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
Weight Loss Jab Side Effects — What to Know
Weight loss jabs, or slimming injections, are prescription medications licensed in the UK for weight management. The most commonly prescribed options are Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Like all medications, they can cause side effects — though many are mild and temporary. This guide explains what side effects are possible, how long they typically last, and what you should know before speaking to your GP about prescription weight loss options in the UK.
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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.
Side Effects of UK Weight Loss Injections
Mounjaro (tirzepatide) and Wegovy (semaglutide) are GLP-1 receptor agonists — prescription-only injections licensed for weight management in the UK. Saxenda (liraglutide) is an older GLP-1 option given by daily injection rather than weekly.
The most common side effects across all three medications include nausea, diarrhoea, constipation, and stomach pain. These tend to be worst during the first few weeks and when doses increase — many people find they settle as their body adjusts.
- Nausea: Very common; often improves within 1–2 weeks
- Diarrhoea or constipation: Both can occur; staying hydrated helps
- Stomach pain: Usually mild; severe pain needs urgent GP attention
- Fatigue: Some people feel tired initially; typically resolves
- Headache and dizziness: Less common but reported
- Hair loss: Reported in some users, often temporary
- Injection site reactions: Redness or bruising at injection site
Most side effects are not serious and do not require stopping treatment. However, you should contact your GP if side effects are severe, persistent, or concerning.
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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 and Getting Prescription Weight Loss in the UK
In the UK, weight loss medications are available through NHS specialist weight management services (tier 3) or through private online prescribing with a UK-licensed prescriber. Access varies significantly by region — some areas have long waiting lists, while others have limited availability.
To explore whether prescription weight loss treatment is suitable for you, speak to your GP. They can assess your weight management uk needs, discuss your medical history, and either refer you to NHS services or discuss private options. Your GP may also suggest trying our free BMI calculator to establish your current health metrics before your appointment. Eligibility typically requires a BMI of 30 or above, or 27+ with weight-related health conditions like type 2 diabetes or high blood pressure.
- 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 jab side effects permanent?
No. Most side effects are temporary and settle within days or weeks as your body adjusts to the medication. Nausea, diarrhoea, and stomach discomfort are common initially but usually improve by week 2–3. Persistent side effects should be discussed with your prescriber.
Can I stop taking a weight loss jab if side effects are too much?
Yes. If side effects are intolerable, you can pause treatment or stop — always discuss this with your prescriber first. Sometimes lowering the dose or slowing the increase helps. Never stop suddenly without guidance from your doctor or prescriber.
What serious side effects should I watch for with weight loss jabs?
Seek urgent medical attention if you experience severe stomach pain (possible pancreatitis), gallbladder pain, serious allergic reactions (swelling of face or throat), or changes in heart rate or vision. These are rare but require immediate GP or hospital assessment.
Do all weight loss jabs have the same side effects?
Weight loss jabs in the UK (Mounjaro, Wegovy, Saxenda) are all GLP-1 agonists and share similar side effect profiles. Mounjaro is a dual GIP/GLP-1 agonist, so effects can vary slightly. Your prescriber will explain which medication is most suitable based on your health.
How can I reduce side effects from weight loss jabs?
Eat smaller meals, avoid greasy or heavy foods, stay hydrated, and move gently after meals. Starting at the lowest dose helps — doses increase gradually over weeks. Using our free progress tracker can help you log how you feel alongside your weight journey and identify patterns.
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.