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 Delay Your Period?
Mounjaro (tirzepatide) can potentially affect your menstrual cycle, including causing delays or changes in period timing. This is not uncommon with weight loss medications and is usually temporary. If you're considering Mounjaro or currently taking it, understanding how it might affect your period is important for your health. This guide explains what research shows, what to expect, and when to speak to your GP about changes.
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 (tirzepatide) is a prescription-only, once-weekly injection licensed in the UK for weight management. Changes to your menstrual cycle can occur for several reasons when taking weight loss medications:
- gradual weight management: Losing weight quickly can temporarily disrupt hormone levels, which regulate your period. The body may interpret rapid change as stress.
- Calorie deficit: Mounjaro works alongside a reduced-calorie diet. Very low calorie intake can affect reproductive hormones and delay or skip periods.
- Gastrointestinal side effects: Nausea, vomiting, and diarrhoea (common Mounjaro side effects) may reduce nutrient absorption, affecting menstrual regularity.
- Hormonal changes: Weight loss itself alters oestrogen and other hormones involved in the menstrual cycle.
Period changes are usually temporary and often resolve as your body adjusts. However, individual experiences vary significantly. Using our GLP-1 weight loss calculator can help you understand realistic weight loss timelines and pace.
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 Speak to Your GP About Period Changes
Contact your GP if you experience:
- Periods stopping completely for more than three months (amenorrhoea)
- Very heavy or prolonged bleeding
- Severe period pain or cramping not typical for you
- Periods becoming extremely irregular or unpredictable
- Other concerning symptoms alongside period changes
Your GP can assess whether changes are due to Mounjaro, your diet, weight loss pace, or another health condition. Never stop taking Mounjaro without speaking to your prescriber first. They may adjust your dose, diet, or recommend temporary pauses. Many women find their cycles normalise after the first few months as the body adjusts.
- 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 period delay a common side effect of Mounjaro?
Period changes including delays are not listed as a direct side effect of tirzepatide in clinical trials, but they do occur in some users. This is more often related to gradual weight management, calorie deficit, and stress on the body rather than the medication itself. If concerned, speak to your GP or prescriber.
Can Mounjaro affect my contraceptive pill?
Yes. Mounjaro can slow how quickly oral medications are absorbed in your stomach. If you take the contraceptive pill, add a barrier method (condom) or switch to a non-oral contraceptive for 4 weeks after starting Mounjaro and 4 weeks after each dose increase. Always tell your prescriber about your contraception.
How long does it take for periods to return to normal after Mounjaro?
This varies. For most people, periods normalise within 2–3 months as the body adjusts to medication and weight loss stabilises. If delays persist beyond 3 months or you're concerned, contact your GP. They can check whether other factors are involved.
Does Wegovy cause period delays like Mounjaro?
Wegovy (semaglutide) is a GLP-1 injection, while Mounjaro is a dual GIP/GLP-1 agonist. Both can contribute to period changes through weight loss and calorie deficit rather than the medication directly. The experience is similar across weight loss injections. Individual results vary.
Should I stop Mounjaro if my period is delayed?
No — do not stop Mounjaro without speaking to your prescriber. Contact them or your GP to discuss the delay. They can assess the cause, check your dose and diet pace, and decide whether any adjustments are needed. Stopping suddenly without guidance is not advisable.
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.