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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Your First Week on Mounjaro: How It Compares to Wegovy
Starting your first week on Mounjaro is a significant step in your weight management journey. Whether you're beginning treatment with tirzepatide (Mounjaro) or considering how it compares to semaglutide (Wegovy), understanding what to expect can help you feel more prepared. Both are prescription-only, once-weekly GLP-1 type injections available in the UK, but they work slightly differently and may feel different during those early days. This guide explains the key differences and what typically happens in week one with either medication.
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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.
How Mounjaro and Wegovy Differ: Mechanism and Trial Data
Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist, meaning it activates two natural hormone pathways to help regulate appetite and support weight management. Wegovy (semaglutide) is a GLP-1 receptor agonist only, activating a single pathway. In clinical trials, this dual mechanism made a measurable difference: SURMOUNT-1 trial data showed participants on Mounjaro lost approximately 15–21% of their body weight, while STEP-1 trial data showed Wegovy users lost approximately 15% on average. Both trials involved reduced-calorie diets and increased physical activity alongside the medication.
- Mounjaro starting dose: 2.5 mg once weekly
- Wegovy starting dose: 0.25 mg once weekly
- Mounjaro escalation: increases every 4 weeks up to 15 mg maximum
- Wegovy escalation: increases every 2–3 weeks up to 2.4 mg maximum
Both require a UK prescription following clinical assessment. Your prescriber will consider your individual health profile, eligibility, and medical history to determine which is more suitable for you.
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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.
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What to Expect in Your First Week: Side Effects and Adjustments
In your first week on either medication, nausea is the most commonly reported side effect—often felt within the first 24–48 hours after your injection. This typically eases within a few days as your body adjusts. Other common first-week experiences include mild stomach discomfort, reduced appetite (which is intended), and occasional fatigue. Most people take their weekly injection on the same day and time each week, storing their Mounjaro or Wegovy KwikPen in the fridge between doses.
- Nausea often peaks on days 2–3, then improves
- Drink plenty of water and eat light, bland foods if nauseous
- Avoid rich, fatty, or spicy meals during week one
- Report severe pain, persistent vomiting, or allergic signs to your GP immediately
Use our free GLP-1 weight loss calculator to understand realistic expectations for your weight loss journey ahead.
- 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.
Can I switch from Wegovy to Mounjaro during week one?
No—switching medications requires your prescriber's guidance. Both require ongoing assessment and doses are adjusted over weeks, not days. If you're experiencing troublesome side effects in your first week, contact your GP or prescribing clinic rather than self-switching. Most side effects settle within 3–7 days.
Which is better for the first week—Mounjaro or Wegovy?
Neither is objectively "better"—suitability depends on your individual health profile, medical history, and NHS Nice criteria or private prescriber assessment. Mounjaro's dual mechanism may suit some; Wegovy's single pathway may suit others. Your prescriber will recommend the most appropriate option for you.
Will I lose weight in my first week on Mounjaro?
Not typically—the first week is about your body adjusting to the medication. Weight loss usually becomes apparent from week 2 onwards, but results vary significantly between individuals. Alongside the injection, a reduced-calorie diet and increased physical activity support sustainable weight loss. Track your progress with our free progress tracker.
What's the difference between Mounjaro's KwikPen and how Wegovy is delivered?
Both come as pre-filled injection pens (KwikPen for Mounjaro, pre-filled pen for Wegovy). Both are stored in the fridge, injected once weekly under the skin, and rotated between injection sites (stomach, thigh, upper arm). Administration is essentially the same—the difference is the medication inside.
Is the first week dose the same as later weeks?
Yes—week one is your starting dose. With Mounjaro, you begin at 2.5 mg and stay on this dose for 4 weeks before increasing. With Wegovy, you start at 0.25 mg and typically increase every 2–3 weeks. Your prescriber will guide your escalation schedule based on how you respond.
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.