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 Jabs Side Effects in the UK
Weight loss jabs, commonly known as slimming injections, are prescription-only medications available to eligible UK adults for weight management. The most commonly prescribed options are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Like all medications, these GLP-1 and GIP/GLP-1 receptor agonist injections can cause side effects, though many people tolerate them well. Understanding what to expect helps you make an informed decision with your GP about whether prescription weight loss medication is right for you.
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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 Injections and Their Common Side Effects
Three prescription weight loss jabs are currently licensed in the UK for weight management:
- Mounjaro (tirzepatide) — a once-weekly dual GIP and GLP-1 receptor agonist, doses from 2.5 mg to 15 mg
- Wegovy (semaglutide) — a once-weekly GLP-1 agonist, doses from 0.25 mg to 2.4 mg
- Saxenda (liraglutide) — a once-daily GLP-1 agonist, doses from 0.6 mg to 3 mg
Very common side effects affecting more than 1 in 10 people include nausea, diarrhoea, vomiting, constipation, and stomach pain. These tend to be most noticeable when starting treatment or increasing doses, and often improve over time as your body adjusts. Common side effects (affecting up to 1 in 10 people) include indigestion, headache, fatigue, dizziness, and injection site reactions like redness or bruising. Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder problems, and allergic reactions requiring immediate medical attention.
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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 Access vs Private Prescription Weight Loss
In the UK, prescription weight loss medication is available through two routes. NHS access is through specialist tier 3 weight management services — your GP can refer you if you meet criteria, though regional availability and waiting times vary significantly. Private access is through GPhC-registered online pharmacies following a consultation with a UK-licensed prescriber, offering faster access without NHS referral. Eligibility generally requires a BMI of 30 or above, or 27 to 29.9 with weight-related health conditions like type 2 diabetes or high blood pressure. Lower BMI thresholds apply for adults of South Asian, Chinese, Black African, or Black Caribbean backgrounds. Your GP can advise which route suits your circumstances and whether you're eligible for treatment.
- 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 weight loss jabs safe to use in the UK?
Yes, Mounjaro, Wegovy, and Saxenda are approved and licensed by the MHRA for weight management in eligible UK adults. They undergo rigorous safety testing before approval. However, they are prescription-only medicines and require clinical assessment to ensure they are suitable for you. Like all medications, they carry potential side effects, and some people may not be eligible due to medical history or other medications. Always discuss safety with your prescriber.
Which weight loss jab has the fewest side effects?
Individual tolerance varies significantly — what causes side effects for one person may not affect another. Clinical trial data shows similar side effect profiles across Mounjaro, Wegovy, and Saxenda, with nausea and digestive issues being most common initially. Saxenda, being a daily injection rather than weekly, may suit some people better. Your prescriber can help choose based on your medical history and preferences. Use our free BMI calculator to check your starting point, then discuss options with your GP.
How long do weight loss jab side effects last?
Most side effects improve within the first few weeks as your body adjusts, particularly gastrointestinal symptoms like nausea. Some people find symptoms reduce as they progress through dose escalation. Injection site reactions typically resolve within hours. However, some side effects like hair loss or fatigue can persist longer. If side effects are severe or don't improve, inform your prescriber — dose adjustment or medication change may help.
Can I stop weight loss jabs if side effects are too much?
Yes, you can discuss stopping with your prescriber at any time. Stopping suddenly is generally safe — there is no withdrawal effect. However, weight regain often occurs after stopping, which is why your prescriber will discuss managing side effects (such as eating smaller meals, staying hydrated, or dose adjustment) before recommending cessation. Your prescriber may suggest alternative weight loss medication or NHS weight management support instead.
Do I need to see my GP before starting weight loss jabs?
Yes. Weight loss jabs are prescription-only medicines requiring clinical assessment. If accessing through NHS, your GP refers you to specialist tier 3 services for evaluation. If accessing privately, you must have a consultation with a UK-licensed prescriber (usually online) who assesses your medical history, medications, and eligibility before issuing a prescription. Your GP should also be informed you are taking weight loss medication. Try our free GLP-1 weight loss calculator to understand projected outcomes alongside your GP discussion.
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.