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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What Are Mounjaro Injection Site Reactions?
Injection site reactions are a common side effect of Mounjaro (tirzepatide), the prescription-only weekly weight loss injection licensed in the UK. These reactions occur at the location where you inject the medicine and may include redness, bruising, itching, or mild pain. Most injection site reactions are mild and temporary, typically improving within a few hours or days. Understanding what to expect can help you feel more confident managing your Mounjaro treatment. This guide explains the types of reactions, how to minimise them, and when to contact your GP or prescriber.
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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.
Types and Symptoms of Injection Site Reactions
Injection site reactions are listed as a common side effect of Mounjaro, meaning they may affect up to 1 in 10 people using the medication. Common symptoms include:
- Redness or erythema at the injection site
- Bruising or haematoma (blood under the skin)
- Itching or pruritus
- Swelling or mild oedema
- Mild pain or soreness
- A small lump or nodule under the skin
These reactions are usually mild to moderate and resolve without treatment within hours to days. The symptoms mirror those seen in the SURMOUNT clinical trials, where injection site reactions were among the most frequently reported adverse effects. Severity tends to decrease over time as your body adjusts to the weekly tirzepatide injections. If you experience severe reactions—such as significant swelling, severe pain, or signs of infection—contact your GP or prescriber immediately.
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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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How to Reduce Injection Site Reactions
Several practical steps can help minimise injection site discomfort and reactions:
- Rotate injection sites weekly — use a different site each time (stomach, thigh, or upper arm) to avoid repeated trauma to the same area
- Allow the pen to reach room temperature before injecting if stored in the fridge
- Clean the injection site with an alcohol wipe and let it dry completely before injecting
- Inject into clean, healthy skin free from scars, bruises, or areas with existing reactions
- Use a fresh needle from a new KwikPen each week — never reuse needles
- Apply gentle pressure after injection rather than rubbing the site
Proper injection technique and site rotation are the most effective ways to prevent repeated reactions. Your prescriber can provide detailed injection guidance when you first start Mounjaro.
- 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.
Are Mounjaro injection site reactions serious?
Most injection site reactions are mild and resolve within days without treatment. Redness, bruising, and itching are common but temporary. However, seek medical advice if you experience severe swelling, signs of infection (warmth, pus, increasing pain), or an allergic reaction (difficulty breathing, facial swelling).
How can I tell the difference between a normal reaction and an infection?
Normal reactions improve within a few days. Signs of infection include increasing redness and warmth over 2–3 days, pus or drainage, fever, or spreading warmth beyond the injection site. Contact your GP immediately if you suspect infection.
Should I stop using Mounjaro if I have injection site reactions?
Most injection site reactions do not require stopping treatment. Site rotation, proper technique, and allowing pens to warm up typically resolve reactions. Always discuss persistent or severe reactions with your prescriber before making any changes to your treatment.
Can I use the same injection site twice?
It's recommended to rotate sites weekly to prevent repeated irritation. Space injections at least 1–2cm apart on the same body area if needed, but varying between stomach, thigh, and upper arm is ideal for minimising cumulative irritation.
Do injection site reactions get better over time?
Yes. Many people experience fewer or milder reactions after the first few weeks as their body adjusts to the weekly injections. Consistent site rotation and proper technique help this process. If reactions persist, discuss with your prescriber or GP.
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