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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Can You Take Mounjaro A Day Early?
If you're taking Mounjaro (tirzepatide) in the UK, you may wonder whether it's safe to take your injection a day or two before your scheduled date. The answer is important for your safety: no, you should not take Mounjaro early. Mounjaro is a once-weekly GLP-1 and GIP receptor agonist injection designed to be taken on the same day each week. Taking it early or too frequently can affect how the medication works and may increase the risk of side effects. Your prescriber will give you specific dosing instructions. Always follow the schedule you've been given, and speak to your GP or prescriber if you're unsure about timing or have concerns about your dose.
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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.
Mounjaro Dosing Schedule: Why Timing Matters
Mounjaro is designed as a once-weekly injection, meaning you should wait a full seven days between doses. The minimum time between doses is 72 hours (3 days) — taking it sooner than this risks over-dosing on active medication. Each Mounjaro KwikPen contains four weekly doses, and your prescriber will help you establish a regular injection day (for example, every Monday morning). Consistency is key because the medication builds up in your system over time. Taking doses too close together or at irregular intervals can cause nausea, stomach upset, and other side effects to worsen. If you're tempted to take Mounjaro early because you're worried about missing a dose or want to start seeing results faster, it's important to understand that accelerating the schedule won't speed up weight loss — it will just increase your risk of adverse effects. Your dose escalation is carefully timed: typically increasing from 2.5 mg every four weeks up to 15 mg maximum. Rushing through these intervals won't improve outcomes and may delay your treatment.
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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.
Mounjaro vs Wegovy: Dosing Schedule Comparison
Both Mounjaro (tirzepatide) and Wegovy (semaglutide) are once-weekly injections licensed in the UK for weight management, so the fundamental timing rule is the same: take each dose on your scheduled day, with a minimum of 72 hours between doses. The key difference is in their mechanisms and dose escalation. Mounjaro is a dual GIP and GLP-1 receptor agonist, starting at 2.5 mg and escalating to a maximum of 15 mg. Wegovy is a GLP-1-only agonist, starting at 0.25 mg and reaching a maximum of 2.4 mg. Both require at least four weeks at each dose level before increasing. Clinical trial data shows Mounjaro (SURMOUNT-1) achieved average weight loss of approximately 20.9% body weight at 15 mg over 72 weeks, while Wegovy (STEP-1) showed approximately clinically studied weight loss at 2.4 mg over 68 weeks — though individual results vary significantly. Regardless of which medication you're prescribed, the dosing principle remains: take it once weekly on your scheduled day, not early, to ensure safe and effective treatment.
- 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.
What should I do if I accidentally take Mounjaro a day early?
If you take Mounjaro less than four days early, it's generally not a medical emergency, but contact your GP or prescriber immediately for advice. They may ask you to adjust your schedule or monitor for side effects. Never take a double dose to compensate. If you experience severe nausea, stomach pain, or other concerning symptoms, seek medical attention. In future, set calendar reminders on your phone to avoid accidental early dosing.
Can I take Mounjaro every six days instead of every seven days to speed up results?
No. Taking Mounjaro more frequently than prescribed will not speed up weight loss and increases the risk of side effects including severe nausea, vomiting, and stomach problems. Mounjaro's efficacy comes from consistent, weekly dosing with carefully timed dose escalations. Your prescriber determines the schedule that's safe and effective for your individual situation. Accelerating it could harm your treatment.
What's the minimum time between Mounjaro doses?
The minimum time between Mounjaro doses is 72 hours (3 days). This means if your normal injection day is Monday, the earliest you should take your next dose is Thursday. However, the standard recommendation is to inject on the same day each week (for example, always on Monday morning) to maintain consistent medication levels and minimise side effects. Consistency is more important than flexibility.
Is Mounjaro dosing different from Wegovy dosing?
Both are once-weekly injections, so the fundamental timing rule is identical: inject once per week, minimum 72 hours between doses. The difference is in dose strength and escalation schedules. Mounjaro starts at 2.5 mg and goes up to 15 mg (a dual GIP+GLP-1 mechanism), while Wegovy starts at 0.25 mg and reaches 2.4 mg (GLP-1 only). Your prescriber will explain the specific schedule for whichever medication you're prescribed.
What should I do if I miss my scheduled Mounjaro injection?
If you miss your Mounjaro dose and remember within four days, take it as soon as possible and resume your normal weekly schedule from there. If more than four days have passed, skip that dose and take your next one on your regular scheduled day — never double-dose to catch up. For example, if you normally inject on Monday and realise on Friday (four days late), inject on Friday. If you realise on Tuesday of the following week, skip that dose and inject on your normal Monday the next week. Always contact your prescriber if you're unsure.
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