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
Abdominal Pain With Weight Loss — When It's Serious
Abdominal pain can occur during weight loss, especially when using prescription weight management medication like GLP-1 injections. While some stomach discomfort is common, it's important to know when pain is expected and when you should contact your GP. This guide covers the causes of abdominal pain during weight loss, what's normal with weight loss medication, red flag symptoms requiring urgent medical attention, and practical steps to manage discomfort safely while pursuing your health goals through prescription weight loss uk or lifestyle changes alone.
10,000+ calculations today · NHS-aligned thresholds · 100% free
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.
What Causes Abdominal Pain During Weight Loss?
Stomach pain during weight loss happens for several reasons. When you reduce your calorie intake or use GLP-1 weight loss medication like Wegovy (semaglutide), your digestive system adjusts. GLP-1 injections work by slowing how quickly food leaves your stomach — this delayed gastric emptying can cause bloating, cramping, and mild discomfort, especially in the first few weeks. Common causes include:
- Reduced food intake: Your stomach may feel cramped from smaller portions
- Medication side effects: GLP-1 medications commonly cause nausea, indigestion, and stomach pain as your body adapts
- Dietary changes: Switching to healthier, higher-fibre foods can cause temporary bloating and gas
- Increased physical activity: More exercise puts stress on abdominal muscles, especially if you're not accustomed to it
- Dehydration: Not drinking enough water during weight loss can worsen cramping
Most mild abdominal discomfort resolves within 1–2 weeks as your body adjusts to reduced food intake or medication. Using our free BMI calculator can help you set realistic weight loss targets that feel sustainable without excessive dietary shock.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care — from a UK-regulated online pharmacy.
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.
Red Flag Symptoms — When to Contact Your GP
While mild stomach pain is common during weight loss, certain symptoms require urgent medical attention. Contact your GP or seek emergency care immediately if you experience severe, persistent abdominal pain spreading to your back — this may indicate pancreatitis, a serious inflammation of the pancreas. Other red flags include vomiting that lasts more than a few hours, signs of gallbladder problems (sharp pain under the right rib cage), inability to keep fluids down, blood in vomit or stools, fever, or sudden yellowing of skin or eyes. If you're taking prescription weight loss medication like Wegovy or Mounjaro through NHS weight loss programmes or private prescribing, always inform your GP of your medication when describing symptoms. Keep track of your weight loss progress and any symptoms using our free progress tracker — this helps your healthcare provider understand your journey.
- 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 abdominal pain normal when starting Wegovy or Mounjaro?
Yes, mild stomach pain, nausea, and indigestion are very common side effects when starting GLP-1 or GLP-1/GIP medication. Most people experience some digestive discomfort in the first 2–4 weeks, which typically improves as your body adapts. Nausea affects more than 1 in 10 people on these medications. However, severe pain is not normal — contact your GP if pain is persistent or severe.
How long does abdominal pain last with weight loss medication?
Most side effects from GLP-1 medication improve within 1–4 weeks. However, some people experience ongoing mild discomfort throughout treatment. If stomach pain persists beyond 4 weeks at the same dose, inform your prescriber — they may adjust your dose or provide additional support. Individual responses vary significantly.
Can diet changes reduce stomach pain during weight loss?
Yes. Eat smaller, more frequent meals rather than three large ones. Choose bland, easily digestible foods initially — avoid fatty, spicy, or high-fibre foods when starting medication. Stay well hydrated with water throughout the day. Eating slowly and chewing thoroughly also helps. As your body adapts, you can gradually reintroduce a wider variety of foods.
What should I do if I have severe abdominal pain with weight loss medication?
Seek immediate medical attention if you have severe pain spreading to your back, persistent vomiting, inability to keep fluids down, fever, or yellowing of skin. These may indicate pancreatitis or gallbladder problems — serious conditions requiring urgent assessment. Do not delay contacting your GP or attending A&E if symptoms are severe.
Is abdominal pain the same whether I lose weight through diet or medication?
Not entirely. Dietary weight loss typically causes mild discomfort from reduced food intake and increased fibre. Medication like Wegovy causes additional symptoms from slowed gastric emptying — nausea and indigestion are more common with injections. Most people find managing their diet alongside medication works better than either approach alone.
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.