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 Week 2 Side Effects vs Wegovy
Week 2 is when most people experience the first side effects of Mounjaro, particularly if they've just started the 2.5 mg starting dose. This guide compares how Mounjaro (tirzepatide, a dual GIP and GLP-1 receptor agonist) and Wegovy (semaglutide, a GLP-1 agonist only) differ in early-stage side effects. Both are prescription-only weight management injections available in the UK following clinical assessment. Understanding what to expect can help you manage symptoms and determine whether either is suitable for you.
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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 Week 2 Side Effects Compare Between Mounjaro and Wegovy
Mounjaro week 2: The most commonly reported side effects at this stage are mild to moderate nausea (affecting around 1 in 5 users), and loose stools or mild diarrhoea. Stomach discomfort and reduced appetite are also typical. Most people find these effects settle within a few days or by week 3 as the body adjusts to the dual GIP and GLP-1 mechanism.
Wegovy week 2: Nausea is similarly common, though some users report it may be slightly less intense in the first two weeks compared to Mounjaro. Diarrhoea and constipation are frequent, as is indigestion. The GLP-1-only mechanism works differently, so individual experiences vary considerably.
- Both: nausea, appetite suppression, mild stomach pain
- Mounjaro: dual receptor activation may trigger slightly more intense early nausea
- Wegovy: side effects often build more gradually across the dose escalation phase
- Both improve significantly by week 3–4 in most cases
The SURMOUNT and STEP clinical trials showed that side effects are generally mild to moderate and transient—meaning they typically resolve as your body adapts. Prescribers can adjust dosing schedules if early effects are problematic.
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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.
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What Influences Your Week 2 Experience
Your individual side effect profile depends on several factors: baseline stomach sensitivity, food intake, hydration levels, and whether you're taking the injection on an empty stomach. Some people with slower gastric emptying may experience more pronounced nausea.
Mounjaro UK prescriptions are managed through NHS specialist weight management services or private online prescribing. Your prescriber will assess whether either medication is suitable based on your health history and tolerability expectations. If week 2 side effects are significant, your GP or prescriber may recommend staying on the starting dose longer before escalating, or switching to the alternative if one was the original choice.
Use our free GLP-1 weight loss calculator to project realistic outcomes and understand what to expect across the full dose range—side effects and results improve significantly by weeks 3–4 as you progress through the dose titration schedule.
- 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.
Are Mounjaro side effects worse in week 2 than Wegovy?
Not necessarily worse—just different. Mounjaro's dual mechanism (GIP + GLP-1) may trigger slightly more intense early nausea for some users, but SURMOUNT trial data shows most side effects resolve by week 3–4. Wegovy side effects often build more gradually. Individual tolerance varies significantly regardless of which medication you take.
What should I eat in week 2 if I have nausea?
Eat small, frequent meals of bland, easily digestible foods: toast, crackers, rice, chicken, banana, or soup. Avoid fatty, spicy, or rich foods. Stay hydrated with water or electrolyte drinks. Never skip meals—low blood sugar can worsen nausea. If nausea persists beyond week 3, contact your prescriber.
Can I stop Mounjaro if week 2 side effects are severe?
Yes—you can pause or stop at any time. However, most people find side effects improve naturally by day 7–10. Contact your prescriber before stopping rather than discontinuing immediately. They may suggest staying on the starting dose longer, adjusting timing, or discussing Wegovy as an alternative with a different mechanism.
Is it normal to have no side effects in week 2?
Yes, entirely normal. Not everyone experiences side effects, especially if they have a naturally slower gastric response or are already familiar with GLP-1 medications. Some people report mild appetite reduction with no nausea at all. There's wide individual variation—no side effects doesn't mean the medication isn't working.
How does the dose titration schedule affect side effects?
Mounjaro starts at 2.5 mg (week 1–4), then escalates to 5 mg, 7.5 mg, and up to 15 mg maximum. Starting low minimises shock to the system. Most prescribers keep you on each dose for 4 weeks, allowing your body to adjust before escalating. This schedule is why side effects typically peak in week 2–3 of each new dose, then settle.
Ready to take control of your weight loss journey?
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