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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Weight Loss Medicine Side Effects in the UK
Prescription weight loss medicines are now available through the NHS and private prescribing in the UK for eligible adults. Understanding the potential side effects of these medications is important before starting treatment. This guide covers the most common prescription weight management options available in the UK, what side effects you may experience, and how to access them safely through proper clinical assessment.
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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.
UK-Licensed Weight Loss Medicines and Their Side Effects
Three prescription weight loss medicines are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All work by mimicking natural hormones that regulate appetite and blood sugar.
Mounjaro and Wegovy are once-weekly injections; Saxenda is a daily injection. The most commonly reported side effects across these medications include nausea, diarrhoea, constipation, vomiting, and stomach pain — particularly when starting or increasing doses. These side effects often improve within a few weeks as your body adjusts. More serious side effects are rare but can include gallbladder problems, pancreatitis symptoms (severe stomach pain), or changes in vision. Individual responses vary significantly, and not everyone experiences side effects. A prescriber will assess your medical history and discuss potential risks before prescribing any weight loss medicine.
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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.
NHS vs Private Access and Getting Started
In the UK, weight loss medicines are available through both NHS specialist tier 3 weight management services and private online prescribing. NHS access typically requires referral from your GP to a specialist weight management clinic, though availability varies significantly by region and waiting times can be lengthy. Private access is available through UK-licensed prescribers without NHS referral, following an online clinical assessment.
To find out if you're eligible, speak to your GP about your weight management options. You'll be assessed on your BMI and any weight-related health conditions — such as type 2 diabetes, high blood pressure, or cardiovascular disease. Use our free BMI calculator to check your starting point, then discuss prescription weight loss medicine options with your healthcare provider if appropriate.
Eligibility generally includes adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions. For people of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds (27.5+) may apply due to higher health risks at lower weights. Prescription weight loss medicines work most effectively alongside a reduced-calorie diet and increased physical activity — they are not replacements for lifestyle changes. Your prescriber will monitor you regularly to assess effectiveness and manage any side effects, adjusting your dose if needed.
- 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 are the most common side effects of weight loss medicines in the UK?
Nausea, diarrhoea, constipation, vomiting, and stomach pain are the most frequently reported side effects of Mounjaro, Wegovy, and Saxenda. These often occur when starting treatment or increasing doses, and typically improve within weeks as your body adjusts. Not everyone experiences side effects, and they vary between individuals.
Are weight loss medicines available on the NHS?
Yes. Mounjaro, Wegovy, and Saxenda are available through NHS specialist tier 3 weight management services. Access requires referral from your GP, and availability varies by region. Private prescribing is also available through UK-licensed online prescribers for eligible adults without NHS referral.
How do I know if I'm eligible for prescription weight loss medicine?
Eligibility typically includes adults with a BMI of 30 or above, or 27–29.9 with weight-related health conditions (such as type 2 diabetes or high blood pressure). Use our BMI calculator to check your current BMI, then speak to your GP about whether weight loss medicine might be suitable for you based on your individual health.
Do weight loss medicines work better than diet and exercise alone?
Prescription weight loss medicines are designed to work alongside diet and exercise, not replace them. Clinical trials show that combining medication with a reduced-calorie diet and increased physical activity produces better results than either approach alone. Your prescriber will discuss realistic expectations during your assessment.
What serious side effects should I watch for?
While rare, contact your GP immediately if you experience severe stomach pain (possible pancreatitis), persistent gallbladder pain, serious allergic reactions (swelling of face or throat), or significant changes in vision. Always inform your prescriber of any existing medical conditions before starting weight loss medicine.
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.