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
Quick Weight Loss Side Effects — What You Need to Know
gradual weight management through prescription medication is an option for eligible UK adults, but it's important to understand potential side effects before starting treatment. The most commonly prescribed weight management medications in the UK—Mounjaro and Wegovy—are GLP-1 and dual GIP/GLP-1 receptor agonists that work by reducing appetite and helping you feel fuller. While clinical trials have shown these medications can support significant weight loss alongside lifestyle changes, they do come with side effects that vary between individuals. This guide explains what side effects you might experience, how they typically develop, and what UK eligibility criteria apply.
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Calculate Your BMI Instantly
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 Medications and Their Side Effects
Three prescription medications are currently licensed for weight management in the UK. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist administered once weekly, with clinical trials showing average weight loss of 15–21% of body weight. Wegovy (semaglutide) is a GLP-1 receptor agonist, also once weekly, with trial data showing approximately 15% average weight loss. Saxenda (liraglutide) is a daily GLP-1 injection with slightly lower average weight loss outcomes.
The most common side effects across these medications include nausea, diarrhoea, vomiting, and constipation—particularly during the dose escalation phase. Nausea typically peaks in the first 2–4 weeks and often improves as your body adjusts. Gastrointestinal effects are more pronounced when doses are increased. Less common but important side effects include injection site reactions, fatigue, dizziness, headaches, and hair loss. These medications may also reduce appetite more dramatically than expected, which can lead to reduced calorie intake beyond the intended level.
Serious side effects are rare but require immediate medical attention: severe stomach pain (possible pancreatitis), gallbladder problems, serious allergic reactions, or unexplained changes in heart rate. Use our GLP-1 weight loss calculator to understand projected results and discuss realistic expectations with your prescriber.
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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 Eligibility and Getting Started Safely
To be eligible for prescription weight loss medication through the NHS or private prescribing in the UK, you typically need a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes, high blood pressure, or obstructive sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply due to higher health risks at lower weights.
Side effect management starts before you begin: your prescriber will assess your full medical history, current medications, and any contraindications. Starting at the lowest dose allows your body to adjust gradually. Many people find that eating smaller, more frequent meals and staying hydrated reduces nausea significantly. If side effects are severe or persistent, your prescriber can pause dose escalation or switch to an alternative medication. Always report new or worsening symptoms to your GP or prescriber—they can offer practical solutions or consider whether the medication remains appropriate for you. Check your BMI using our free BMI calculator to understand your starting point and eligibility.
- 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 injections in the UK?
The most common side effects of Mounjaro, Wegovy, and Saxenda are nausea, diarrhoea, vomiting, and constipation. These are typically gastrointestinal effects that often improve as your body adjusts to the medication. Nausea is most pronounced during dose escalation and usually peaks in the first 2–4 weeks on each new dose level.
How long do side effects last when taking weight loss medication?
Side effects vary between individuals. Most gastrointestinal effects improve within 2–4 weeks as your body adapts. Some people experience minimal side effects from the start, whilst others notice improvements only after several weeks. If side effects remain severe beyond 4 weeks on a stable dose, discuss alternatives with your prescriber.
What should I do if I experience severe side effects?
Seek immediate medical attention if you develop severe, persistent stomach pain (possible pancreatitis), signs of a serious allergic reaction, gallbladder problems, or unexplained changes in heart rate. For milder side effects, contact your prescriber or GP—they can suggest management strategies, adjust your dose, or consider switching medications.
Am I eligible for weight loss medication on the NHS in the UK?
You may be eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions. Access is typically through NHS tier 3 specialist weight management services via GP referral, though availability varies significantly by region and waiting lists can be lengthy.
Can I get weight loss medication privately if I'm not eligible on the NHS?
Yes. Private prescribing is available through GPhC-registered online pharmacies following a clinical assessment with a UK-licensed prescriber. This is faster than NHS referral but involves a cost (typically £129–£320+ per month depending on dose). Your prescriber will still assess eligibility based on your individual health circumstances.
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.