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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Injection Site Reactions with Mounjaro
Injection site reactions are common when using Mounjaro (tirzepatide), the prescription-only weekly weight loss injection available in the UK. These reactions typically occur at the injection site and are usually mild and temporary. Understanding what to expect, how to manage them, and when to seek medical advice can help you use this medication safely and comfortably alongside your reduced-calorie diet and increased physical activity.
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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.
What Are Mounjaro Injection Site Reactions?
Injection site reactions are local responses at the point where you inject Mounjaro using the KwikPen device. They are among the most common side effects reported during clinical trials and in clinical practice across the UK.
- Redness (erythema) — mild pink or red discolouration around the injection area
- Bruising — small purple or blue marks that develop after injection
- Swelling (oedema) — slight puffiness or tenderness at the site
- Pain or discomfort — mild soreness during or immediately after injection
- Itching — temporary itchiness at the injection site
These reactions typically appear within minutes to hours of injection and usually resolve within a few hours to a few days. They are classified as common side effects, affecting approximately 1 in 10 people using Mounjaro. Most people find they become less frequent as their body adjusts to the medication over the first few weeks of treatment.
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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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Managing and Preventing Injection Site Reactions
You can reduce the risk of injection site reactions by rotating your injection sites weekly — use your stomach (abdomen), upper thigh, or upper arm, and avoid injecting in exactly the same spot each time. Ensure the KwikPen is at room temperature before use, as cold injections may cause more discomfort. Apply a cold compress for 10–15 minutes after injection if you experience bruising, or a warm compress if you notice stiffness.
Never rub or massage the injection site immediately after use. Allow the skin to dry completely after cleaning with an alcohol wipe, and inject slowly into clean, healthy skin. If you notice signs of infection — increasing warmth, pus, or spreading redness — contact your GP. Most injection site reactions require no treatment and resolve on their own, but your GP or prescriber can advise if symptoms persist beyond a few days.
When to Seek Medical AdviceContact your GP or prescriber if you experience severe pain, significant swelling that doesn't reduce after 48 hours, signs of infection (warmth, redness, pus, or fever), or allergic reactions (swelling of lips or face). If you're concerned about managing injection site reactions, ask your GP about strategies for your next dose or whether you should adjust your injection technique. Your prescriber may recommend over-the-counter pain relief such as paracetamol if needed. Remember to discuss any concerns at your next review appointment — injection site reactions are a known side effect, and your healthcare team can provide personalised guidance based on your experience with the weekly weight loss injection.
- 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 injection site reactions with Mounjaro dangerous?
No — injection site reactions are generally mild and not dangerous. Redness, bruising, and mild swelling typically resolve within hours to days without treatment. However, if you notice signs of infection (warmth, pus, spreading redness, or fever), contact your GP immediately.
Can I reduce injection site reactions by changing my technique?
Yes. Rotate injection sites weekly (stomach, thigh, upper arm), let the KwikPen warm to room temperature, and ensure your skin is clean and dry before injecting. Allow the skin to dry completely after cleaning with an alcohol wipe. Avoid injecting in exactly the same spot each week.
How long do injection site reactions typically last?
Most injection site reactions resolve within a few hours to a few days. Redness and mild swelling usually disappear within 24–48 hours. Bruising may take slightly longer but typically fades within 5–7 days. Reactions tend to decrease over the first few weeks of treatment.
What's the difference between a normal reaction and infection at the injection site?
Normal reactions cause mild redness, bruising, or tenderness that fades quickly. Signs of infection include increasing warmth, pus, spreading redness, or fever. If you suspect infection, contact your GP immediately — don't wait for it to resolve on its own.
Are injection site reactions more common with Mounjaro than other weight loss injections?
Injection site reactions occur with most injectable medications. Mounjaro (tirzepatide) and Wegovy (semaglutide) both report injection site reactions as common side effects. Your GP or prescriber can discuss which weekly weight loss injection might be most suitable for you based on your individual health.
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