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
What to Expect in Week 2 on Mounjaro
By week 2 on Mounjaro, you're in the early adjustment phase of your treatment. Most people are still on the starting dose of 2.5 mg, and this is when side effects like nausea often peak before gradually improving. If you're wondering how Mounjaro compares to Wegovy at this stage, or whether your experience is normal, this guide covers what happens during the second week, how the two medications differ in their early phases, and when you can expect changes. Use our free GLP-1 weight loss calculator above to understand projected outcomes, and our BMI calculator to track your baseline health context as you begin your weight management journey.
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.
Mounjaro vs Wegovy: Mechanism and Early Response Differences
Week 2 marks an important phase where both Mounjaro and Wegovy begin affecting your appetite and hunger signals, but they work differently. Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two hormone pathways simultaneously — this dual mechanism may produce more pronounced appetite suppression for some people, though not universally. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway.
Clinical trial data shows this distinction matters: SURMOUNT-1 (Mounjaro) showed average weight loss of around 15% at the 5 mg dose by week 8, whilst STEP-1 (Wegovy) demonstrated approximately 14.9% average weight loss at the maximum 2.4 mg dose by week 68. At week 2, you're unlikely to see significant weight loss yet — both medications typically take 4–6 weeks to show measurable results. Early side effects like nausea and constipation are more common in week 1–3 as your body adjusts to the Mounjaro KwikPen injections or Wegovy pen.
Individual response varies widely. Some people notice reduced appetite immediately; others take longer to feel the medication's effects. Your prescriber will assess your tolerability before increasing your dose — minimum 4 weeks on each dose is standard practice in the UK.
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.
Side Effects at Week 2 and Which Medication May Suit You Better
At week 2, nausea peaks for many people on either medication, often improving by week 3–4 as your body adapts. Constipation, diarrhoea, stomach pain, and reduced appetite are also common. These side effects are similar between Mounjaro and Wegovy, though some people report that the dual mechanism of Mounjaro produces more intense early nausea — but this is individual and not assured.
Choosing between them isn't about which is "better" — both require UK prescription and clinical assessment from a licensed prescriber. Your suitability depends on your medical history, other medications (particularly diabetes treatments or contraceptives), and how your body responds. The NHS NICE guidance and your prescriber will determine which medication is appropriate for you. Many people find that if one doesn't suit them, switching to the other is an option. Track your weight progress using our free progress tracker alongside your 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.
Will I lose weight by week 2 on Mounjaro?
Unlikely. Most weight loss begins after 4–6 weeks. Week 2 is about adjusting to the medication and managing side effects. You're on the starting dose (2.5 mg), which is designed for tolerability, not maximum effect. Measurable weight loss typically appears after your first dose increase to 5 mg around week 5.
Is Mounjaro or Wegovy better for early-stage side effects?
Both cause similar side effects in week 2 — nausea, constipation, reduced appetite. Some people report Mounjaro's dual GIP+GLP-1 mechanism produces stronger early nausea, but individual variation is significant. Your prescriber will help you manage side effects regardless of which medication you're on.
Why is Mounjaro dose titration slower than Wegovy?
Mounjaro increases by 2.5 mg increments every 4 weeks (2.5 → 5 → 7.5 mg etc.), whilst Wegovy increases by smaller amounts (0.25 → 0.5 → 1 mg). This slower titration allows your body to adjust to Mounjaro's dual mechanism. Both medications follow safety protocols; the schedules simply reflect different dosing strategies.
Should I expect better results from Mounjaro than Wegovy by week 2?
No — by week 2, neither medication shows significant weight loss differences. Both are in their early adjustment phase. Clinical trials show Mounjaro averages slightly higher long-term weight loss (~15–21% vs ~14.9%), but individual results vary widely. Your prescriber will choose based on your health profile, not predicted outcomes.
Can I switch from Mounjaro to Wegovy if week 2 side effects are unbearable?
Switching medications is possible, but week 2 is extremely early. Most side effects improve by week 3–4 as your body adjusts. Discuss any concerns with your prescriber before considering a switch — they can advise on managing side effects or adjusting your routine rather than changing medication.
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.