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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How To Take Semaglutide — Step-by-Step UK Guide
Semaglutide (brand name Wegovy) is a prescription-only weekly injection used for weight management in UK adults. If your GP or a licensed prescriber has assessed you as eligible, here's exactly how to take semaglutide safely and effectively. This guide covers the dosing schedule, injection technique, storage, and what happens if you miss a dose. Always follow your prescriber's instructions, as individual dosing may vary based on your health circumstances and how your body responds to treatment.
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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.
Semaglutide Dosing Schedule and Weekly Injections
Semaglutide is a GLP-1 receptor agonist that comes as a once-weekly injectable pen. Your dosing schedule will start low and increase gradually over several weeks:
- Weeks 1–4: 0.25 mg once weekly
- Weeks 5–8: 0.5 mg once weekly
- Weeks 9–12: 1 mg once weekly
- Weeks 13–16: 1.7 mg once weekly
- Week 17 onwards: 2.4 mg once weekly (maximum dose)
Each pen contains 4 doses (one month's supply). You must wait at least 7 days between injections — never inject more frequently. Your prescriber may adjust your schedule if you experience side effects. Unlike Ozempic (which is licensed for type 2 diabetes at doses up to 2 mg), Wegovy is specifically licensed for weight management at the higher 2.4 mg dose. The gradual increase helps your body adjust and reduces nausea.
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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.
Injection Technique and Storage Instructions
Injection sites: You can inject semaglutide into your stomach (abdomen), upper thigh, or upper arm — rotate your injection site each week to avoid irritation. Clean the skin with an alcohol wipe before injecting. The pre-filled pen is easy to use; simply dial the dose, place the needle against your skin at a 90-degree angle, and press the button until you hear a click.
Storage: Keep unopened pens in a refrigerator at 2–8°C in the original carton. Do not freeze. Once you start using a pen, it can be kept at room temperature (below 30°C) for up to 30 days. After 30 days, dispose of the pen safely in a sharps bin. Use our free GLP-1 weight loss calculator to track your projected progress as you progress through your dosing schedule.
- 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.
What if I miss my weekly semaglutide injection?
If you remember within 4 days, inject as soon as possible and continue your normal schedule the following week. If more than 4 days have passed, skip that dose and inject on your next scheduled day. Never take a double dose. Always maintain at least 3 days between injections. Contact your prescriber if you're unsure.
Can I switch between semaglutide and other weight loss medications like Mounjaro?
Semaglutide (Wegovy) and tirzepatide (Mounjaro) are different medications with different mechanisms. Semaglutide is a GLP-1 agonist only, whilst Mounjaro is a dual GIP and GLP-1 agonist. Any switch between medications must be managed by your prescriber — do not stop or change on your own.
What are the most common side effects when taking semaglutide?
Very common side effects include nausea, diarrhoea, constipation, vomiting, and stomach pain — especially when starting or increasing doses. These usually improve over time. If side effects are severe or persistent, contact your GP. Serious side effects like severe stomach pain or allergic reactions require immediate medical attention.
Does semaglutide work better if I inject at a specific time of day?
No — semaglutide works the same regardless of injection time. However, injecting on the same day each week helps you remember your dose. You can choose any day that suits you; just maintain a 7-day interval between injections. Some people prefer injecting in the morning, others in the evening.
Will semaglutide affect my other medications?
Semaglutide may slow the absorption of oral medications taken at the same time — this is particularly relevant for oral contraceptives. If taking the pill, add a barrier contraceptive method for 4 weeks after starting and after each dose increase. Always inform your prescriber of all medications you're taking so they can advise on potential interactions.
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.