Over the last few years, weight loss treatments like Mounjaro have generated an enormous buzz.
Mounjaro reduces your appetite and helps you feel fuller for longer, meaning you’ll reduce your calorie intake without trying to. The upshot in most cases is weight loss, which has been demonstrated time and again in clinical trials. Here’s exactly how it works.
What is Mounjaro?
Mounjaro is a once-weekly injectable medicine containing an active ingredient called tirzepatide. It was first approved for type 2 diabetes in 2022, before becoming available for weight loss in November 2023 following promising clinical trial results.
Mounjaro works by mimicking two hormones called GLP-1 and GIP (produced naturally by your gut when you eat), which help to control blood sugar and create feelings of fullness after a meal.
Although it is considered safe and effective, it isn’t suitable for everyone. In the UK, you’re eligible to take Mounjaro if you have a BMI of 30 or more.
You shouldn’t take it if you’re pregnant, breastfeeding, have gallbladder or pancreas problems, are allergic to any of the ingredients, aged under 18, or have a family history of certain types of thyroid cancer. And needless to say, Mounjaro isn’t a quick fix or a substitute for a healthy lifestyle.
How does Mounjaro work for weight loss?
When you eat a meal, the digestive system breaks down carbohydrates into glucose (a type of sugar). That glucose enters the bloodstream, causing blood sugar levels to rise. This in turn triggers a feedback loop: two hormones in the gut (GIP and GLP-1) set to work on regulating appetite and getting your blood sugar under control.
Known as incretins, these hormones stimulate the pancreas to release insulin, which moves glucose into the cells where it’s used for energy. Your blood sugar levels return to normal. GLP-1 also suppresses glucagon, a hormone that raises blood sugar levels when they fall too low.
Tirzepatide—the active ingredient in Mounjaro—works by mimicking the actions of GIP and GLP-1. When the molecule enters the bloodstream, it binds to GIP and GLP-1 receptors, generating the same reactions as the actual hormones. A simple way to picture this is by thinking about a lock and key. If the receptors are the lock, and the hormones are the master key, then taking a drug like Mounjaro is a bit like getting another key cut.
Because Mounjaro targets the GIP and GLP-1 receptors, it goes one step further than Saxenda and Wegovy, which act on the GLP-1 receptors alone. For a long time, scientists thought that mimicking GIP as well wouldn’t have any additional effect. However, the evidence now suggests it has added benefits. So far, Mounjaro has shown better results for weight loss than its rivals.
Let’s look more closely at what happens in the body when the GLP-1 and GIP ‘locks’ are opened:
Your appetite drops
Tirzepatide binds to GLP-1 receptors in the hypothalamus, which is the part of the brain responsible for hunger and fullness. This suppresses the appetite and tells the stomach you’re full. Tirzepatide also prompts the body to suppress a hormone called ghrelin, which is responsible for hunger signals.
If your appetite hasn’t changed much yet, here’s why you might still feel hungry on Mounjaro and how to fix it.
Your insulin sensitivity improves
As well as prompting the pancreas to release more insulin, tirzepatide increases your cells’ sensitivity to insulin. That’s why it works so well for type 2 diabetes, a condition in which the cells don’t respond to insulin as they should. But you don’t have to be diabetic for this to matter. In one study, four in ten young American adults showed some degree of insulin resistance.
When the body becomes insulin resistant, it responds by producing even more of the hormone, which may in turn encourage the body to store extra calories as fat. Therefore, it is possible that improving insulin sensitivity can play a role in weight loss.
Your cravings reduce
Tirzepatide binds to GLP-1 receptors in brain regions that are associated with pleasure and reward. In one study, patients were shown images of fatty, sugary foods while lying in an fMRI scanner. A placebo group, who hadn’t taken tirzepatide, showed activation in four different parts of the brain. The patients who had taken the drug showed significantly less activation, suggesting that they weren’t craving these foods.
Many people taking weight loss drugs have reported exactly that—they think about food less, experience less “food noise” (constantly thinking about food), and eating loses its emotional impact.
The passage of food through your stomach slows down
Tirzepatide also slows down the movement of food through the stomach, which lowers the rate at which nutrients move into your bloodstream. This stops your blood sugar from spiking and helps you feel fuller for longer.
Taken together, these mechanisms make Mounjaro remarkably effective. One large trial (SURMOUNT-3) looked at people who had already lost 5% of their body weight through lifestyle changes. After taking Mounjaro for 72 weeks, they lost a further 21% of their starting weight, as well as noting improvements in other areas like blood pressure. That’s a similar success rate to gastric band surgery.
It should be noted that, as with any weight loss medication, Mounjaro works best with diet and exercise changes. You are unlikely to see the full benefit if you eat the same diet as before. And taking Mounjaro without committing to healthy lifestyle changes is more likely to lead to rebound weight gain once you stop the treatment.
Check out our Mounjaro-friendly snack ideas and meal prep guide if you need a bit of inspiration to stay on track.
How quickly does Mounjaro work?
Though you might notice a drop in appetite within hours of your first injection, the real benefits will take a while longer to kick in. Most of the time, you’ll start to notice the scales budge within a month or two.
In one clinical trial (SURMOUNT-1), patients on the 5mg dose had lost 16% of their body weight by 72 weeks (around a year and a half). Those on the 10mg and 15mg dose lost 21.4% and 22.5% respectively. On average people lose just under 4% of their body weight after one month.
By the end of the trial, 36.2% of the 15mg patients had lost a quarter of their body weight or more.
Though it’s very effective on average, Mounjaro doesn’t work for everyone. Results will vary depending on individual factors like diet and adherence, and no specific amount of weight loss is guaranteed. Talk to your doctor if you’re not losing weight a few months down the line.
Can you expect to lose weight only by taking Mounjaro?
Mounjaro is not a standalone solution. If you want the best results possible, you should also maintain a balanced diet, regular exercise, good hydration, and sustainable lifestyle habits.
In 2024, the journal Obesity published nutritional recommendations for people taking medicines like Mounjaro. They suggest that most women should strive for 1,200-1,500 kcal a day, rising to 1,500-1,800 kcal a day for most men. But everyone’s different and it’s important to factor in individual requirements such as your activity levels. Because calorie intake is so low, it’s really important to get the right kinds of nutrients—focusing on protein and vegetables while reducing sugar. That way, you can safely lose weight while avoiding nutritional deficiencies.
Benefits of Mounjaro
Mounjaro shows impressive weight loss results compared to lifestyle changes alone. The placebo group in the SURMOUNT-1 trial only lost 2.4% of their body weight in 72 weeks, compared to 16% for those on the lowest dose of Mounjaro.
Mounjaro also helps regulate your blood sugar levels, meaning your energy will be more balanced throughout the day and you won’t succumb to the same crashes and spikes. Finally, it reduces hunger and curbs cravings, quietening down any mental chatter around food.
Mounjaro side effects
Like any medication, Mounjaro comes with potential side effects, although in most cases these are short-lived, mild and manageable.
The most common side effects include:
- Gastrointestinal side effects (burping, flatulence, constipation, diarrhoea, nausea and vomiting)
- Mild weakness or fatigue
- Feelings of malaise
- Dizziness
- Hair loss
- Low blood pressure
- Injection site reactions like irritation, itching and bruising.
Most of the time, these side effects will subside as your body gets used to the medication.
Very rarely, users may experience more serious side effects including:
- Gallbladder disorders
- Very low blood sugar
- Pancreatitis
- Kidney disease
- Allergic reactions.
Side effects support at Voy
"If you're using Voy and you experience any side effects, rest assured that our expert clinicians are just a message or phone call away. They can adjust your dose, prescribe medication to ease your symptoms, or simply offer reassurance.
Everyone responds differently to weight loss treatment and we'll help you find what feels right for you."
How to take Mounjaro
Mounjaro is taken as a once-weekly injection, using your pre-filled four-dose KwikPen. The Mounjaro should be injected into your upper leg, abdominal area or upper arm. You can opt for the same body part every time, but make sure to choose a different injection site within that area. Wash your hands before starting, then follow the ‘Instructions for Use’ to administer your dose.
Although it’s important to try to take Mounjaro on the same day every week, don’t worry too much if you miss a dose. You can take it as soon as you remember, so long as it’s within four days of your scheduled day. Any longer than that, and you should skip the dose.
Make sure to only take the dosage recommended by your doctor, and store Mounjaro properly. Typically, you’ll start on 2.5mg a week for the first four weeks, before adjusting your dose in 2.5mg increments. If you accidentally take too much, speak to your doctor.
Is Mounjaro available on the NHS?
Healthcare professionals at specialist NHS weight management services can prescribe Mounjaro if they feel it’s the right treatment for you. It’s also possible to get Mounjaro from your GP. But it’s currently only available on the NHS to patients with the highest clinical need—those with a BMI of 40 or more with at least four weight-related health conditions such as type 2 diabetes, high blood pressure, heart disease, abnormal blood fats and obstructive sleep apnoea. After that, it will be offered to other groups in priority order of clinical need.
In the meantime, Mounjaro can be accessed privately through clinics or weight loss management services like Voy. Voy combines clinically proven medication and lifestyle coaching to help you achieve weight loss that sticks.
Alternatives to Mounjaro
Mounjaro is not the only weight loss drug on the market, and in rare cases you may be advised to switch to a different medication. Or perhaps you’re here because you’ve been advised to switch from Mounjaro to Wegovy.
Wegovy (semaglutide), contains the molecule semaglutide. It launched in the UK in September 2023 and is available on the NHS to certain patients. Like Mounjaro, it is administered by injection once a week. While the currently approved maximum dose is 2.4mg, clinical research has shown that participants taking a higher dose of 7.2mg achieved an average weight loss of 20.7%. It is known by the brand name Ozempic when used to treat type 2 diabetes.
Saxenda, which contains the molecule liraglutide, is injected once a day. It launched privately in the UK in 2017, before becoming available on the NHS in 2021. Unlike Wegovy and Mounjaro, it is not prescribed for type 2 diabetes.
Both these drugs are GLP-1 receptor agonists. They work much the same way as Mounjaro does, but Mounjaro is unique in targeting the GIP receptor too.
Mounjaro and weight management
While Mounjaro is not a silver bullet, it is a ground-breaking way of approaching weight management. Many people have achieved significant weight loss with the drug, with health benefits to match.
However, it’s important not to set expectations too high. Your journey with Mounjaro is a long-term one, and it’s not a quick fix. For best results, Mounjaro needs to be taken alongside a healthy diet and exercise routine. Remember, the Voy app enables you to reach out for support whenever you need it. Whatever happens, our expert coaching team is on hand to help.
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