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
Weight Loss Capsules Side Effects — What You Need to Know
Weight loss capsules and injections are prescription-only medicines available in the UK for eligible adults. The most commonly prescribed weight loss medications are GLP-1 receptor agonists like semaglutide (Wegovy), which work by reducing appetite and helping you feel fuller after smaller meals. While these medications can support weight management alongside diet and exercise, they do come with potential side effects that vary between individuals. Understanding what to expect helps you make an informed decision with your GP or prescriber.
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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.
Common Side Effects of Weight Loss Medications
Prescription weight loss capsules and injections may cause side effects, particularly during the first few weeks as your body adjusts. Very common side effects include nausea, diarrhoea, vomiting, constipation, and stomach pain — these affect more than 1 in 10 people taking GLP-1 medications. Common side effects (affecting up to 1 in 10 people) include indigestion, acid reflux, headache, fatigue, dizziness, and injection site reactions if using an injectable form.
Most side effects are mild to moderate and tend to improve as your body adjusts over the first few weeks of treatment. However, some people experience persistent nausea or digestive changes that may affect quality of life. This is why starting at a lower dose and gradually increasing helps your body adapt. Your prescriber will monitor your response and adjust your dose if side effects become bothersome. For weight management in the UK, your GP or specialist can assess whether these side effects are manageable for you personally.
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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.
When to Seek Medical Attention
While most side effects are minor, some symptoms require urgent medical attention. Seek immediate care if you experience severe, persistent stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, tongue, or throat), changes in vision, or signs of gallbladder problems such as sharp pain in your upper stomach area. These serious side effects are rare but do require prompt evaluation.
Your GP should review your suitability for weight loss medication before you start treatment and monitor you regularly while taking it. If you develop side effects that concern you, contact your prescriber rather than stopping medication abruptly. They can adjust your dose, switch you to a different medication, or provide support to manage symptoms. Always report all side effects honestly — your prescriber needs this information to help you safely.
- 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 tablets and injections?
Nausea, diarrhoea, vomiting, and constipation are the most common side effects of GLP-1 weight loss medications in the UK. These affect more than 1 in 10 people and typically improve within the first few weeks as your body adjusts. Starting at a lower dose helps minimise these effects.
Do side effects of weight loss capsules go away?
For most people, yes. Initial side effects like nausea and stomach discomfort usually improve within 2–4 weeks as your body adjusts to the medication. However, some people experience persistent digestive changes. Your prescriber can adjust your dose or explore alternatives if side effects don't improve.
Can you take weight loss medication if you have a sensitive stomach?
GLP-1 medications do affect digestion, so they may not be ideal if you have a sensitive stomach. However, your GP can assess your individual situation and decide whether the benefits outweigh the risks. Conditions like severe gastroparesis or IBS may make these medications unsuitable.
Is nausea from weight loss injections dangerous?
Nausea itself is not dangerous, though persistent nausea can affect your nutrition and quality of life. However, severe stomach pain (particularly if it spreads to your back) is a red flag and requires urgent medical attention as it may indicate pancreatitis.
How do I manage side effects while taking weight loss medication?
Start with smaller, frequent meals rather than large ones. Stay hydrated, eat slowly, and avoid fatty or heavy foods. Your prescriber may suggest over-the-counter remedies like ginger for nausea. Keep a symptom diary and report changes to your GP. Using our free BMI calculator and progress tracker helps you monitor whether the medication is working effectively despite any side effects you experience.
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.