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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Understanding Cancer-Related Weight Loss and UK Treatment Options
Unintended weight loss during cancer treatment is a common concern for many UK patients. Weight loss can occur due to the cancer itself, side effects of treatment, reduced appetite, or changes in metabolism. If you're experiencing rapid or unexplained weight loss, it's essential to discuss this with your GP or oncology team immediately, as they can assess the underlying cause and recommend appropriate support. This guide explains why cancer-related weight loss happens and explores prescription weight management options available to eligible adults in the UK who are managing their health during or after treatment.
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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 Prescription Weight Management Options
For adults in the UK managing their weight during or after cancer treatment, prescription medications may be considered as part of a comprehensive health plan alongside medical supervision. Three GLP-1 and dual-action receptor agonist medications are licensed for weight management in the UK:
- Mounjaro (tirzepatide) — a dual GIP and GLP-1 agonist injected once weekly, with clinical trials showing average weight loss of 15–21% of body weight
- Wegovy (semaglutide) — a GLP-1 agonist injected once weekly, with evidence of approximately 15% average weight loss
- Saxenda (liraglutide) — a GLP-1 agonist injected daily, another licensed option
These are prescription-only medicines. Eligibility typically requires a BMI of 30 kg/m² or above, or BMI 27–29.9 with weight-related health conditions. Cancer patients should always consult their oncology team and GP before considering any weight management medication, as individual health circumstances vary significantly.
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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.
NHS vs Private Access and Next Steps
In the UK, prescription weight loss medication can be accessed through NHS specialist tier 3 weight management services (via GP referral) or private prescribing (following an online clinical assessment). NHS access varies by region and may involve waiting lists; private options are faster but involve costs. If you're undergoing cancer treatment, your GP or oncology team should be your first point of contact — they understand your full medical history and can advise whether prescription weight management is appropriate for you. Use our free BMI calculator to check your current BMI, and discuss eligibility with your healthcare provider. Your GP can also refer you to NHS dietitian services, which provide evidence-based nutritional support during and after cancer treatment.
- 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.
Is gradual weight management during cancer treatment normal?
Some weight loss during cancer treatment is common due to reduced appetite, treatment side effects, nausea, or the cancer itself affecting metabolism. However, any significant or unexplained weight loss should be discussed with your oncology team immediately. They can assess whether it's treatment-related, nutritional, or requires intervention. Your medical team is best positioned to advise.
Can I use weight loss injections like Mounjaro during cancer treatment?
Prescription weight loss medications like Mounjaro and Wegovy are only suitable in specific circumstances and require careful assessment. Your oncology team and GP must be involved in any decision, as cancer medications and treatments may interact with weight loss injections. Never start any new medication without explicit approval from your cancer care team.
What's the difference between cancer-related weight loss and intentional weight loss?
Cancer-related weight loss is often unintended and caused by the disease or treatment side effects. Intentional weight loss involves deliberate calorie reduction or lifestyle changes. Cancer patients experiencing unintended loss need medical supervision and nutritional support, not weight loss medications. Focus on maintaining adequate nutrition during treatment.
How fast should weight loss happen safely?
The NHS recommends a safe rate of 0.5–1 kg (1–2 lbs) per week for sustainable, health-protective weight loss. Faster loss can result in muscle loss and nutritional deficiencies. If you're losing weight rapidly without intending to, this is a medical concern — contact your GP or oncology team.
Where can I get nutritional support if I'm losing weight during cancer treatment?
Ask your GP for a referral to NHS dietitian services, which specialise in cancer nutrition. Your hospital may also have an oncology dietitian as part of your care team. They can provide practical advice on managing appetite, nutrition, and side effects. This is the recommended first step before considering any medications.
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.