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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What are the safest weight loss drugs in the UK?
Weight loss medication can be a helpful tool for managing excess weight, but safety depends on clinical assessment, individual health, and medical supervision. In the UK, several prescription weight loss drugs are licensed and approved by the MHRA. The safest options are those prescribed through NHS weight management services or following a clinical assessment with a qualified healthcare professional. This guide explains the main prescription weight loss medications available, how they work, and what safety considerations matter. Always consult your GP before starting any weight loss medication—they can assess whether a drug is suitable for your health profile.
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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 prescription weight loss medications
Three main prescription weight loss drugs are currently licensed in the UK for adults with obesity or overweight plus weight-related health conditions:
- Mounjaro (tirzepatide) — a once-weekly injection that works on two hormone pathways (GIP and GLP-1 receptors). Clinical trials showed participants lost an average of 15–21% of body weight alongside a reduced-calorie diet and increased physical activity.
- Wegovy (semaglutide) — a once-weekly GLP-1 injection. The STEP clinical trials demonstrated average weight loss of around 15% at the maximum dose of 2.4 mg over 68 weeks.
- Saxenda (liraglutide) — a daily GLP-1 injection. It has a longer safety record in the UK and may be offered through some NHS weight management services.
All three are prescription-only medicines requiring a clinical assessment to confirm eligibility. Safety is ensured through medical evaluation, dose escalation protocols, and regular monitoring by healthcare professionals.
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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
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How to access safe weight loss medication in the UK
Access to prescription weight loss medication in the UK follows two main routes. NHS access: Ask your GP for a referral to a tier 3 specialist weight management service. These teams assess your health, monitor you throughout treatment, and manage any side effects. Waiting lists vary by region but specialist services provide ongoing support.
Private access: Licensed online prescribers can assess your suitability for weight loss medication following a clinical consultation. You must be eligible (BMI ≥30 or ≥27 with weight-related conditions) and honest about your medical history. Always use a GPhC-registered pharmacy to dispense your prescription. Track your progress with our free progress tracker to monitor weight changes and health improvements over time.
Safety depends on three key factors: medical assessment (your GP or prescriber reviews your full health history), dose escalation (all weight loss medications start at the lowest dose and increase gradually over weeks), and monitoring (regular check-ins to assess effectiveness and manage side effects). Calculate your current BMI and weight management targets using our free BMI calculator to discuss personalised options with your GP. Common side effects include nausea, diarrhoea, and constipation—these usually improve as your body adjusts. Serious side effects are rare when medication is prescribed and monitored appropriately. The safest approach combines prescription medication with lifestyle changes: reduced-calorie diet, increased physical activity, and behaviour 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.
Are weight loss injections safe?
Yes, when prescribed and monitored by a healthcare professional. Mounjaro, Wegovy, and Saxenda are MHRA-approved and extensively clinically tested. Safety requires medical assessment to confirm eligibility, gradual dose increases, and monitoring for side effects. Always use a licensed prescriber and regular GP check-ins.
What's the difference between Mounjaro and Wegovy safety?
Both are safe when used appropriately. Mounjaro uses a dual GIP+GLP-1 mechanism and showed slightly higher average weight loss in trials. Wegovy (semaglutide) is a GLP-1 agonist with a longer UK safety history. Neither is inherently "safer"—suitability depends on individual health, other medications, and prescriber assessment.
Can I get weight loss medication through the NHS?
Yes. Ask your GP for a referral to a tier 3 weight management service. NHS access requires BMI ≥30 (or ≥27 with weight-related conditions) and assessment by a specialist team. Availability varies by region and waiting lists can be long, but NHS services provide ongoing monitoring and support.
What side effects should I expect from weight loss drugs?
Most common side effects are nausea, diarrhoea, vomiting, and constipation—these usually improve as your body adjusts. Serious side effects (severe stomach pain, gallbladder problems, allergic reactions) are rare. Your prescriber will discuss all side effects before you start and monitor you throughout treatment.
How long do I need to take weight loss medication?
Duration varies. Some people take medication for 12–24 months, others longer. Your prescriber will assess whether to continue based on your progress and health. Weight loss medication works best alongside sustained dietary changes and physical activity. Use our GLP-1 weight loss calculator to project potential results and discuss realistic timelines with your healthcare team.
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.