. Using the same site repeatedly can cause lipohypertrophy (thickened fat deposits) and reduced medication absorption.
Does a lump mean my Mounjaro injection went wrong?
Not necessarily. A small lump is normal even with correct technique. However, if you inject too shallowly (intradermally) or too deeply, you may get unusual reactions. Your prescriber can review your injection technique at your next appointment.
What's the difference between a normal lump and an injection site reaction I should worry about?
Normal: small, soft, painless or mildly tender, resolves within days. Concerning: large, hard, painful, spreading, warm, red, or accompanied by pus or fever. When in doubt, contact your GP — they can assess via NHS services.
Can ice or heat help a lump after Mounjaro?
Gentle ice (wrapped in cloth, not directly on skin) for 10–15 minutes in the first hour can reduce immediate swelling. After 24 hours, a warm compress may soothe tenderness. Most lumps don't require treatment and resolve naturally.
Ready to take control of your weight loss journey?
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