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 From Your Mounjaro First Dose
Starting Mounjaro (tirzepatide) can feel daunting, but understanding what happens during your first dose makes the experience clearer. Your first injection will be 2.5 mg—the lowest starting dose—given as a once-weekly injection under the skin. Most people tolerate it well, though some experience mild nausea or digestive changes. This guide explains what the first few days and weeks involve, based on NHS and clinical guidance. If you're accessing Mounjaro through the NHS, you'll have been assessed by a specialist tier 3 weight management service. Use our free BMI calculator and GLP-1 weight loss calculator to track your baseline measurements before starting.
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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.
Your First Injection: What Happens During and After
Your first Mounjaro injection is straightforward. You'll use a pre-filled KwikPen device to inject 2.5 mg under your skin—typically into your stomach, thigh, or upper arm. The needle is small and the injection takes just a few seconds. Most people report minimal pain or discomfort at the injection site, though slight redness or bruising may appear and usually fades within hours.
What to expect immediately after:
- Mild injection site reactions are common—redness, swelling, or slight bruising at the injection point
- Some people feel slightly lightheaded immediately after, so sitting down for a few minutes is sensible
- Nausea may begin within hours of the injection, though it's often mild at this starting dose
- Most side effects are mild and temporary at the 2.5 mg dose
Keep the injection site clean and dry. Rotate your injection sites weekly to reduce irritation. If you experience severe pain, spreading redness, or signs of infection, contact your GP.
Clinically BackedWeight Loss
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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.
Free Weight Loss Tools & Guides
Everything you need to understand your weight, explore medication options, and track your progress — completely free.
The First Week: Side Effects and What's Normal
During your first week on Mounjaro, your body is adjusting to the medication. Nausea is the most common side effect, reported in clinical trials by more than 1 in 10 people starting treatment. It typically peaks within 24–48 hours and often improves by day 4–5. Appetite suppression may also begin, which is actually the intended effect—you may notice food is less appealing or you feel satisfied sooner.
Managing the first week:
- Eat small, frequent meals and avoid fatty or heavy foods
- Stay hydrated—drink water regularly throughout the day
- Get adequate rest—your body is adjusting
- Mild diarrhoea or constipation can occur; these usually settle by week 2
- Contact your GP if nausea is severe, persistent vomiting occurs, or you cannot keep food or fluids down
Most side effects at the 2.5 mg starting dose are mild. Serious side effects are rare at this dose level.
- 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 feel very sick after my first Mounjaro injection?
Nausea is common but varies widely. Some people experience mild nausea, others none at all. At the 2.5 mg starting dose, side effects are usually mild. Nausea typically peaks at 24–48 hours and improves by day 4–5. Eating small meals, staying hydrated, and resting help. Contact your GP if nausea is severe or prevents eating and drinking.
How long does the first Mounjaro dose stay in your system?
Mounjaro's active effects last about 7 days, which is why it's a once-weekly injection. However, tirzepatide remains detectable in your system for several weeks as it gradually clears. Side effects from your first dose typically resolve within 7–14 days as your body adjusts.
Can I eat normally after my first Mounjaro injection?
Yes, but eat small portions of easily digestible foods. Avoid fatty, fried, or heavy meals for the first few days, as these can worsen nausea. Many people naturally eat less because appetite is suppressed—this is normal and part of how Mounjaro works. Drink plenty of water throughout the day.
What if I experience severe side effects after my first dose?
Severe side effects from the 2.5 mg starting dose are uncommon, but contact your GP immediately if you experience severe stomach pain, persistent vomiting, serious allergic reactions, or cannot keep fluids down. Tier 3 NHS services also provide ongoing support—your prescriber can advise on whether a dose adjustment is needed.
When should I expect to see weight loss after my first Mounjaro dose?
Weight loss typically begins within the first 2–4 weeks as appetite suppression takes effect. However, results are gradual and individual. Clinical trials showed significant results over 12–16 weeks of treatment. Track your progress using our free progress tracker and remember that sustainable weight loss is approximately 0.5–1 kg per week alongside a reduced-calorie diet and physical activity.
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.