Everything you need to know about Mounjaro
Learn how Mounjaro works, what results to expect, and side effects
Mounjaro has quickly become one of the most talked-about weight-loss treatments in the UK. It began as a treatment for type 2 diabetes, but its effects on hunger were also found to have a striking effect on weight loss.
In a major clinical trial, people taking the highest dose lost around a fifth of their starting weight over 72 weeks [1]. So, how does Mounjaro work, and when should you expect results?
In this article, we cover:
- What is Mounjaro and who’s it for?
- How does it work?
- How much weight can you lose?
- How long does it take to work?
- What are the doses?
- Is it safe?
- Can you drink alcohol while taking Mounjaro?
- What happens when you stop?
What is Mounjaro and who’s it for?
Mounjaro is the brand name for tirzepatide. It’s a prescription medicine taken as a once-weekly injection.
It was originally developed to treat type 2 diabetes, but now it’s also licensed in the UK for weight loss in adults with:
- A BMI of 30 or above
- A BMI of 27 or above alongside a weight-related health condition, like high blood pressure, high cholesterol, or prediabetes [2]
These are the licensed criteria. NHS access is currently more restricted. In every case, treatment requires an individual clinical assessment.
Mounjaro should not be used during pregnancy or breastfeeding. Women who could become pregnant should discuss contraception with their prescriber and inform them if pregnancy occurs or is planned. If you’re on HRT, make sure you let your prescriber know as Mounjaro can reduce the absorption of progesterone, reducing its protective effects.
Important: Mounjaro is used alongside changes to diet and physical activity, not instead of. It can make lifestyle changes considerably easier by reducing hunger, but it doesn’t replace them entirely.
How does Mounjaro work?
After you eat, your gut releases hormones that help your body respond to the meal. Two of these hormones are called GLP-1 and GIP. Mounjaro mimics the effect of both.
Mounjaro’s effect on the body:
- Helps you feel fuller after eating
- Reduces hunger and food cravings
- Slows the rate at which food leaves your stomach
- Helps the pancreas release insulin when blood sugar rises
- Improves insulin sensitivity and reduces the amount of glucose released by your liver [2]
This dual action is what sets Mounjaro apart from other treatments like Wegovy, which acts on GLP-1 alone.
How much weight can you lose with Mounjaro?
In the 72-week SURMOUNT-1 trial, assuming participants completed their treatment, average weight loss was:
- 16.0% with 5mg
- 21.4% with 10mg
- 22.5% with 15mg [1]
Participants also received support to reduce their calorie intake and increase physical activity. While it’s not guaranteed, most people see substantial weight loss with consistent use.
A useful first milestone is losing 5% of your starting weight. Even this relatively modest reduction can improve blood pressure, blood sugar, and other measures of metabolic health.
How long does Mounjaro take to work?
Mounjaro gets to work in your body quite quickly, but the changes on the scales take longer.
First 3 days: Mounjaro reaches its highest level in your bloodstream after 8 to 72 hours [2]. That doesn’t mean you’ll lose weight in three days, but some people may begin noticing hunger less. Others feel very little at first.
Weeks 1 to 4: You’ll usually remain on the 2.5mg starting dose while your body adjusts. Mounjaro reaches a steady level in your body after around four weeks, although weight loss may still be limited at this stage [1,2].
From week 5: Your dose normally increases to 5mg. As treatment continues, appetite control and weight loss may become more noticeable.
Over the following months: Weight loss tends to build gradually. In clinical trials, from around 14 weeks onwards, most people lose around 10% of their starting body weight and continue to lose more over the following months [1].
What are the Mounjaro doses?
Mounjaro is available in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg, and 15mg.
Treatment starts at 2.5mg once a week. After four weeks, the dose can be increased to 5mg. Further increases can be made in 2.5mg steps with at least four weeks at each dose [2].
There’s no need to reach 15mg; many people respond well to 5mg or 10mg and keep this as their maintenance dose.
When should you increase your Mounjaro dose?
Four weeks is the minimum time between dose increases, but it doesn’t mean you have to move up.
Your dose should only increase if you’re tolerating the treatment well. If you’re experiencing side effects, it’s best to wait until they’ve resolved or become manageable.
Which is better: Wegovy or Mounjaro?
Both are once-weekly injections for weight loss, but they contain different medicines.
Wegovy contains semaglutide and acts on GLP-1 receptors. Mounjaro contains tirzepatide and acts on both GLP-1 and GIP receptors.
In a direct 72-week comparison, participants taking tirzepatide lost an average of 20.2% of their weight compared with 13.7% among those taking semaglutide [3]. This suggests Mounjaro produces greater average weight loss, but that doesn’t necessarily mean it’s the best treatment for you.
Your medical history, previous treatment, side effects, treatment goals, and cost can all affect the choice. Some people tolerate one medicine better than the other.
Is Mounjaro safe?
Mounjaro has been studied in large clinical trials and is licensed for weight loss in the UK. The majority tolerate Mounjaro very well. Side effects are usually mild and manageable, though a few can be more serious.
The most common side effects include:
- Nausea and vomiting
- Diarrhoea or constipation
- Indigestion, acid reflux, or bloating
- Abdominal discomfort
- Injection-site reactions
Gastrointestinal side effects are by far the most common and can affect up to half of users taking the highest dose [4]. These side effects are most noticeable when treatment begins or the dose increases. They tend to settle as the body adjusts.
Persistent vomiting or diarrhoea can cause dehydration. Contact your clinical team if your symptoms aren’t settling or if you’re struggling to keep fluids down.
Less common but important risks include:
- Pancreatitis: Seek immediate help if you develop severe, persistent abdominal pain, especially if it spreads to your back.
- Gallbladder problems: Speak to a doctor urgently if you develop severe upper abdominal pain, fever, or yellowing of the skin or eyes.
This list isn’t exhaustive. Always read the patient information leaflet supplied with your pen and report any worrying symptoms.
Can you drink alcohol while taking Mounjaro?
There’s no ban on alcohol, but drinking may aggravate nausea, indigestion, and dehydration. Alcohol also contains calories and may make weight loss more difficult.
It’s sensible to limit or avoid alcohol when starting Mounjaro or increasing your dose.
If you’re taking Mounjaro for diabetes, you should be particularly careful because alcohol can affect your blood-sugar levels.
How quickly should you lose weight on Mounjaro?
There isn’t one perfect rate of weight loss, but both very slow and very rapid weight loss may prompt a review of your treatment.
If you’ve lost less than 5% of your starting weight after six months on your highest tolerated dose, your prescriber will usually review whether continuing Mounjaro is worthwhile. This doesn’t necessarily mean you’ll have to stop, as they’ll also consider any other benefits, side effects, and your individual circumstances.
At the other end of the scale, losing more than around 1% of your body weight each week, or roughly 4% a month, may be too fast. Your prescriber may pause further dose increases, reduce your dose, or recommend stopping treatment. Rapid weight loss can increase the likelihood of gallstones, which can occasionally lead to pancreatitis. It may also make it harder to maintain muscle mass and meet your nutritional needs.
The aim isn’t to lose weight as quickly as possible. A steady, sustainable response at a well-tolerated dose is generally preferable to chasing the maximum dose.
What happens when you stop Mounjaro?
Mounjaro doesn’t cause a withdrawal syndrome when you stop. However, its effects on appetite wear off after treatment stops, so hunger and food cravings are likely to return.
Weight regain is common. In the SURMOUNT-4 trial, participants who stopped regained a substantial proportion of the weight they had lost, despite continuing with lifestyle support [5].
Speak to your prescriber before stopping. It helps to have a realistic maintenance plan covering food, physical activity, and ongoing support. Many people use Mounjaro as a long-term treatment while it remains safe, effective, and affordable for them.
Is Mounjaro right for you?
Mounjaro can produce substantial weight loss and may make managing hunger feel far less exhausting. But it still requires appropriate prescribing, realistic expectations, and ongoing clinical support.
The aim isn’t to reach the highest dose as quickly as possible. It’s to find a treatment and dose that helps you make sustainable progress while continuing to feel well.
References
- Jastreboff AM, Aronne LJ, Ahmad NN, Wharton S, Connery L, Alves B, et al. Tirzepatide once weekly for the treatment of obesity. N Engl J Med. 2022;387:205–216. doi:10.1056/NEJMoa2206038
- Electronic Medicines Compendium. Mounjaro KwikPen: Summary of Product Characteristics. Eli Lilly and Company Limited. Available: https://www.medicines.org.uk/emc/product/1585/smpc
- Aronne LJ, Horn DB, le Roux CW, Ho W, Falcon BL, Gomez Valderas E, et al. Tirzepatide as compared with semaglutide for the treatment of obesity. N Engl J Med. 2025;393:26–36. doi:10.1056/NEJMoa2416394
- Mishra R, Raj R, Elshimy G, Zapata I, Kannan L, Majety P, et al. Adverse events related to tirzepatide. J Endocr Soc. 2023;7:bvad016. doi:10.1210/jendso/bvad016
- Horn DB, Linetzky B, Davies MJ, Laffin LJ, Wang H, Murphy MA, et al. Cardiometabolic parameter change by weight regain on tirzepatide withdrawal in adults with obesity. JAMA Intern Med. 2026;186:157–167. doi:10.1001/jamainternmed.2025.6112