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
Free tracker included with every download this week
How Do Weight Loss Injections Work in the Stomach?
Weight loss injections like Wegovy and Mounjaro are administered by injecting into the fatty tissue of your stomach (abdomen), thigh, or upper arm. These prescription-only medications work by activating natural hormones that regulate appetite and blood sugar. The injection itself is quick and painless — the needle is very fine. Understanding how these injections work and where they're given can help you decide if weight management medication is right for you alongside lifestyle changes.
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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.
How Weight Loss Injections Are Administered
Weight loss injections are given once weekly as a simple subcutaneous injection — meaning the needle goes into the fatty tissue just under your skin, not into muscle or vein. The most common injection site is the stomach (abdomen), but you can also inject into the outer thigh or back of the upper arm. Rotating your injection site each week helps prevent skin irritation.
The injection itself takes seconds. Both Wegovy (semaglutide) and Mounjaro (tirzepatide) come in pre-filled pens that make self-injection straightforward. Your prescriber or pharmacist will show you how to inject correctly. Many people report minimal discomfort — the needles are very fine (29 gauge, similar to insulin pens). After injection, there may be mild redness or bruising at the site, but this usually resolves within hours.
Each pen contains enough medication for 4 weeks of weekly injections. Storage is simple: unopened pens go in the fridge (2–8°C), and after first use, they can be kept at room temperature for up to 30 days.
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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.
How These Injections Affect Your Stomach and Appetite
Weight loss injections work by mimicking GLP-1 (glucagon-like peptide-1), a natural hormone your gut produces after eating. This hormone signals to your brain that you're full, helping reduce appetite and food cravings. The injection also slows how quickly food leaves your stomach, which supports a feeling of fullness after smaller meals.
Mounjaro works slightly differently — it activates both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) pathways, offering a dual mechanism. In clinical trials, Mounjaro helped participants lose an average of 15–21% of their body weight, while Wegovy showed average weight loss of around 15%. However, individual results vary significantly. These medications are most effective when combined with a reduced-calorie diet and increased physical activity. Your GP or prescriber can assess whether prescription weight loss medication is suitable for your health profile.
- 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 the stomach injection painful?
No — weight loss injection needles are very fine and the injection is quick. Most people experience minimal to no discomfort. Some report a slight pinch or pressure. Any mild redness or bruising at the injection site usually resolves within hours. Your prescriber will demonstrate correct injection technique to minimise discomfort.
Can I inject weight loss medication anywhere on my stomach?
Yes, you can inject anywhere on your abdomen where there's fatty tissue — the lower abdomen, sides, or upper belly are all suitable. Rotate your injection site each week to avoid irritation or lipodystrophy. Never inject into areas with scars, bruises, or hardened tissue. The thigh and upper arm are also approved injection sites.
What happens to my stomach when I take these injections?
The injection itself doesn't damage your stomach — the medication enters the fatty tissue under your skin. The active ingredient then affects how your gut hormones work, slowing gastric emptying and reducing appetite signals. This typically leads to eating smaller portions and feeling fuller sooner. The slowing effect often reduces over time as your body adapts.
How long does a weight loss injection take to work?
Most people notice reduced appetite within the first 1–2 weeks of starting. Significant weight loss typically becomes visible after 4–8 weeks. Weight loss continues over months as you progress through doses. The STEP and SURMOUNT clinical trials measured results over 68–72 weeks. Your prescriber will monitor your progress and adjust your dose based on your response.
Do I need a prescription to buy weight loss injections in the UK?
Yes — Wegovy, Mounjaro, and other weight loss injections are prescription-only medicines in the UK. You cannot buy them without a prescription from a UK-licensed prescriber. Access is available through NHS specialist weight management services (though waiting lists vary by region) or through private online prescribing following a clinical assessment. Your prescriber will check your eligibility and health before issuing a prescription. ```
Ready to take control of your weight loss journey?
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