If you're deciding between two of the most commonly prescribed weight loss medications, this comparison covers how they work, what the clinical data shows, and what to consider when choosing.
Key Facts:
Wegovy (Semaglutide) targets one hormone pathway, whereas Mounjaro (Tirzepatide) targets two different ones.
In a clinical trial over 72 weeks, the highest dose (7.2mg) of Wegovy led to an average weight loss of 19-21% of baseline body weight (1).
In a clinical trial over 72 weeks, the highest dose (15mg) of Mounjaro led to an average weight loss of 20–22.5% of baseline body weight (2).
On the NHS, both cost the standard prescription charge. Private costs vary by medication, dose and pharmacy.
Switching from one to the other is possible under medical supervision which may require you to drop back down to the initial starting dose.
In this article:
What is the main difference between these two medications?
What are the weight loss results for Wegovy and Mounjaro?
What are the side effects?
Can I switch if my current medication isn't working?
How much do they cost?
Who is a good candidate for each?
What happens if I stop taking my medication?
Frequently asked questions
What is the main difference between these two medications?
The two most commonly prescribed weight loss medications have different active ingredients and work in slightly different ways.
Wegovy (Semaglutide; a single-receptor medication) works on one pathway, which involves a gut hormone your body releases after eating, called GLP-1. It tells your brain that you're full and slows down how quickly food leaves your stomach.
Mounjaro (Tirzepatide; a dual-receptor medication) mimics both this hormone and a second hormone called GIP, which affects how your body stores fat and keeps energy levels in check. This drug may have stronger effects on appetite and metabolism as it works on both pathways.
Both are injectable medications that are given once a week and both follow a dose escalation approach, which means that the dose starts low and slowly increases over several months.
What are the weight loss results for Wegovy and Mounjaro?
A 2025 clinical study compared the effect of Tirzepatide (Mounjaro) and Semaglutide (Wegovy) on weight loss. It found that, over a 72-week period, participants lost an average of 21.6% of their starting body weight while using Mounjaro (15mg), and an average of 15.4% of their starting body weight when using Wegovy (2.4mg) (3).
But the picture has changed with the introduction of a new, higher dose of Wegovy. Results from a recent clinical trial found that over 72 weeks, people using the 7.2mg dose of Wegovy lost an average of 19-21% of baseline body weight (1).
What are the side effects?
Both medications may cause side effects in 77 to 79% of patients, mainly gastrointestinal ones such as nausea, diarrhoea, constipation and vomiting. Most of them are mild to moderate in severity, and they go away within a few weeks to months of taking the medication (1,2).
Side effect | Wegovy | Mounjaro |
|---|---|---|
Any side effect | 79% | 77% |
Nausea | 17-22% | 17-22% |
Vomiting | 21% | 15% |
Diarrhoea | Common | Common |
Constipation | Common | Common |
Discontinuation due to side effects | 8% | 6% |
In clinical trials, Mounjaro has slightly lower rates of stopping treatment (discontinuation) due to side effects (6% vs 8% for Wegovy), suggesting it may be slightly better tolerated overall. Both carry rare but serious side effect risks, including pancreatitis and thyroid tumours. These weight loss medications are contraindicated in people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia Type 2 (MEN2).
Can I switch if my current medication isn't working?
Yes, switching is generally safe when done under medical supervision. There are a number of reasons why you might want to switch. The most common are tolerability, price or not seeing the results you’d like with your current weight loss medication. If after 6 months of taking these medications at their highest doses you lose less than 5% of your body weight, then switching to an alternative may be your best option.
Your prescriber will recommend how long you should wait between the last dose of your previous medication and the first dose of the new medication but this is typically 7 days. At this point, your prescriber will help you decide what the correct dosage should be for your needs.
When switching, you will also need to go through a second round of dose escalations (which could take anywhere from 16 to 20 weeks) before you reach your target maintenance dose. You should not change medication without first consulting your prescriber.
How much do they cost?
If you qualify for one of these medications on the NHS, both have the same charge as other prescriptions in the UK (the standard prescription fee at the time). However, the charges for private prescriptions vary by medication, dose, and pharmacy you are getting them from. Find our Mounjaro pricing guide here and Wegovy pricing guide here.
NHS access to either of these medications varies by region and whether or not you meet the body mass index (BMI) and weight-related health condition requirements for eligibility. While some regions in the UK have already begun to prescribe these medications for weight loss, others have not yet done so. Waiting times for NHS prescribed medications can also be lengthy, whereas private prescriptions can allow for quicker access to these medications.
Who is a good candidate for each?
Both medicines are suitable for those who are 18 years or older with a BMI of 30 or more, or a BMI of 27 or more with at least one weight-related condition (e.g. type 2 diabetes, high blood pressure, high cholesterol, etc.).
Mounjaro may be the better fit for you if: you have type 2 diabetes or metabolic syndrome (it might also help lower your blood sugar as well as your weight), if you’ve tried Wegovy before but the results were not as good as you were expecting. Mounjaro has also been shown to improve cardiovascular health and many users report easing of joint pain from reduced weight.
Wegovy may be the better fit for you if you want the most well-known option with a longer track record, a lower cost or if you have tried this treatment before without experiencing bad side effects. Wegovy is also licensed for reducing the risk of major adverse cardiovascular events and many users find reduced joint pain, as with Mounjaro.
The right choice for you will be based on your past medical history, goals and how you personally respond to the treatment. Talking to your prescriber or pharmacist can help you weigh up your options.
What happens if I stop taking my medication?
If you stop taking the medication without maintaining the positive lifestyle changes you made during treatment, you may regain the weight you lost gradually over the next 6 to 12 months.
The people who successfully keep their weight loss after stopping medication are those who developed sustainable eating, exercise and protein habits during the treatment period. The medication gives you invaluable insights into your eating habits and the opportunity to create more positive ones. If those habits stick, then regaining weight will be much slower and often partial.
Long-term success for many people means either continuing treatment at a maintenance dose or transitioning to a structured plan that maintains the behavioural changes the medication has helped develop. Make sure to discuss your long-term approach with your prescriber before discontinuing it.
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Frequently asked questions
Are these two medications the same?
No. Wegovy (Semaglutide) and Mounjaro (Tirzepatide) contain different active ingredients. Wegovy targets one hormone pathway (GLP-1), whereas Mounjaro targets two hormone pathways simultaneously (GIP and GLP-1). This difference in the way they work may explain the difference in weight loss results seen with Mounjaro compared to Wegovy in clinical trials.
Which medication causes more side effects?
Both have similar overall side effect rates (77 to 79% of people experiencing side effects). Wegovy tends to have higher vomiting rates (21% vs 15% for Mounjaro), while Mounjaro may cause more nausea at its highest doses. Stopping treatment due to side effects was 8% for Wegovy vs 6% for Mounjaro, suggesting that Mounjaro may be slightly better tolerated. Again, it’s always up to the individual and decided with your prescriber.
Can I switch from one to the other if it isn't working?
Yes, you can switch from one to the other but people typically wait at least 7 days between their last injection and first dose of the alternative. You must always switch under medical supervision and your prescriber will determine the appropriate waiting time and starting dose for the new medication.
What is the cost difference?
On the NHS, both cost the standard prescription charge. Private costs vary by medication, pen dose and pharmacy supplying it to you.
How long does it take to see weight loss results with each?
Both medications usually lead to noticeable weight loss results within 4 to 8 weeks of reaching the maintenance dose. Maximum weight loss is achieved between 12 and 18 months. Individual results vary based on how good you are at taking the medication regularly, as well as how much you change your diet and increase exercise.
Is Mounjaro safer?
Both medications have similar side effect profiles. Mounjaro had slightly lower discontinuation rates (6% vs 8% for Wegovy), but both are well-tolerated overall. Safety may depend on your individual medical history. Any side effects can be discussed with your prescriber.
Will I regain weight if I stop?
Regaining some weight is possible after stopping either medication. People who maintain the positive dietary and exercise habits they developed during treatment are less likely to regain their original body weight prior to starting medication but this isn’t guaranteed. Long-term success depends on sustained behaviour change or ongoing maintenance therapy and varies person to person.
How do I know which medication is right for me?
The best choice depends on your medical history, weight loss goals, budget and whether you have type 2 diabetes. A pharmacist or prescriber can assess your individual circumstances and recommend the most appropriate option for you.
If you have questions about which prescription weight management medication may be appropriate for you, speak to your prescriber or pharmacist. They can review your medical history and help you make an informed decision.
References
(1) Wharton S, et al. Once-weekly Semaglutide 7.2 mg in adults with obesity (STEP UP): a randomised, controlled, phase 3b trial. Lancet Diabetes Endocrinol. 2025;S2213-8587(25):00226-8.
(2) Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al.; SURMOUNT-1 Investigators. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387(3):205–216. doi: 10.1056/NEJMoa2206038.
(3) Aronne LJ, Bade Horn D, le Roux CW, Ho W, Falcon BL, Gomez Valderas E, Das S, Lee CJ, Glass LC, Senyucel C, Dunn JP; SURMOUNT-5 Trial Investigators. Tirzepatide as compared with Semaglutide for the treatment of obesity. N Engl J Med. 2025. DOI: 10.1056/NEJMoa2416394.