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 Pills Side Effects — What to Expect in the UK
Weight loss pills can be effective for some people, but like all medications, they come with potential side effects. In the UK, prescription weight loss medication is available through both NHS specialist services and private prescribing for eligible adults. Understanding the side effects, who can access these medications, and how they work alongside lifestyle changes is essential before starting treatment. This guide covers the UK-licensed options and what you need to know before speaking to your GP or prescriber.
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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 Medications and Their Side Effects
Three prescription weight loss medications are licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All are GLP-1 receptor agonists (Mounjaro is a dual GIP/GLP-1 agonist), meaning they work by mimicking natural hormones that regulate appetite and blood sugar.
Common side effects across all three medications include:
- Nausea and vomiting
- Diarrhoea or constipation
- Stomach pain or indigestion
- Headache and dizziness
- Injection site reactions (redness, bruising)
- Fatigue and hair loss (less common)
Most people experience these side effects as the body adjusts during the first few weeks, and they often improve over time. However, some people find them difficult to manage. Serious side effects are rare but can include pancreatitis (severe stomach pain), gallbladder problems, and allergic reactions. Always report persistent or severe symptoms to your GP or prescriber immediately.
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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 Next Steps
Prescription weight loss medication is available in the UK if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like type 2 diabetes, high blood pressure, or sleep apnoea. Lower eligibility thresholds apply if you are of South Asian, Chinese, Black African, or Black Caribbean heritage.
NHS access: Referral to a specialist tier 3 weight management service through your GP. Availability varies by region and waiting lists can be long.
Private access: Online consultation with a UK-licensed prescriber, prescription issued if appropriate, dispensed by a registered pharmacy.
Use our free BMI calculator above to check if you may be eligible, then speak to your GP about options. Your prescriber will assess whether medication suits you and discuss expected side effects and weight loss outcomes, which vary between individuals.
- 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 pills in the UK?
Nausea, diarrhoea, vomiting, constipation, and stomach pain are very common during the first few weeks of treatment with GLP-1 medications. These usually improve as your body adjusts. Injection site reactions are also common. Less common side effects include headache, dizziness, and fatigue. Most people manage these well with time.
Are weight loss pills safe in the UK?
Prescription weight loss medications available in the UK (Mounjaro, Wegovy, Saxenda) are licensed and have undergone rigorous safety testing. Serious side effects are rare. However, they are not suitable for everyone—pregnant women, those with a history of pancreatitis, and people with certain medical conditions cannot take them. Your prescriber will assess whether it's safe for you.
How do I know if I'm eligible for weight loss medication on the NHS?
You're typically eligible if you have a BMI of 30 or above, or 27 or above with weight-related health conditions like diabetes or high blood pressure. South Asian, Chinese, Black African, and Black Caribbean adults may be eligible at a BMI of 27.5 or above. Speak to your GP about referral to a specialist weight management service.
What's the difference between Mounjaro and Wegovy side effects?
Both cause similar common side effects (nausea, diarrhoea, constipation, stomach pain), as they're both GLP-1-based medications. Mounjaro is a dual GIP/GLP-1 agonist, so it works slightly differently, but side effect profiles are comparable. Individual tolerance varies. Your prescriber will help you choose what's right for you.
Can I get weight loss pills on the NHS or do I need to pay privately?
Both are possible in the UK. NHS access is through specialist tier 3 weight management services, though waiting lists vary by region. Private access is available through online consultations with UK-licensed prescribers. Costs for private treatment typically range from £130–£320 per month depending on dose and provider.
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.