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 the injection site after using Mounjaro is common and usually temporary. This page explains what causes injection site reactions with tirzepatide (Mounjaro), how they compare to Wegovy (semaglutide), and when to contact your GP. Both Mounjaro and Wegovy are prescription-only weight management injections used in the UK, and understanding what's normal after injection can help you feel more confident using them. If you're considering either medication, use our free GLP-1 weight loss calculator to explore potential outcomes, and our BMI calculator to understand your current health context.
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Calculate Your BMI Instantly
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.
Injection Site Lumps — Mounjaro vs Wegovy: Key Differences
Mounjaro (tirzepatide) and Wegovy (semaglutide) both cause injection site reactions in some users, but the frequency and severity can differ slightly due to their different mechanisms and formulations.
- Mounjaro: In clinical trials, injection site reactions occurred in approximately 1–2% of users. Lumps, bruising, redness, and itching are classified as "common" side effects.
- Wegovy: Injection site reactions are also common, with similar rates reported in STEP trials. Redness, pain, and bruising at the injection site are expected in a minority of users.
- Why the difference? Mounjaro is a dual GIP + GLP-1 agonist with a slightly thicker formulation, whilst Wegovy is a single GLP-1 agonist. Both use fine needles and are injected into subcutaneous tissue (under the skin), so reactions depend more on individual skin sensitivity and injection technique than on the drug itself.
- What's normal: Small lumps, mild bruising, and temporary redness typically fade within 24–48 hours. These are not dangerous and do not mean you should stop using the medication.
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How It Works
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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
When to Worry — Red Flags vs Normal Injection Site Reactions
Most injection site lumps after Mounjaro or Wegovy are harmless, but contact your GP or prescriber if:
- The lump becomes increasingly larger or firmer after 3–5 days (may indicate infection or lipohypertrophy — thickened tissue from repeated injections)
- You develop signs of infection: warmth, pus, increasing redness, or fever
- Pain at the injection site is severe or worsens
- You develop an allergic reaction: swelling of lips, face, or throat (seek urgent medical attention)
For ongoing lumps with repeated injections, rotate your injection sites carefully: stomach, upper thigh, and upper arm. Reusing the same spot can cause lipohypertrophy. Your prescriber can advise on site rotation best practice.
- 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, bruising, or mild swelling at the injection site are common and typically harmless. They usually resolve within 24–48 hours. Mounjaro is a weekly injection given into subcutaneous tissue, and minor tissue reactions are expected in some users. However, if the lump persists or worsens after several days, contact your GP.
Is Wegovy safer for injection site lumps than Mounjaro?
No. Both Mounjaro and Wegovy cause similar rates of injection site reactions. The choice between them depends on clinical suitability assessed by your prescriber, not on injection site risk. Both are safe prescription-only medications. Individual responses vary considerably.
What causes lumps at the injection site with GLP-1 medications?
Lumps result from the needle puncture, the injected liquid, and mild inflammation as your body responds to the injection. Lipohypertrophy (thickened fatty tissue) can also develop if you inject into the same spot repeatedly. Rotating injection sites—stomach, thigh, upper arm—helps prevent this. Your prescriber can advise on proper site rotation.
When should I contact my GP about an injection site lump?
Contact your GP if the lump becomes increasingly large or painful after 3–5 days, if you notice signs of infection (warmth, pus, fever), or if you develop an allergic reaction. Minor bruising and temporary swelling are normal and do not require medical attention. Always inform your prescriber if you have concerns.
Can I keep using Mounjaro if I have a lump at the injection site?
Yes, in most cases. If the lump is mild and painless, you can continue your prescription as normal and simply rotate to a different injection site. If you experience severe pain, signs of infection, or worsening swelling, contact your prescriber before your next injection. Never stop medication without medical guidance.
Ready to take control of your weight loss journey?
Start with a free BMI check, explore your GLP-1 options, or begin tracking your progress today. No sign-up. No cost. Just results.