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 Mess With Your Period?
Mounjaro (tirzepatide) can affect your menstrual cycle, but the relationship is complex. Some women experience changes including lighter or irregular periods, whilst others notice no difference. These effects may be related to weight loss itself rather than the medication directly. If you're concerned about changes to your period whilst taking Mounjaro, speak to your GP or the prescriber managing your treatment. It's important to track any changes and discuss them during your regular health reviews.
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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 is a prescription-only GIP and GLP-1 receptor agonist — it works by affecting appetite and how your body processes food. Whilst the medication itself doesn't directly target hormones that control your period, several mechanisms may cause changes:
- gradual weight management: Losing weight quickly can temporarily disrupt menstrual cycles because hormones like oestrogen are stored in fat tissue. Significant weight loss can affect oestrogen levels and cycle regularity.
- Nutritional changes: If calorie intake drops too sharply, your body may reduce reproductive hormone production as a survival mechanism.
- Stress and metabolic changes: Any significant change to your body's metabolism can influence cycle timing.
In clinical trials of Mounjaro, menstrual disturbances were not listed as a primary side effect, but individual women report varied experiences. Most changes tend to normalise as weight stabilises. Our GLP-1 weight loss calculator can help you track expected weight loss at different doses — understanding your anticipated progress may help you anticipate cycle changes.
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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.
When to Speak to Your GP
Contact your GP if you experience severe period changes such as: absence of periods for more than three months, extremely heavy bleeding, or severe pain. You should also speak to your prescriber immediately if you're trying to become pregnant or think you might be pregnant — Mounjaro is not recommended during pregnancy.
Minor changes like lighter periods or a slightly delayed cycle are often normal during significant weight loss and typically resolve once your weight stabilises. However, if you feel concerned about any change, don't hesitate to ask your GP for reassurance. They can assess whether the change is related to Mounjaro, weight loss, or another cause. Keep a simple record of your cycle changes to share with your healthcare team.
- 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 missed periods?
Weight loss itself can cause temporary irregular or missed periods, particularly if weight drops rapidly. Mounjaro causes weight loss in some users, so missed periods may be an indirect effect of loss of weight rather than the medication acting directly on your reproductive hormones. This usually normalises as weight stabilises. Always inform your GP if you miss periods for more than three months.
Will my period return to normal after stopping Mounjaro?
In most cases, menstrual cycles return to normal after stopping Mounjaro, particularly as weight stabilises. However, timing varies by individual. If you're concerned about ongoing cycle changes after stopping treatment, discuss this with your GP, who can rule out other causes and offer support.
Is Mounjaro safe if I'm trying to get pregnant?
Mounjaro is not recommended if you're pregnant or planning to become pregnant. Additionally, women taking oral contraceptives should use a barrier method or switch to non-oral contraception for 4 weeks after starting Mounjaro and 4 weeks after each dose increase, as the medication may reduce oral contraceptive absorption. Speak to your prescriber before starting if you're planning pregnancy.
Can Mounjaro cause hormonal imbalances?
Mounjaro doesn't directly cause hormonal imbalances, but weight loss can temporarily affect oestrogen levels since this hormone is stored in fat tissue. This usually resolves as weight stabilises. If you notice signs of hormonal imbalance beyond period changes — such as mood changes or hot flushes — discuss these with your GP.
What if my periods become very heavy on Mounjaro?
Unusually heavy bleeding is not a common side effect of Mounjaro listed in clinical trials. If your periods become significantly heavier, this may be unrelated to the medication. Speak to your GP to rule out other causes such as hormonal changes, nutritional deficiency, or other medical conditions. Use our BMI calculator to track your overall health progress alongside any period 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.