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 Medication Can You Not Take With Mounjaro?
Mounjaro (tirzepatide) can interact with certain medications, which is why a clinical assessment is essential before prescription. The most important interaction involves oral contraceptives—these may be less effective when you start Mounjaro. Other medications may have interactions depending on your health conditions. Your prescriber will review all your current medicines before approving Mounjaro and will advise on any adjustments needed. This guide explains the key medication interactions and what to expect when accessing this prescription-only weight management injection in the UK.
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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.
Key Drug Interactions With Mounjaro
Oral contraceptives are the most significant interaction. Mounjaro slows gastric emptying (how quickly food and medications leave your stomach), which may reduce how well oral contraceptive pills are absorbed. If you take a combined oral contraceptive or progestogen-only pill, your prescriber will recommend adding a barrier contraceptive method (condoms) or switching to a non-oral contraceptive for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase. This reduces the risk of unintended pregnancy.
Diabetes medications including sulphonylureas and insulin may need dose adjustments when you start Mounjaro, as Mounjaro itself helps regulate blood sugar. Your GP or prescriber will monitor your blood sugar levels and adjust diabetes medication doses accordingly to avoid hypoglycaemia (low blood sugar).
Other oral medications may have reduced absorption due to slower gastric emptying. Always inform your prescriber of all medications you take—including supplements and over-the-counter medicines—before starting tirzepatide.
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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.
Accessing Mounjaro in the UK: NHS vs Private
NHS access: Mounjaro (tirzepatide) is approved by NICE for NHS specialist weight management services. You'll need a GP referral to a tier 3 or tier 4 specialist service. Availability varies by region, and waiting lists can be lengthy.
Private access: You can access Mounjaro through UK-licensed online prescribers without NHS referral. An online clinical assessment will include a full medication review—your prescriber will ask about all current medicines to identify any interactions. If Mounjaro is unsuitable due to drug interactions or other health factors, alternatives like Wegovy (semaglutide) may be suggested. Costs typically range from £129–£320 monthly depending on your dose. The Mounjaro KwikPen is then dispensed by a registered pharmacy.
Whichever route you choose, medication interaction screening is a standard part of the assessment process.
- 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 I take Mounjaro with birth control pills?
Oral contraceptives may be less effective when taking Mounjaro due to slowed gastric emptying. Your prescriber will recommend adding condoms or switching to a non-oral contraceptive (implant, IUD, injection) for 4 weeks after starting and after each dose increase. This is a standard precaution for safety.
What happens if I take insulin with Mounjaro?
Mounjaro helps regulate blood sugar, so your insulin doses may need adjustment to prevent low blood sugar (hypoglycaemia). Your prescriber will review your diabetes medication and monitor your levels throughout treatment. Never adjust insulin doses yourself—always follow your prescriber's guidance.
Should I tell my prescriber about over-the-counter medicines?
Yes, absolutely. Tell your prescriber about all medications including vitamins, supplements, and OTC medicines like painkillers or allergy tablets. Mounjaro may affect absorption of oral medications, and some combinations should be avoided or monitored.
Does Mounjaro interact with antidepressants?
Most antidepressants do not have significant direct interactions with Mounjaro, but your prescriber will review your specific medication. If you're taking any antidepressant, inform your prescriber during your clinical assessment so they can confirm it's safe to use alongside tirzepatide.
What if I'm taking multiple medications?
A comprehensive medication review is part of every clinical assessment—whether through NHS or private prescription. Your prescriber will check each medicine for interactions with Mounjaro and advise on any adjustments needed. This is why full honesty about all medications is essential.
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.