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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Medications With Weight Loss Side Effects
Several prescription medications can cause weight loss as a side effect, either unintended or as a primary therapeutic benefit. In the UK, weight loss medication is available through both NHS specialist services and private prescribing for eligible adults. Understanding which medications support weight management—and how they work—can help you make informed decisions about your health. This guide covers prescription weight loss options, how they function, and what to expect.
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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.
Prescription Weight Loss Medications in the UK
GLP-1 receptor agonist injections are the primary prescription medications licensed specifically for weight management in the UK. These include Wegovy (semaglutide), Mounjaro (tirzepatide), and Saxenda (liraglutide). GLP-1 medications work by mimicking a natural gut hormone that regulates appetite and blood sugar, helping you feel fuller on smaller portions. In clinical trials, adults lost an average of 14–21% of their body weight over several months, though individual results vary significantly.
- Wegovy (semaglutide): Once-weekly injection, doses from 0.25 mg to 2.4 mg. STEP trial data showed approximately 15% average weight loss at 68 weeks.
- Mounjaro (tirzepatide): Once-weekly injection, doses from 2.5 mg to 15 mg. SURMOUNT trial data showed 15–21% average weight loss depending on dose.
- Saxenda (liraglutide): Daily injection, maximum 3 mg. Also a GLP-1 agonist with evidence for sustainable weight loss.
All three are prescription-only and require a clinical assessment before being prescribed. Access varies by region—some NHS areas have limited availability, and waiting lists can be lengthy. Private prescribing through registered UK services offers faster access without NHS referral.
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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 and How to Access Weight Loss Medication
In the UK, you may be eligible for prescription weight loss treatment if you have a BMI of 30 or above, or 25–29.9 with a weight-related health condition such as type 2 diabetes, high blood pressure, or sleep apnoea. For adults of South Asian, Chinese, Black African, or Black Caribbean heritage, lower BMI thresholds apply due to higher health risks at lower weights.
Access routes include NHS tier 3 specialist weight management services (requiring GP referral) or private online prescribing following a clinical assessment. Your GP can advise on NHS availability in your area. Use our free BMI calculator to check your starting point, and the GLP-1 weight loss calculator to see projected outcomes at different doses. Alongside medication, sustainable weight management requires a reduced-calorie diet and increased physical activity—medication supports these efforts but does not replace them.
- 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 there other medications that cause weight loss as a side effect?
Yes. Some antidiabetic medications (metformin, SGLT2 inhibitors), antidepressants (bupropion), and stimulant medications can cause weight loss. However, GLP-1 injections are the primary prescription treatments specifically licensed for weight management in the UK. Always discuss any medication changes with your GP.
How quickly will I lose weight on Wegovy or Mounjaro?
Weight loss is gradual and individual. Clinical trials show average losses over several months. Most people do not see significant results in the first 4 weeks—your prescriber gradually increases your dose, and weight loss typically accelerates after several weeks. Sustainable loss (0.5–1 kg per week) is safer than rapid loss.
What are the most common side effects of GLP-1 weight loss injections?
Nausea, diarrhoea, constipation, and reduced appetite are very common, especially when starting or increasing doses. Most side effects ease over time. Serious side effects (severe stomach pain, allergic reactions) are rare but require immediate medical attention. Report all concerns to your prescriber.
Can I get Wegovy or Mounjaro on the NHS?
Availability varies significantly by region and NHS area. NICE has approved tirzepatide (Mounjaro) for NHS use through specialist weight management services, but access is limited and waiting lists can be very long. Private prescribing offers faster access. Check with your GP about local NHS options.
Do I need to diet and exercise while taking weight loss medication?
Yes. Prescription weight loss medications work best alongside a reduced-calorie diet and increased physical activity. Medication alone is not sufficient—these lifestyle changes are essential for sustainable results and long-term health benefits.
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.