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 Are the Downsides of Mounjaro?
Mounjaro (tirzepatide) is a prescription-only weight management injection that can help with weight loss, but like all medications, it has significant downsides worth understanding before considering it. Common side effects include nausea, diarrhoea, and vomiting, particularly when starting or increasing doses. Beyond side effects, there are practical considerations including cost, access through NHS specialist services or private prescription, ongoing commitment, and the fact that weight regain can occur if treatment stops. Understanding both benefits and drawbacks helps you make an informed decision with your GP or prescriber.
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 Side Effects and Health Considerations
Gastrointestinal side effects are the most frequent downsides of Mounjaro. Nausea affects more than 1 in 10 people, particularly in the first weeks or after dose increases. Diarrhoea and constipation commonly alternate, and some users experience persistent stomach pain or indigestion. These often ease over time but can be disruptive.
Other common side effects include fatigue, dizziness, headaches, and injection site reactions such as bruising or redness. Hair loss has been reported by some users. In rare cases, severe stomach pain spreading to the back may indicate pancreatitis, which requires urgent medical attention.
Special considerations: Mounjaro may reduce the absorption of oral contraceptives — additional contraception is needed for 4 weeks after starting and after each dose increase. If you take diabetes medications like insulin or sulphonylureas, your doses may need adjustment under medical supervision. Anyone with a history of gallbladder problems, pancreatitis, or severe gastroparesis should discuss Mounjaro carefully with their GP.
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.
Access, Cost, and Long-Term Commitment Issues
NHS access is limited. Mounjaro is available through NHS specialist tier 3 weight management services following GP referral, but waiting lists in many UK regions are lengthy (sometimes 6–12 months or longer), and not all ICBs fund it equally. Private prescription costs £180–£320+ per month depending on dose, which is ongoing and not covered by most insurance.
Dose titration takes time. Starting at 2.5 mg weekly, doses increase every 4 weeks up to a maximum of 15 mg. This means 16–20 weeks to reach full dose, during which side effects may persist. Weight regain is a real concern: Clinical trials show that stopping Mounjaro typically results in weight return within months. This means long-term commitment and cost, not a one-off treatment. Use our free GLP-1 weight loss calculator to understand potential outcomes and timescales for your situation.
It requires lifestyle changes alongside medication. Mounjaro is not a substitute for diet and exercise — clinical trials show weight loss only when combined with reduced-calorie eating and physical activity. Without these changes, results are significantly reduced. Individual results vary considerably: While clinical trials showed average weight loss of 15–21% of body weight, some people lose less, and a small minority lose very little. There is no way to predict individual response before starting. Additionally, not everyone tolerates the medication well — some people experience side effects severe enough to require stopping treatment. Track your progress with our free progress tracker once you begin any weight loss journey.
- 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 Mounjaro available on the NHS in the UK?
Yes, but access is limited. Mounjaro is available through NHS specialist tier 3 weight management services following GP referral. However, waiting times vary significantly by region — some areas have waits of 6–12 months or longer. Private prescription is available but costs £180–£320+ per month depending on dose.
What are the most common downsides of Mounjaro?
Nausea, diarrhoea, and constipation are the most common side effects, affecting more than 1 in 10 people. Other downsides include fatigue, dizziness, headaches, and the need for long-term commitment since weight typically returns if you stop taking it.
Can I stop taking Mounjaro if I lose the weight I want?
Technically yes, but weight regain often occurs within weeks to months of stopping. Some people continue long-term to maintain weight loss. This ongoing cost and commitment is a significant downside — it's not a one-off treatment but rather a long-term medication.
How does Mounjaro compare to Wegovy as weight loss injections?
Both are prescription-only GLP-1-based medications for weight management. Mounjaro uses a dual GIP and GLP-1 mechanism and showed slightly higher average weight loss (15–21%) than Wegovy (approximately 15%) in clinical trials. Both have similar side effects and require clinical assessment. The choice depends on individual factors and prescriber recommendation.
What if I can't tolerate the side effects of Mounjaro?
If side effects are severe, discuss with your prescriber — stopping is an option, though gastrointestinal symptoms often ease after the first few weeks. Your GP may recommend dose reduction, switching to a different medication like Wegovy, or pausing temporarily. Some people find side effects manageable; others cannot tolerate them. Always report concerns to your healthcare provider.
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.