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 Injection Site Reactions — UK Health Guide
Injection site reactions are among the most common side effects of Mounjaro (tirzepatide), the prescription-only weekly weight loss injection licensed in the UK. Most reactions are mild and resolve within a few hours to days, but knowing what's normal and when to seek help from your GP is important for safe use. This guide explains local injection site effects, how to manage them, and when medical advice is needed.
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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.
Common Mounjaro Injection Site Reactions
Local injection site reactions affect many people using Mounjaro and are generally not serious. The most common reactions include:
- Redness or erythema — pink or red discolouration around the injection site, usually fading within hours to 24 hours
- Bruising — small bruises from the needle puncturing small blood vessels; typically resolve within 7–14 days
- Swelling — mild puffiness or slight swelling around the injected area, usually minor and temporary
- Itching or mild irritation — localised itching that typically subsides quickly without treatment
- Pain or tenderness — mild soreness at the injection site, similar to other weekly injections
These reactions are considered common side effects in clinical trials and occur in a significant proportion of users. They are not signs of an allergy in most cases and do not usually require medical intervention. Rotating injection sites between your stomach, thigh, and upper arm (with assistance) helps minimise repeated trauma to the same area and may reduce reaction frequency.
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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
Most injection site reactions resolve independently, but contact your GP if you experience:
- Severe swelling that spreads beyond the immediate injection site or affects your hand, face, or throat — this may indicate a serious allergic reaction
- Warmth, pus, or signs of infection at the site (increasing redness, heat, or drainage after 24 hours)
- Severe or spreading bruising or persistent swelling lasting more than a few days
- Persistent pain that doesn't improve with over-the-counter painkillers
- Signs of cellulitis (spreading redness, increased warmth, fever)
Keep the injection site clean and dry. If you're concerned about any reaction, speak to your GP or the prescriber who issued your Mounjaro prescription — they can assess whether the reaction is normal or requires further investigation.
- 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 normal with Mounjaro?
Yes, injection site reactions like redness, bruising, and mild swelling are common side effects of Mounjaro reported in clinical trials. They're usually mild and resolve within hours to days without treatment. Most do not indicate an allergy or serious problem.
How can I reduce injection site reactions?
Rotate your injection site each week between your stomach, upper thigh, and upper arm (with help). Allow the skin to warm to room temperature before injecting, insert the needle at 90 degrees, and hold steady. Avoid injecting into areas with bruising or irritation. Ice the site briefly after injection if needed.
What's the difference between a normal reaction and an allergic reaction?
Normal reactions are localised (redness, bruising, mild swelling around the injection site only) and resolve within days. Allergic reactions involve swelling of the face, lips, tongue, or throat; difficulty breathing; hives across the body; or severe itching. Allergic reactions require immediate medical attention.
Is it safe to continue using Mounjaro if I have injection site reactions?
Mild local reactions are normal and don't usually mean you should stop treatment. Continue using different injection sites each week. Contact your GP if reactions are severe, spreading, or show signs of infection, or if you develop signs of allergy.
Can I use Mounjaro if I bruise easily?
Yes, but inform your GP or prescriber if you take blood thinners, have a bleeding disorder, or bruise very easily. Most people using Mounjaro can safely manage minor bruising with site rotation and proper injection technique. Your prescriber can advise on your individual situation.
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