A medical expert has described exactly what happens in the first four weeks of injecting yourself with Mounjaro, including side effects and how your eating habits will change.
In January, it was reported that 500,000 people in the UK were either taking the popular weight-loss drug Wegovy or Mounjaro.
The latter - referred to by The Guardian as the ‘King Kong’ of weight loss drugs - was initially authorised for weight management and weight loss in the UK in November 2023.
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Two months later, the ever-popular, four-dose KwikPen version of Mounjaro for both diabetes and weight management was approved for sale.
But what actually is Mounjaro, how does it help you lose weight, and what can you expect to happen when you’re on it?
What is Mounjaro?
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Mounjaro is an injectable prescription medicine that improves and lowers blood sugar levels in adults with type 2 diabetes.
This drug works by activating two receptors (GLP-1 and GIP) to increase the level of hormones in the body, according to Diabetes.org.
Not only does it help your body to produce more insulin and lower your blood sugar levels, but it slows down how quickly food is digested - making it a popular weight-loss solution.
As of June 2025, people with obesity can gain access to Mounjaro via their GP services in England.
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However, it’s unlikely the injection will be handed over unless your BMI is over 40 and you have four weight-related conditions.
You can also purchase it privately.
How do you take Mounjaro?
Those on Mounjaro are advised to inject the drug under the skin once per week. This can either be in your thigh, your upper arm, or in your stomach, Boots’ Online Doctor reports.
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It’s important that you follow dosage instructions strictly. Usually, these doses increase every four weeks until you reach a maximum maintenance dose of either 5mg, 10mg, or 15mg.
According to Boots, Mounjaro should start working right away. But be warned: it could take several weeks before you notice an improvement where your blood sugar level is concerned.
First week on Mounjaro
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If you’re contemplating taking Mounjaro, then you may be wondering what you can expect. According to Scott McDougall, a registered pharmacist and co-founder of The Independent Pharmacy, it takes around eight to 72 hours for the medication to reach peak levels in your system.
You may feel a reduction in your appetite during this time, and you could also begin experiencing common side effects, such as diarrhoea and headaches.
Your body will become more used to Mounjaro in the future, but it’s expected that this first week could be a little rough. If you’re concerned about how you're feeling during this stage, it’s worth contacting your GP for more information.
Second week on Mounjaro
McDougall writes that your sweet, sugary cravings could be reduced dramatically after taking your second dose of Mounjaro.
He states that you’ll probably be eating and drinking less than before, and that side effects may diminish due to your body becoming used to the substance you’re injecting into it.
To stop becoming tired or hungry, it’s recommended you eat a healthy, balanced diet, and that you continue to consume plenty of liquids.
Third week on Mounjaro
The third dose of Mounjaro is where users’ bodies begin to become accustomed to the drug, according to McDougall.
“At this point most people notice their side effects becoming milder or disappearing completely, although some may find that their symptoms are more stubborn,” the pharmacist explains.
“Though you may not notice any weight loss just yet, you could find that your appetite is consistently reduced and you’re eating less than normal.”
One month on Mounjaro
According to clinical trials, people who make it to week four on Mounjaro are likely to lose up to 4 percent of their starting weight, with 6 percent being reported after two months, as per The Care Pharmacy.
Your body is also likely to be adjusted to the weight-loss drug at this stage.
If you haven’t reported any significant side effects to your GP, then your doctor may now increase your dose of the medication.
By the four month mark, you may also be making healthier meals and increasing your level of exercise, building better habits for the future.
What are the side effects of Mounjaro?
Most of the early side effects Mounjaro can cause in the body are gastrointestinal in nature, writes Boots Online Doctor.
These include nausea, vomiting, diarrhoea and constipation. And as your body adapts to the drug dosage, these side effects will reportedly settle.
Other common side effects could include: fatigue, feeling lightheaded and gallstones.
Users on Mounjaro have also previously reported suffering from gastroesophageal reflux, injection site reactions and hair loss.
It’s important that you contact a medical professional if you experience a rash after injecting Mounjaro, or if your lips, tongue and/or throat begin to swell.
Shortness of breath and dizziness, and/or fainting are also causes for concern.
In an exclusive interview with Tyla, a Mounjaro user named Christian reported some other side effects.
“I've got loads of energy - like loads of energy. The doctor actually said to me that I'd be surprised,” he told us. “It kind of subdues your stomach but it also helps to burn your fat cells so it gives you loads of energy.
"One of the other side effects is I've been struggling to get to sleep, but I don't feel that's a very common one."
How long can you be on Mounjaro for?
Morrisons Clinic states that this drug is designed for ‘long-term treatment’ and that it can be prescribed for up to two years.
How long you take it depends on your weight loss goals as well as your individual circumstances.
What happens when you come off Mounjaro?
Health experts from PhloClinic say that Mounjaro can take up to 30 days to leave your system, depending on your regular dosage.
Once you’ve finished your medication, the GLP-1 and GIP hormone receptors will no longer help regulate your appetite, and it's thought that they usually return to pre-treatment levels.
This could cause you to experience an increased appetite again and could regain some weight.
If you do want to quit, it’s advised you consult your clinical team so they can help you come off Mounjaro safely.
Ella Scott