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
Do Weight Loss Pills Work?
Yes, prescription weight loss pills can work — but results vary significantly between individuals, and they work best alongside lifestyle changes. Clinical trials show that medications like Wegovy (semaglutide) and Mounjaro (tirzepatide) help many people lose weight when used as part of a structured weight management approach. However, "work" doesn't mean studied results. This guide explains what the evidence actually shows, how these medications function, and what to realistically expect if prescribed weight loss medication in the UK through the NHS or private prescribing.
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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.
What Evidence Shows About Weight Loss Medication
Clinical trials demonstrate that prescription weight loss injections can be effective. The STEP trial for Wegovy (semaglutide) showed participants lost an average of approximately 15% of their body weight over 68 weeks at the maximum dose of 2.4mg, combined with a reduced-calorie diet and increased physical activity. Mounjaro (tirzepatide) trials showed slightly higher average weight loss — around 15–21% of body weight — as it uses a dual GIP and GLP-1 mechanism rather than single GLP-1 activation.
- Individual results vary considerably — some people lose significantly more, others less
- These medications work by reducing appetite, helping you feel fuller after smaller meals, and slowing digestion
- Weight loss typically occurs gradually over several months, not overnight
- Results are most consistent when combined with reduced-calorie diet and exercise
However, approximately 1 in 4 people prescribed these medications do not achieve significant weight loss, which is why they're not a assured solution. Use our free GLP-1 weight loss calculator to see projected results at different doses based on trial averages.
Clinically BackedWeight Loss
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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.
When Weight Loss Pills Don't Work as Expected
Several factors affect whether weight loss medication will be effective for you individually. These include genetics, how your body responds to the specific hormone mechanism, adherence to dose escalation schedules, dietary compliance, and underlying health conditions. Some people experience side effects (nausea, constipation, reduced appetite) that are manageable; others find them too severe to continue treatment.
Important context: weight loss medication is prescribed alongside lifestyle changes, not instead of them. If someone returns to their previous eating patterns after stopping the medication, weight typically returns. This is why prescription weight loss treatment in the UK is part of a structured weight management programme, whether through NHS specialist services or private prescribing with ongoing support.
- 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.
How long does it take to see weight loss results from prescription pills?
Weight loss typically becomes noticeable after 4–8 weeks of consistent use. Most significant results appear between 12–24 weeks. Clinical trials tracked results over 68 weeks, showing that continued use over several months produces the most substantial outcomes. Results are gradual, not rapid.
Are weight loss pills safe?
Prescription weight loss medications like Wegovy and Mounjaro are licensed and undergo rigorous safety testing. However, they're not suitable for everyone — contraindications include pregnancy, personal or family history of thyroid cancer, and certain other conditions. Always discuss safety with your GP or prescriber, who will assess whether it's appropriate for you.
Will I regain weight if I stop taking weight loss pills?
Weight regain is common after stopping medication if lifestyle habits return to previous patterns. Clinical evidence shows maintaining weight loss requires continued healthy eating and physical activity. Some people continue medication longer-term for sustained weight management, whilst others use it as a temporary support tool.
How much weight can you lose with prescription pills?
Average weight loss in clinical trials ranges from 15–21% of body weight depending on the medication and maximum dose reached. However, "average" means some people lose more, others less. Individual factors including starting weight, adherence, diet, and exercise all influence outcomes. Check your realistic range with a prescriber.
Are weight loss pills available on the NHS?
Yes — Mounjaro and Wegovy are NHS-licensed and available through specialist tier 3 weight management services. However, NHS access varies significantly by region, and waiting lists can be lengthy. Private prescription is available through online prescribers without NHS referral, though costs apply. Your GP can advise on local NHS availability.
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.