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 Pills Side Effects — What You Need to Know
Weight loss pills prescribed in the UK for weight management may cause side effects, particularly when you first start taking them or when doses increase. The most common prescription weight loss medications available in the UK are GLP-1 receptor agonist injections like Wegovy (semaglutide) and Mounjaro (tirzepatide), which work by regulating appetite and how your body processes food. Understanding potential side effects helps you decide whether prescription weight loss is right for you and what to expect during treatment. Your GP or prescriber will discuss these with you before you start, and most side effects are temporary.
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Calculate Your BMI Instantly
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 Prescription Weight Loss Medication
The most frequently reported side effects of GLP-1 weight loss injections like Wegovy and prescription weight loss medication in general include:
- Nausea — the most common, often worse when starting or increasing doses, typically improves within days or weeks
- Diarrhoea or constipation — digestive changes are very common as these medications slow how food moves through your stomach
- Stomach pain or indigestion — mild to moderate abdominal discomfort, particularly after eating
- Vomiting — less common but may occur, especially if nausea is severe
- Loss of appetite — this is actually how the medication helps with weight loss, though it can make eating feel challenging
- Fatigue or dizziness — some people feel tired initially, usually settling within 1–2 weeks
These side effects are typically mild to moderate and tend to reduce over time as your body adjusts. Most people find side effects manageable and temporary, particularly if doses are increased gradually as recommended.
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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 When to Seek Help
Whilst common side effects are usually mild, you should seek immediate medical attention if you experience severe stomach pain spreading to your back (possible pancreatitis), serious allergic reactions (swelling of face, lips, or throat), gallbladder problems (sharp pain in the upper stomach area), or changes in vision (which may indicate complications if you have diabetes). These are rare but serious. Contact your GP or NHS 111 immediately if concerned. Additionally, if side effects are severely affecting your quality of life or not improving after 2–3 weeks, speak to your prescriber — you may need a dose adjustment or alternative. Never stop taking weight loss medication without medical guidance, as this can affect your treatment plan. Track your side effects and any other symptoms to discuss with your healthcare provider at follow-up appointments.
- 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.
How long do weight loss pill side effects last?
Most common side effects improve within 1–2 weeks as your body adjusts. Nausea and digestive symptoms typically reduce significantly after the first few doses. If side effects persist beyond 3–4 weeks or worsen, contact your prescriber.
Can I reduce side effects of weight loss medication?
Yes. Eating smaller, lower-fat meals helps reduce nausea and stomach pain. Stay hydrated, avoid rich foods, and take your injection on the same day each week. Gradual dose increases also help — your prescriber will increase slowly to minimise side effects.
Are weight loss pills safe for NHS weight loss programmes?
Yes, prescription weight loss medications approved in the UK (like Wegovy and Mounjaro) are safe when prescribed by your GP or a specialist weight management service following clinical assessment. Use our free BMI calculator to check if you may be eligible for NHS or private prescription weight loss treatment.
What's the difference between Wegovy side effects and other weight loss injections?
Wegovy (semaglutide) and Mounjaro (tirzepatide) have similar common side effects — nausea, diarrhoea, constipation. Mounjaro uses a dual GIP and GLP-1 mechanism which may produce slightly different responses between individuals, but side effect profiles are broadly comparable.
Should I stop taking weight loss pills if I get side effects?
Do not stop without consulting your prescriber. Most side effects improve on their own. Your GP may adjust your dose, recommend dietary changes, or switch you to a different medication. Stopping abruptly without guidance can affect your weight loss progress and health plan.
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.