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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Weight Loss Drugs Side Effects — What You Need to Know
If you're considering prescription weight loss medication in the UK, understanding potential side effects is essential. Weight management drugs like Mounjaro, Wegovy, and Saxenda are prescription-only medicines that may help adults lose weight alongside lifestyle changes — but they do come with side effects you should be aware of. This guide explains what to expect from UK-licensed weight loss medications, how they compare, and how to access them through the NHS or privately.
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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 Weight Loss Medications and Their Side Effects
Three prescription-only weight management medications are currently licensed in the UK: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). All three work by activating GLP-1 pathways in the body, helping reduce appetite and support feeling fuller after smaller meals.
Common side effects across all three medications include:
- Nausea (very common, especially when starting)
- Diarrhoea or constipation
- Vomiting
- Stomach pain or indigestion
- Fatigue or dizziness
- Injection site reactions (redness, bruising)
- Headache
- Hair loss (reported by some users)
Most side effects are mild to moderate and tend to improve as your body adjusts. However, some people may experience persistent nausea or stomach issues. Clinical trials show that side effects are often the reason people reduce their dose or stop treatment. Your prescriber will monitor you closely and can adjust your dose if side effects become problematic.
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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.
Serious Side Effects and NHS vs Private Access
While rare, serious side effects requiring urgent medical attention include severe stomach pain (possible pancreatitis), gallbladder problems, serious allergic reactions, or vision changes. These are uncommon but must be reported to your GP immediately.
In the UK, you can access weight loss medication through two routes: NHS specialist weight management services (tier 3/4) or private online prescribing following a clinical assessment. NHS access requires a GP referral and typically involves waiting lists — availability varies significantly by region. Private prescribing is faster but costs between £129–£320 monthly depending on dose and provider. Whichever route you choose, a licensed prescriber will assess whether medication is appropriate for your health and discuss potential side effects before prescribing. Use our GLP-1 weight loss calculator to explore potential outcomes with different doses, and our BMI calculator to check your current weight category for NHS eligibility.
- 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.
Which weight loss drug has the fewest side effects?
All three UK-licensed medications (Mounjaro, Wegovy, Saxenda) have similar side effect profiles. Nausea and digestive issues are common with all. Individual tolerance varies — some people experience minimal side effects whilst others find them significant. Your prescriber will help find the right medication and dose for you.
How long do weight loss drug side effects last?
Most side effects improve within 1–2 weeks as your body adjusts. Nausea typically settles as you progress through dose escalations. Some people experience persistent mild symptoms. If side effects remain bothersome after 4 weeks, inform your prescriber — dose adjustments or switching medications may help.
Can I take weight loss medication if I have other health conditions?
This depends on your specific conditions. Weight loss medications are not suitable for people with a history of pancreatitis, thyroid cancer, gallbladder disease, or severe gastroparesis. If you take diabetes medications or oral contraceptives, dose adjustments may be needed. Always discuss your full medical history with your GP or prescriber.
What's the difference between Mounjaro and Wegovy side effects?
Both cause similar common side effects (nausea, diarrhoea, stomach pain). Mounjaro is a dual GIP/GLP-1 agonist and Wegovy is GLP-1 only, but side effect profiles are comparable. Clinical trial data shows both are generally well-tolerated. Individual experiences vary — your prescriber can advise which might suit you.
How do I know if a side effect needs urgent attention?
Seek immediate medical attention for severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, throat), sudden vision changes, or severe chest pain. Contact your GP for persistent vomiting, severe diarrhoea, or side effects that won't improve after several weeks.
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.