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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Side Effects Of Weight Loss Pills — What You Need to Know
Weight loss pills and injections approved in the UK can help with weight management for eligible adults, but like all prescription medications, they may cause side effects. Understanding what to expect helps you make an informed decision with your GP. This guide covers the common side effects of UK-licensed weight loss medication options, how they compare, and when to seek medical attention. Most side effects are mild and temporary, but it's important to know the full picture before starting any 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-Licensed Weight Loss Medications and Their Side Effects
Three prescription-only weight loss medications are licensed in the UK for weight management: Mounjaro (tirzepatide), Wegovy (semaglutide), and Saxenda (liraglutide). Each works through similar hormone pathways but has slightly different side effect profiles.
Mounjaro and Wegovy are once-weekly injections with broadly comparable side effects. The most common include nausea, diarrhoea, constipation, and stomach discomfort — particularly when starting or increasing doses. Clinical trials show these typically ease after a few weeks as your body adjusts. Less common side effects include headaches, fatigue, dizziness, and injection site reactions.
Saxenda (liraglutide) is a daily injection with a similar side effect pattern. Because it requires daily injections rather than weekly, some people find compliance more challenging, though side effects are comparable.
Serious side effects are rare but include severe stomach pain (possible pancreatitis), gallbladder problems, and serious allergic reactions. You must report any persistent or severe symptoms to your GP or healthcare provider immediately.
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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 Managing Side Effects
In the UK, prescription weight loss medication is available through NHS specialist tier 3 weight management services (following GP referral) or private online prescribing following a clinical assessment. Side effect profiles are the same regardless of where you access the medication.
Most people experience the worst side effects in the first 1–2 weeks of starting or increasing their dose. Practical steps help: eating smaller, lighter meals, staying hydrated, and taking the injection at a consistent time each week. Your prescriber may also adjust your dose schedule if side effects are troublesome.
Important: If you experience severe stomach pain, changes in vision, gallbladder pain, or signs of an allergic reaction (swelling of face, lips, or tongue), seek urgent medical attention. Your GP or healthcare provider can assess whether the medication is right for you and discuss managing side effects alongside a reduced-calorie diet and increased physical activity.
- 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.
What are the most common side effects of weight loss pills in the UK?
Nausea, diarrhoea, constipation, and stomach pain are very common, especially when starting or increasing doses. These typically ease within 1–2 weeks. Headaches, fatigue, and dizziness are less common. Most people adjust well, though individual experiences vary.
Are weight loss injection side effects permanent?
No. Most side effects are temporary and resolve as your body adjusts to the medication. Some people experience persistent mild nausea, but severe or lasting side effects are uncommon. If you have concerns, speak to your prescriber — they can help adjust your dose or management plan.
Which prescription weight loss medication has fewer side effects?
Mounjaro, Wegovy, and Saxenda have similar side effect profiles. No single medication is definitively "safer" — individual tolerance varies. Your prescriber will assess which is most suitable for you based on your health history and preferences. Some people tolerate one better than another.
When should I contact my GP about weight loss pill side effects?
Contact your GP if you experience severe or persistent stomach pain, changes in vision, gallbladder pain (upper stomach area), signs of an allergic reaction, or any side effects that significantly impact your daily life. Most mild side effects do not require medical attention but should be reported at your next review.
Can I use the BMI calculator to check if I'm eligible for weight loss medication?
Yes. Use our free BMI calculator above to determine your current BMI. UK eligibility requires a BMI of 30 or above, or 27–29.9 with weight-related health conditions like diabetes or high blood pressure. Your GP or prescriber will assess your full health picture during a clinical consultation.
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.