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
Why Have I Gained Weight on Mounjaro?
Weight gain while taking Mounjaro can be frustrating and confusing, especially when you're expecting weight loss. This can happen for several reasons — some related to the medication itself, others to lifestyle factors or how your body responds individually. Understanding what's causing the gain helps you work with your prescriber to adjust your approach. Check your personal progress using our free GLP-1 weight loss calculator to track patterns over time.
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.
Common Reasons for Weight Gain on Mounjaro
Several factors may explain weight gain while on this weekly weight loss injection. Insufficient dose: If you're on a lower dose (2.5 mg, 5 mg, or 7.5 mg), the appetite-suppressing effect may not be strong enough for you individually — weight loss typically increases as the Mounjaro dose is titrated upward. Eating more than expected: Some people find that as nausea side effects reduce after a few weeks, appetite returns and calorie intake rises without realising it. Water retention: Some users experience temporary fluid retention, which can mask fat loss on the scales. Medication interactions: Certain medications can affect weight, particularly some antidepressants or steroids. Medical conditions: Thyroid problems, PCOS, or hormonal changes can reduce weight loss effectiveness. Use our free BMI calculator to check whether your overall health metrics have improved even if the scale hasn't moved.
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.
When to Contact Your GP or Prescriber
If you've been on Mounjaro for 12 weeks or longer without any weight change, or if you're actively gaining weight, contact your prescriber. Do not adjust your own dose — dose escalation must be managed by your prescriber and should follow the standard schedule (minimum 4 weeks at each dose before increasing). Your prescriber may: increase your dose if you're below 15 mg, review other medications you're taking, check for underlying health conditions like thyroid dysfunction, or discuss whether this medication is the right choice for you. You may also benefit from discussing your diet and activity levels with your GP or a registered dietitian, who can assess whether a reduced-calorie approach needs adjustment.
- 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 Mounjaro cause weight gain?
Direct weight gain from Mounjaro is uncommon, but it can happen if appetite suppression is insufficient at your current dose, if you're eating more than you realise, or if water retention masks fat loss. Your prescriber can review your dose and approach.
How long before Mounjaro starts working for weight loss?
Weight loss typically begins within 4–8 weeks, though individual results vary. Full effects of each dose are usually seen after 4 weeks, which is why the standard protocol waits 4 weeks before increasing. If no change after 12 weeks, discuss with your prescriber.
Does water retention happen on Mounjaro?
Some users report temporary fluid retention, particularly in the first few weeks. This can mask fat loss on the scales. Staying hydrated, reducing sodium intake, and waiting a few weeks often helps. The scale may not move but body composition may improve.
What if I'm eating too much on Mounjaro?
If nausea has settled and appetite has returned, you may be consuming more calories without realising. Track your food intake for a week using a food diary or app, and discuss your diet with a dietitian. You may need support to maintain a calorie deficit alongside the medication.
Should I stop taking Mounjaro if I'm gaining weight?
Do not stop taking Mounjaro without speaking to your prescriber first. Suddenly stopping can cause rebound hunger. Instead, contact your GP or prescriber to review your dose, discuss your lifestyle, and explore whether adjustments to your treatment plan would help.
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.