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
Does Mounjaro Affect Your Period?
Mounjaro (tirzepatide) may cause changes to your menstrual cycle, including irregular periods, lighter or heavier flow, or temporary missed periods. These changes are not uncommon with weight loss medications and can occur due to gradual weight management, hormonal shifts, and changes to how your body absorbs oral medications like contraceptives. If you experience significant period changes after starting Mounjaro, it's important to inform your GP so they can assess whether this is a normal response or requires investigation.
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.
How Mounjaro May Affect Your Menstrual Cycle
Mounjaro works as a GIP and GLP-1 receptor agonist, which means it affects appetite regulation and how your stomach empties. When you lose weight rapidly—particularly within the first few weeks of starting the injection—your body's hormone balance can shift, potentially affecting oestrogen and progesterone levels. Additionally, Mounjaro may reduce how quickly oral medications are absorbed in your stomach, including oral contraceptives like the combined pill. This reduced absorption means the contraceptive pill may be less effective, and your period can become irregular as a result. Common changes include lighter periods, skipped periods, or innovative bleeding between periods. Most people find these changes settle after a few weeks, but some experience ongoing irregularity.
- gradual weight management: Sudden weight reduction can temporarily disrupt hormone balance
- Reduced medication absorption: Slower gastric emptying may affect contraceptive pill effectiveness
- Appetite suppression: Reduced calorie intake can influence menstrual regularity
- Individual variation: Some people experience no period changes at all
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 About Period Changes
Not all period changes on Mounjaro require medical attention, but you should contact your GP if you experience severe pain, unusually heavy bleeding lasting longer than a week, or complete absence of periods for more than three months. You should also inform your GP if you're taking the combined oral contraceptive pill, as you may need to use additional contraception (such as condoms) during the first 4 weeks after starting Mounjaro and for 4 weeks after each dose increase—this is standard guidance for GLP-1 medications. Your GP can assess whether your period changes are a normal response to weight loss or whether further investigation is needed. Keep a brief log of any changes to discuss with your healthcare provider.
- 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 irregular periods a common side effect of Mounjaro?
Period irregularities are reported by some people using Mounjaro, particularly during the first few weeks. They are not listed as a "very common" side effect in the official information, but irregular periods are a recognised response to gradual weight management and hormonal shifts. Most changes settle within 4–8 weeks as your body adjusts. If irregularity persists beyond this, contact your GP.
Will my period return to normal after stopping Mounjaro?
Most people's menstrual cycles return to their pre-Mounjaro pattern within a few weeks or months after stopping the injection, though this varies individually. Your body's hormone balance typically stabilises once weight loss slows and stops. If period irregularity continues after stopping, discuss this with your GP to rule out other causes.
Can Mounjaro affect my contraceptive pill?
Yes. Mounjaro slows gastric emptying, which may reduce how quickly your body absorbs the oral contraceptive pill. You should use additional contraception (such as condoms) for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. After this period, the pill typically works normally again. Always inform your prescriber if you're using hormonal contraception.
Should I stop taking Mounjaro if my periods become irregular?
No—do not stop Mounjaro without speaking to your prescriber first. Most period changes are temporary and not dangerous. However, contact your GP or your prescriber to discuss the changes so they can confirm whether your symptoms are expected and advise whether any action is needed.
How long do period changes typically last on Mounjaro?
Most people experience period changes for 4–8 weeks after starting Mounjaro as their body adjusts to weight loss and hormonal shifts. Some experience changes with each dose increase. If irregularity continues beyond 8–12 weeks or becomes severe, contact your GP for further assessment and guidance.
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.