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 is less common than weight loss, but it does happen to some people. This can occur due to dose adjustment, individual body response, lifestyle factors, or underlying health conditions. If you've gained weight on this prescription weight loss injection, it's important to understand why and discuss options with your GP or prescriber. Your response to medication is individual, and there are steps you can take.
10,000+ calculations today ยท NHS-aligned thresholds ยท 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/mยฒ).
While BMI isn't a perfect measure โ it doesn't account for muscle mass, bone density, or fat distribution โ 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 โ 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
Weight gain on Mounjaro can happen for several reasons, even though clinical trials showed most people lose weight on this weekly weight loss injection. Your body may simply not be responding as expected to the GIP and GLP-1 receptor agonist mechanism โ individual results vary significantly. You may also be in the early stages of dose titration; many people experience appetite suppression more strongly at higher doses, so remaining on a lower dose like 2.5 mg or 5 mg may not provide adequate effect. Lifestyle factors are crucial too โ if calorie intake has increased or physical activity has decreased, weight gain can occur regardless of medication. Additionally, some health conditions like thyroid disorders, polycystic ovary syndrome (PCOS), or cortisol imbalances can resist weight loss even with medication. Finally, water retention from the injection or changes in muscle mass can affect the scales without representing true fat gain.
Clinically BackedWeight Loss
No appointments. No waiting rooms. Just care โ 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 โ 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 โ completely free.
What to Do Next โ NHS and Prescriber Options
If you've gained weight on Mounjaro, contact your GP or the prescriber who issued your Mounjaro KwikPen prescription โ do not adjust your dose alone. Your prescriber may recommend continuing to the next dose level, as some people only see appetite suppression benefits at higher doses (up to the maximum 15 mg). They may also review your diet, physical activity, and any underlying health conditions that could be interfering. If Mounjaro is not working after an adequate trial period and dose escalation, your prescriber might suggest switching to a different weight loss injection such as Wegovy (semaglutide) or Saxenda (liraglutide), which work through different mechanisms. This is a standard clinical decision โ Mounjaro vs Wegovy comparisons often show that individual response varies, so a medication switch is a legitimate option if you're not seeing results.
- 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 โ 5mg โ 7.5mg โ 10mg โ 12.5mg โ 15mg
- GLP-1 receptor agonist
- Up to 15โ17% body weight loss (STEP trials)
- Weekly injection, 5-step dose escalation
- NHS & private prescription available
- Doses: 0.25mg โ 0.5mg โ 1mg โ 1.7mg โ 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 | 15โ22.5% | 12โ17% |
| Key trial | SURMOUNT-1 (2022) | STEP 1 (2021) |
| Max dose | 15 mg weekly | 2.4 mg weekly |
| Escalation period | 20โ32 weeks | 16โ20 weeks |
| Administration | Weekly injection (pen) | Weekly injection (pen) |
| NHS availability | Private prescription only | NHS & private |
| Approx. private cost | ยฃ150โยฃ250/month | ยฃ150โยฃ300/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 โ 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 you gain weight while taking Mounjaro?
Yes, though it's less common than weight loss. Weight gain can occur due to insufficient dose, poor response to the medication, increased calorie intake, reduced activity, or underlying health conditions. Clinical trials showed most people lose weight, but individual results vary significantly. Always discuss with your prescriber if you're gaining weight.
Should I increase my Mounjaro dose if I'm gaining weight?
Never adjust your dose without speaking to your prescriber first. Your GP or the clinician who prescribed your medication should review your progress, your diet, activity levels, and any health conditions. They may recommend dose escalation or may suggest switching to a different medication โ this is a clinical decision, not one to make yourself.
What dose of Mounjaro leads to most weight loss?
Clinical trials (SURMOUNT-1) showed higher doses produce greater average weight loss โ approximately 15% at 5 mg, 19.5% at 10 mg, and 20.9% at 15 mg. However, many people experience appetite suppression benefits even at lower doses. If you're on a low dose and not seeing results, your prescriber may escalate gradually over weeks.
Is weight gain on Mounjaro a sign I should switch to Wegovy?
Not necessarily. Your prescriber may first try increasing your Mounjaro dose, as response often improves at higher doses. However, if you've been on an adequate dose for several weeks and still gaining, a switch to Wegovy (a GLP-1 agonist only, vs Mounjaro's dual GIP+GLP-1 mechanism) may be considered โ individual response varies between medications.
Can I use a progress tracker to monitor my weight on Mounjaro?
Yes, tracking your weight weekly is helpful. Our free progress tracker allows you to log your weight over time and identify patterns. This data is valuable to share with your GP or prescriber when reviewing your response to Mounjaro, helping them make informed decisions about dose or medication changes.
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.