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
Achieving Weight Loss Over 4 Months With Prescription Medication
Losing weight over a 4-month period is achievable for many UK adults, especially with the right support and approach. If you're considering prescription weight management options, understanding what's available through the NHS or privately, who's eligible, and realistic timescales can help you make informed decisions. The UK now has several licensed medications to support weight loss, alongside lifestyle changes like diet and exercise. This guide covers medication options, eligibility, and what to expect during a 4-month period.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
UK Prescription Weight Loss Medications Available
Three prescription medications are currently licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). These are GLP-1 and GIP/GLP-1 receptor agonists—medications that work by reducing appetite and helping you feel fuller after meals.
Mounjaro is a dual GIP and GLP-1 agonist dosed once weekly, ranging from 2.5 mg to 15 mg. Clinical trials showed an average weight loss of 15–21% of body weight over 72 weeks. Wegovy is a GLP-1 agonist also dosed once weekly, with an average weight loss of around 15% of body weight. Saxenda is a once-daily GLP-1 injection.
Eligibility generally requires a BMI of 30 kg/m² or above, or 27–29.9 kg/m² with weight-related health conditions 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 may apply. All medications require a prescription and clinical assessment—individual results vary significantly.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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 vs Private Access in the UK
NHS access: Weight loss medications are available through specialist tier 3 and 4 weight management services, typically following a GP referral. However, availability varies greatly by region, and waiting times can be lengthy—sometimes months to years—depending on your local Integrated Care Board (ICB). Not all areas offer all medications.
Private access: Online prescribing services allow UK adults to access prescription weight loss medication after a clinical assessment, without NHS referral. These are available from GPhC-registered pharmacies. Costs typically range from £129–£320 per month depending on dose and medication.
To explore eligibility, start by speaking with your GP. They can assess your individual health, discuss options, and advise whether prescription support is appropriate for you. Use our free BMI calculator above to check where you currently stand, and our GLP-1 weight loss calculator to estimate potential outcomes at different doses over your 4-month timeframe.
- 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 much weight can I realistically lose in 4 months with prescription medication?
Results vary significantly between individuals. Based on clinical trial data, GLP-1 medications typically produce gradual weight loss over time. At 4 months (16 weeks), you might expect a modest percentage loss, building towards the average 15–21% seen over a full year. Weight loss is most effective when combined with reduced-calorie diet and increased physical activity.
What is the difference between Mounjaro and Wegovy?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist with doses up to 15 mg weekly, whilst Wegovy (semaglutide) is a GLP-1 agonist with a maximum dose of 2.4 mg weekly. Clinical trials showed slightly higher average weight loss with Mounjaro (15–21%) compared to Wegovy (around 15%), but individual results vary. Both are once-weekly injections.
Do I need to be referred to the NHS to access weight loss medication?
Access varies. Through the NHS, you typically need a GP referral to a specialist weight management service, with potential waiting lists. Through private GPhC-registered online services, you can access prescription medication after a clinical assessment without NHS referral. Your GP can advise which route is most appropriate for you.
What side effects should I expect with these medications?
Common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain—typically more pronounced when starting or increasing doses. Most people adapt within a few weeks. Serious side effects are rare but include severe stomach pain (possible pancreatitis) or allergic reactions. Always report unexpected symptoms to your prescriber.
Will my weight come back after I stop taking the medication?
Weight management is typically ongoing. Clinical evidence suggests that without continued medication and lifestyle support, weight loss can be regained. These medications are usually intended as longer-term support, not short-term fixes. Your prescriber will discuss ongoing management plans with you based on your individual needs and health goals.
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.