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 Injections Side Effects in the UK
Weight loss injections are prescription medications available to eligible adults in the UK. The most commonly prescribed options — Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide) — can help support weight management alongside diet and exercise. Like all medications, they may cause side effects, which vary between individuals. 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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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.
Common Side Effects of UK-Licensed Weight Loss Injections
The most common side effects of GLP-1 and GIP/GLP-1 weight loss injections are gastrointestinal — affecting your stomach and digestion. These typically occur when starting treatment or increasing doses:
- Nausea — the most frequently reported side effect, affecting more than 1 in 10 people
- Diarrhoea and constipation — both can occur as your body adjusts
- Stomach pain or indigestion — usually mild and temporary
- Vomiting — less common but possible in the first few weeks
- Injection site reactions — redness, bruising, or discomfort at injection sites
- Fatigue and headaches — reported by some users during dose escalation
Most side effects reduce as your body adjusts, typically within a few days or weeks. Your prescriber can advise on managing them or adjusting your dose if needed.
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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.
Eligibility, NHS Access, and Getting Started in the UK
In the UK, weight management medication is available through NHS specialist services or private prescription. You may be eligible if you have a BMI of 30 or above, or a BMI of 27–29.9 with weight-related health conditions such as type 2 diabetes, high blood pressure, or heart disease. Check your personal BMI using our free BMI calculator above. NHS access is through tier 3 specialist weight management services, usually requiring a GP referral, though waiting times vary by region. Private prescription options are also available following an online clinical assessment. Either route requires a prescriber to assess your suitability — side effects and medical history are part of this conversation.
- 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?
Nausea is the most common, affecting over 10% of users. Other frequent side effects include diarrhoea, constipation, stomach pain, and injection site reactions. Most are mild and improve within days or weeks as your body adjusts to the medication. Your prescriber can suggest ways to manage them.
Are weight loss injection side effects permanent?
No — most side effects are temporary and reduce as your body adjusts, typically within the first few weeks. Gastrointestinal effects are most common during dose escalation. Serious side effects are rare, but you should seek medical attention if you experience severe stomach pain or signs of allergic reaction.
Can I manage side effects while taking weight loss injections?
Yes — your GP or prescriber can offer practical suggestions such as eating smaller meals, staying hydrated, and adjusting your diet. Some people find side effects improve by taking injections on a specific day of the week or taking the injection at night. Never adjust your dose without medical guidance.
How do Mounjaro, Wegovy, and Saxenda side effects compare?
All three are GLP-1-based medications (Mounjaro also acts on GIP), so side effects are similar — nausea, diarrhoea, constipation, and injection site reactions are most common. Individual responses vary significantly. Your prescriber will choose based on your eligibility and medical history, not to avoid specific side effects.
Should I worry about serious side effects with weight loss injections?
Serious side effects are uncommon in the UK. Seek urgent medical attention for severe, persistent stomach pain (possible pancreatitis), serious allergic reactions, or sudden vision changes. Always inform your prescriber of any concerning symptoms, even if they seem minor — they can assess whether they require further investigation or medication adjustment.
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.