Mounjaro (tirzepatide) is a once-weekly weight loss injection designed to target natural gut hormones that regulate appetite and blood sugar, helping support steady, clinically-meaningful weight loss over time.
Mounjaro reduces appetite, helping you feel full and eat less
Weight loss builds gradually
Check if the weight loss treatment is right for you
If you’re starting Mounjaro, one of the first questions you’ll have is: what should I actually expect? How quickly will it work, how much weight might you lose, and what changes will you notice in the first few weeks?
Rather than relying on willpower alone, Mounjaro works with your body’s natural gut hormones to reduce hunger and help you feel full for longer, supporting gradual, sustained weight loss as part of a structured, clinician-led programme.1
Tirzepatide works by acting on two natural gut hormones, GLP-1 and GIP, which help to control blood sugar and appetite. Together, these signals help the body release insulin more effectively after meals, reduce hunger, and support feelings of fullness.1
Because it stays active in the body for several days, it’s taken just once a week. In clinical studies, people taking tirzepatide lost a significant amount of weight over time, alongside improvements in markers linked to insulin sensitivity and metabolic health.2
Who can access Mounjaro depends a lot on whether you’re going through the NHS or a private provider.
On the NHS, access to Mounjaro follows strict NICE guidance and is being rolled out in phases. In practice, this means treatment is currently limited to people with more severe obesity and multiple weight-related health conditions, and waiting times can be long.
At present, or over the next few years, NHS eligibility generally requires:2
A BMI of 35 kg/m² or higher, plus at least one weight-related condition, such as high blood pressure, abnormal cholesterol, sleep apnoea, cardiovascular disease, or type 2 diabetes
In many cases, a BMI of 40 kg/m² or higher, often alongside multiple weight-related conditions
Lower BMI thresholds (by around 2.5 kg/m²) apply for people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds
Even if someone meets the criteria, NHS access may be delayed due to phased funding and service capacity. Treatment is also reviewed closely, and continuation usually depends on achieving meaningful weight loss (typically at least 5% within six months at the highest tolerated dose).
Private providers may offer access outside NHS funding criteria, subject to clinical assessment.
With Numan, Mounjaro may be suitable if you:
Are 18 years or older, and
Have a BMI of 30 kg/m² or above, or
Have a BMI of 27 kg/m² or above with at least one weight-related condition (such as high blood pressure, high cholesterol, insulin resistance, or pre-diabetes), and
Complete a medical assessment reviewed by a qualified prescriber
This approach recognises that earlier intervention can reduce long-term health risks, rather than waiting until obesity becomes more severe.
Mounjaro isn’t appropriate for everyone. It may not be suitable if you:
Don’t meet the BMI or clinical criteria
Have certain medical conditions or medication interactions
Are pregnant, breastfeeding, or planning pregnancy
Are seeking a short-term or cosmetic weight-loss solution
Most people don’t lose a large amount of weight in the first week or two. Early changes are often subtle as your body adjusts to the medication and appetite begins to settle. Often, people notice reduced hunger first, before the scales move in a meaningful way.3
Over the first few months, weight loss usually becomes more noticeable and more consistent. In clinical studies, the majority of people taking tirzepatide lost at least 5% of their starting weight over time, with steady progress rather than rapid drops. This slower pace is expected and is generally safer and easier to maintain.3
As treatment continues and doses are gradually increased, weight loss tends to build. By around a year to a year and a half, people taking tirzepatide in the SURMOUNT clinical trial, lost, on average:3
Around 15% of their starting weight on lower maintenance doses
Around 19-21% of their starting weight on higher maintenance doses
Importantly, many people achieved substantial results, with roughly half of those on higher doses losing 20% or more of their body weight. Alongside weight loss, improvements were seen in markers linked to heart and metabolic health.3
Like other appetite-regulating medications, Mounjaro can cause side effects, particularly as your body adjusts. Studies show that most people experience mild effects, if any, and they tend to be temporary, easing as treatment continues or once a dose has settled.
The most commonly reported side effects are digestive and include:1,3
Nausea
Diarrhoea
Constipation
Vomiting or acid reflux
Less commonly, some people may notice:1,3
A faster heart rate
Reactions around the injection site, such as redness or irritation
Symptoms linked to dehydration if fluid intake is low
Rare but important side effects, such as pancreatitis, gallbladder issues, or changes in blood sugar, are uncommon and would be monitored closely by clinicians. Please refer to the patient information leaflet for a more detailed list of side effects.
By working with the body’s appetite and metabolic systems, Mounjaro supports steady, clinically meaningful weight loss when combined with proper medical oversight and lifestyle support. The key is suitability, safety, and long-term thinking, which is why treatment should always start with a thorough assessment and ongoing clinician care, not guesswork or pressure to rush results.
Farzam K, Patel P. Tirzepatide. In: StatPearls. Treasure Island (FL): StatPearls Publishing; 2025.
Tirzepatide for managing overweight and obesity. NICE. [accessed 29 Jan 2026] Available from: https://www.nice.org.uk/guidance/ta1026
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(3):205–16.
Medically reviewed by Hassan Thwaini
Clinical Pharmacist and Copywriter, Master of Pharmacy (MPharm)
Medically reviewed:
