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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Mounjaro and Wegovy: Pain Under the Ribs Explained
Pain under the ribs while taking Mounjaro or Wegovy is a recognised side effect that can occur with both GLP-1 and dual GIP/GLP-1 medications. Understanding the difference between these two injection types, how they work, and what to expect can help you make an informed decision with your prescriber. This guide explains what causes rib pain, how Mounjaro and Wegovy compare, and when to seek medical attention. Always consult your GP before starting either medication.
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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.
How Mounjaro and Wegovy Work Differently
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist — it activates two natural hormone pathways that regulate appetite and digestion. This dual mechanism may be more potent at slowing stomach emptying, which can increase the likelihood of stomach distension and rib cage discomfort, particularly during the dose titration phase. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single hormone pathway. Clinical trial data shows Mounjaro users experienced slightly higher rates of gastrointestinal side effects overall compared to Wegovy users in SURMOUNT vs STEP trials, though individual experiences vary widely.
- Mounjaro: more potent gastric slowing effect initially
- Wegovy: single mechanism, generally slightly gentler on digestion
- Both slow gastric emptying, causing nausea, bloating, and rib pain
- Pain typically improves as the body adapts over weeks
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If your BMI suggests you're overweight or obese, explore our GLP-1 calculators to project potential weight loss on Mounjaro or Wegovy.
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Rib Pain: Causes and When to Seek Help
Pain under the ribs with either medication usually stems from abdominal bloating and gas accumulation due to slowed digestion. Your stomach expands, pressing against the rib cage, causing discomfort or pain. This is most common in the first 4–8 weeks but can persist longer in some users. However, severe or persistent pain spreading to the back may indicate pancreatitis — a rare but serious side effect requiring urgent medical attention.
- Mild rib discomfort: normal, typically settles within weeks
- Severe or spreading pain: contact your GP or call 111 immediately
- Pain with vomiting and fever: seek emergency care
- Keep your prescriber informed of all symptoms
- 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 rib pain more common with Mounjaro than Wegovy?
Clinical trials suggest Mounjaro users experience slightly higher rates of gastrointestinal side effects overall due to its dual mechanism, but individual variation is significant. Some Wegovy users experience rib pain too. Your personal experience depends on your individual physiology, dose, and how quickly your body adapts. Both medications can cause this side effect.
Can I switch from Mounjaro to Wegovy if I have rib pain?
Yes, switching between medications is possible but should only be done under your prescriber's guidance. Some users find one medication suits them better than the other. Your GP or prescriber can assess whether switching is appropriate and manage any dose changes safely.
How long does rib pain typically last on these injections?
Most users experience peak gastrointestinal side effects during the first 4–8 weeks while their body adapts. Pain under the ribs often improves significantly after this adjustment period. However, some users experience longer-lasting discomfort. Staying hydrated, eating smaller meals, and using our free progress tracker can help you monitor patterns.
What's the difference between Mounjaro and Wegovy side effects?
Both cause similar side effects like nausea, diarrhoea, and abdominal pain, but Mounjaro's dual GIP/GLP-1 mechanism may cause slightly more intense initial gastrointestinal effects. Wegovy's single GLP-1 mechanism is often described as marginally gentler, though this varies by individual. Your prescriber can discuss which suits your medical history.
Should I stop Mounjaro or Wegovy if I have rib pain?
Do not stop without consulting your GP. Mild-to-moderate rib discomfort typically settles with time and usually does not require stopping treatment. However, severe pain, especially if spreading to your back, requires urgent medical evaluation. Your prescriber may suggest dose adjustments or supporting strategies rather than stopping treatment entirely.
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