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 Your Second Week on Mounjaro
Your second week on Mounjaro (tirzepatide) marks the continuation of your initial 2.5 mg starting dose. This is a critical adjustment period where your body is adapting to the medication. During week two, you're establishing your injection routine and may notice how side effects evolve. Understanding what's normal during this phase helps you stay confident with your treatment. If you're prescribed Mounjaro through an NHS specialist weight management service or private prescriber in the UK, tracking your experience during these early weeks provides valuable information for your follow-up appointment.
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 Happens During Week Two on Mounjaro
Your second week on Mounjaro involves your second 2.5 mg injection, delivered via KwikPen once weekly. By week two, you've moved past the novelty of injecting and can focus on how your body is responding.
- Appetite changes: Many people report reduced hunger and food cravings intensifying during this period as the medication accumulates in your system
- Stomach symptoms: Nausea, constipation, or diarrhoea may peak during week two as your digestive system adjusts to the dual GIP and GLP-1 receptor agonist action
- Injection site: Rotating between stomach, thigh, and upper arm helps prevent irritation — continue alternating each week
- Energy levels: Some people experience fatigue; others feel energised. This stabilises over the following weeks
Keep notes on how you feel — this information supports your GP or prescriber in deciding whether to continue, maintain, or adjust your dose at week four.
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.
Managing Side Effects and NHS Support
Most people find side effects manageable during week two with simple strategies. Eat smaller, more frequent meals if nausea persists, avoid greasy foods, and stay hydrated. Anti-nausea medication can be prescribed by your GP if needed — don't hesitate to ask. If you're prescribed through an NHS specialist weight management service (tier 3), your clinical team will have given you contact details for support between appointments. Private prescribers via online clinics also provide guidance for troubleshooting. Remember, side effects typically reduce by weeks four to six as your body acclimates. If you experience severe stomach pain, serious allergic reactions, or changes in vision, seek immediate medical attention and inform your healthcare team that you're on Mounjaro.
- 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 nausea normal in the second week on Mounjaro?
Yes, nausea is very common during week two as your body adjusts to tirzepatide. It typically peaks in the first two to three weeks and often improves by week four or five. Eating smaller meals, staying hydrated, and avoiding fatty foods helps. If severe, contact your GP who can prescribe anti-nausea medication.
Should I change my Mounjaro dose if I feel side effects in week two?
No. Week two is part of the adjustment phase — your prescriber will assess whether to maintain, pause, or adjust your dose at week four, not before. Side effects are normal and often ease naturally. Only change your dose on your prescriber's guidance.
Can I use the BMI calculator during my Mounjaro treatment?
Absolutely. Our free BMI calculator helps you track your starting BMI and monitor progress as you progress through your Mounjaro doses. Calculating your BMI at the start, week eight, and every four weeks gives you measurable data alongside the scales.
How does Mounjaro differ from Wegovy if I'm considering alternatives?
Mounjaro (tirzepatide) is a dual GIP and GLP-1 agonist, whereas Wegovy (semaglutide) is a GLP-1-only agonist. Clinical trials show Mounjaro resulted in slightly higher average weight loss (~21% vs ~15% body weight), but both require a prescription and clinical assessment. Your prescriber will advise which suits you.
What if I miss my second Mounjaro injection?
If you miss your week two dose, inject within four days of the missed date and continue your normal weekly schedule from there. If more than four days have passed, skip that dose and take your next injection on your normal scheduled day. Never double-dose.
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.