If you’ve been researching medical weight loss, you’ve probably come across the terms GLP-1, Wegovy and Mounjaro.
These medications have changed the way doctors approach weight management for many people. But how do they actually work, and what can you realistically expect if you’re considering treatment?
The short answer is that GLP-1 medicines work with the body’s natural appetite and blood sugar systems. They can help you feel fuller for longer, reduce hunger and food cravings, and slow the movement of food through the stomach. For people who meet the medical criteria, this can make sustained weight loss more achievable alongside changes to diet and physical activity.
Here’s what you need to know.
What is GLP-1?
GLP-1 stands for glucagon-like peptide-1. It is a hormone naturally produced by your body, particularly after you eat.
One of its jobs is to help regulate appetite and blood sugar. When GLP-1 is released, it sends signals to the brain involved in hunger and fullness. It also helps the pancreas release insulin when blood glucose is elevated and slows gastric emptying, meaning food moves more slowly from the stomach into the small intestine.
This combination can affect how much you want to eat and how satisfied you feel after eating.
GLP-1 medications are designed to mimic some of these effects, but they last much longer in the body than naturally produced GLP-1.
How does GLP-1 cause weight loss?
There are several effects working together.
1. You may feel less hungry
GLP-1 medicines act on areas of the brain involved in appetite regulation. Many people find that persistent hunger becomes easier to manage and that they think about food less often.
2. You may feel full sooner
Because these medications can increase feelings of fullness, you may naturally want smaller portions.
3. Digestion slows down
GLP-1 medicines slow gastric emptying, so food can remain in the stomach for longer. This can contribute to a longer-lasting feeling of fullness.
4. Blood sugar regulation improves
GLP-1 helps the body release insulin when it is needed and reduces the amount of glucose released by the liver. This is particularly important for people with type 2 diabetes or impaired blood sugar regulation.
The result is not simply “burning fat faster.” Rather, these medications can make it easier to eat less by changing some of the biological signals that influence hunger, fullness and food intake.
What is the difference between Mounjaro and Wegovy?
Mounjaro and Wegovy are both prescription medicines used for weight management, but they are not the same drug.
Wegovy contains semaglutide, which is a GLP-1 receptor agonist.
Mounjaro contains tirzepatide, which acts on two hormone pathways: GLP-1 and GIP (glucose-dependent insulinotropic polypeptide). For this reason, tirzepatide is often described as a dual GLP-1/GIP receptor agonist rather than a GLP-1 medicine alone.
Both can reduce appetite, improve feelings of fullness and affect blood sugar regulation. Tirzepatide also acts on the GIP pathway, which has additional effects on appetite and metabolism.
In the UK, both Mounjaro and Wegovy are licensed for weight management in adults who meet specified criteria. Generally, this includes people with obesity, or people who are overweight and have at least one weight-related health condition. They are intended to be used alongside a reduced-calorie diet and increased physical activity.
Mounjaro vs Wegovy for weight loss
It is tempting to look at clinical trial numbers and ask which medication produces more weight loss. However, the major Mounjaro and Wegovy trials were separate studies, involving different participants and study designs, so their results should not be treated as a direct head-to-head comparison.
In the STEP 1 trial, adults taking semaglutide 2.4 mg alongside lifestyle intervention lost an average of 14.9% of their starting body weight after 68 weeks, compared with 2.4% with placebo.
In the SURMOUNT-1 trial, average weight loss after 72 weeks with tirzepatide ranged from 15.0% to 20.9%, depending on the dose, compared with 3.1% with placebo.
These figures give a useful indication of what was achieved in large clinical trials, but individual results vary considerably.
Your starting weight, dose, response to treatment, health conditions, eating habits, activity levels and ability to stay on treatment can all affect the outcome.
How quickly do GLP-1 medications work?
Weight loss does not usually happen overnight.
Both Mounjaro and Wegovy are started at a lower dose and increased gradually. This helps the body adjust to treatment and can improve tolerability, particularly when it comes to digestive side effects. For example, the current UK prescribing information for Mounjaro starts treatment at 2.5 mg once weekly, with increases made at intervals of at least four weeks when appropriate.
Some people notice a change in appetite relatively early. The amount of weight lost during the first few weeks, however, can be modest.
The bigger picture is more important than the number on the scales after two or three weeks. Clinical trials have followed people for many months because weight loss tends to build over time as the dose is gradually adjusted and treatment continues.
This is one reason medical weight loss should be viewed as a longer-term treatment plan rather than a quick fix.
What results can you realistically expect?
There is no single amount of weight that everyone will lose with a GLP-1 medication.
Clinical trials provide useful averages, but they don’t predict exactly what will happen to one individual.
For context, the STEP 1 trial found that around 86% of people taking semaglutide achieved at least 5% weight loss after 68 weeks, while about half achieved at least 15% weight loss.
In SURMOUNT-1, the proportion achieving at least 5% weight loss at 72 weeks was 85% with 5 mg tirzepatide, 89% with 10 mg and 91% with 15 mg.
These results came from controlled clinical trials with structured follow-up, so they should not be treated as a promise of what an individual patient will achieve.
A good medical weight loss programme looks at more than weight alone. Changes in waist measurement, blood pressure, blood sugar, mobility, energy and other health markers can also be important.
Why medical supervision matters
GLP-1 medications are prescription medicines, not ordinary weight-loss supplements.
Before treatment, a healthcare professional should assess whether the medication is appropriate for you. This can include reviewing your medical history, current medicines, weight and health measurements, previous attempts at weight loss and any conditions that could affect treatment.
Follow-up is also important once treatment begins.
Your clinician can monitor your progress, discuss side effects, adjust your dose when appropriate and help you make changes to nutrition and physical activity. If the medication isn’t producing enough benefit or isn’t suitable for you, your treatment plan can be reviewed.
In the UK, the MHRA advises that GLP-1 medicines should only be taken when prescribed by an appropriate healthcare professional. It also warns against buying these medicines from unregulated sellers, beauty salons or social media because counterfeit or inappropriate products can pose serious risks.
What are the common side effects of Mounjaro and Wegovy?
The most common side effects are digestive.
These can include:
- Nausea
- Diarrhoea
- Constipation
- Vomiting
- Abdominal discomfort
- Indigestion or feeling overly full
Side effects are particularly common when starting treatment or increasing the dose. They are often mild to moderate and may improve as your body adjusts.
There are also less common but more serious risks. Acute pancreatitis, for example, is an infrequent but potentially serious side effect associated with GLP-1 and dual GLP-1/GIP medicines. Severe, persistent abdominal pain, particularly if it spreads to the back, needs urgent medical assessment.
Your prescriber should discuss the risks that are relevant to you before treatment begins.
Can everyone take GLP-1 medication?
No.
These medicines are intended for people who meet specific medical criteria. They are not appropriate simply because someone would like to lose a few pounds for cosmetic reasons.
Your medical history matters. Certain health conditions, medicines and circumstances may mean a particular GLP-1 medication is unsuitable or requires additional assessment.
Pregnancy is another important consideration. GLP-1 medicines should not be taken during pregnancy or while trying to become pregnant, and anyone who could become pregnant should discuss contraception and treatment with their healthcare professional.
This is why an assessment with a qualified prescriber is an important first step.
How do you get started with medical weight loss?
If you’re considering Mounjaro, Wegovy or another prescription weight-loss treatment, the first step is a medical consultation.
A clinician can assess whether you meet the relevant criteria and whether treatment is appropriate based on your health and circumstances.
If medication is suitable, treatment will generally start at a lower dose and be increased gradually where appropriate. You’ll also benefit from practical support around food, activity and the habits that help maintain your progress.
The aim isn’t simply to see a lower number on the scales. A well-managed medical weight loss programme should help you make sustainable improvements while keeping your health and safety at the centre of treatment.
Frequently asked questions about GLP-1 weight loss medication
Is Mounjaro a GLP-1?
Mounjaro contains tirzepatide, which acts on both the GLP-1 and GIP receptors. So while it is commonly grouped with GLP-1 weight-loss medicines, technically it is a dual GLP-1/GIP receptor agonist.
Is Wegovy a GLP-1?
Yes. Wegovy contains semaglutide, a GLP-1 receptor agonist licensed for weight management.
How much weight can you lose with Wegovy?
Clinical trial participants taking semaglutide 2.4 mg lost an average of 14.9% of their starting body weight after 68 weeks when combined with lifestyle intervention. Individual results can be higher or lower.
How much weight can you lose with Mounjaro?
In the SURMOUNT-1 trial, average weight loss after 72 weeks ranged from 15.0% to 20.9%, depending on the tirzepatide dose. Individual results vary, and these figures come from a clinical trial rather than a guarantee of treatment outcome.
Do you need to diet and exercise while taking a GLP-1?
Yes. Mounjaro and Wegovy are intended to be used alongside a reduced-calorie diet and increased physical activity. Medication can make appetite and portion control easier, but nutrition, movement, sleep and other lifestyle factors remain important parts of long-term weight management.
Are GLP-1 medications a quick fix?
No. These treatments are designed as part of medical weight management and require appropriate prescribing, dose adjustment and follow-up. Weight loss generally develops over months rather than days, and maintaining results may require ongoing treatment and lifestyle support.
Could GLP-1 medical weight loss be right for you?
If you’re considering Mounjaro, Wegovy or another GLP-1-based treatment, you don’t have to work out whether you’re suitable on your own.
A medical assessment can look at your current weight, health history, medications and goals, then help you understand which options may be appropriate.
If you’d like to learn more about medical weight loss and GLP-1 treatment, get in touch with our team to discuss your options and find out what getting started could look like for you.