Mounjaro has quickly become one of the most talked-about weight loss treatments in the UK, helping thousands of people lose significant weight with medically supported treatment. But what’s actually in it, how does it work, and is it the right option for you?
Many people still don’t realise that Mounjaro is actually the brand name; the active ingredient is tirzepatide, and understanding it properly can make all the difference when you’re deciding whether to take the next step.
Tirzepatide is a once-weekly injectable treatment approved by the MHRA for weight management in adults. It works by targeting the body’s natural hunger and fullness hormones to help reduce appetite, control cravings, and support sustainable weight loss. In clinical trials, patients lost up to 22.5% of their body weight over 72 weeks.
Here at Phlo Clinic, we offer tirzepatide (Mounjaro) as part of our medically supported weight loss service, with prescriptions issued by qualified independent prescribing pharmacists.
If you’re already considering it, here’s everything you need to know about how tirzepatide works, who it’s suitable for, and what to expect from treatment.
What is tirzepatide?
Tirzepatide is the active ingredient in Mounjaro, a prescription weight loss injection manufactured by Eli Lilly. It belongs to a class of medicines known as GLP-1/GIP receptor agonists, which work by mimicking hormones your gut naturally releases after eating.
What makes tirzepatide different from other weight loss injections is that it targets two hormone receptors simultaneously: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Most similar treatments only target one. This dual action is why tirzepatide tends to produce more significant weight loss results than single-receptor alternatives.
The MHRA authorised tirzepatide for weight management in November 2023, making it a fully licensed treatment in the UK. It had previously been used to treat type 2 diabetes, where its effects on appetite and blood glucose were already well established.
Mounjaro vs tirzepatide: what’s the difference?
There’s no difference clinically. Mounjaro is simply the brand name for the medication; tirzepatide is the active ingredient. You may also see it referred to as Mounjaro KwikPen, which is the four-dose version approved by the MHRA in January 2024, offering a month’s supply in a single pen.
How does tirzepatide work for weight loss?
Tirzepatide works in three main ways, all of which contribute to weight loss:
- Reduces appetite: It signals to your brain that you’re full, making you feel less hungry between meals and reducing cravings.
- Slows stomach emptying: Food stays in your stomach longer after eating, which prolongs the feeling of fullness.
- Regulates blood glucose: It stimulates insulin release when blood sugar levels are high, helping to keep glucose levels stable.
The combined effect is that most people naturally eat less without feeling deprived.
What is tirzepatide approved for in the UK?
Tirzepatide has two distinct licensed uses in the UK, both authorised by the MHRA.
Its original approval was for type 2 diabetes, where it helps control blood glucose levels alongside diet and exercise. In November 2023, the MHRA granted a second indication: weight management in adults with obesity or overweight with weight-related health conditions. This made tirzepatide one of the most significant additions to the UK’s weight loss treatment landscape in years.
In terms of eligibility for weight management, the MHRA license covers:
- Adults with a BMI of 30 kg/m² or above
- Adults with a BMI of 27–29.9 kg/m² who have at least one weight-related condition, such as high blood pressure, prediabetes, high cholesterol, or cardiovascular problems
It’s worth noting that NHS access to tirzepatide for weight loss is being rolled out in phases, with priority given to those with higher BMIs and multiple health conditions.
At Phlo Clinic, we follow the same MHRA eligibility and safe prescribing guidelines used across UK healthcare. As a fully regulated GPhC-registered digital pharmacy, every consultation is reviewed individually by one of our UK-qualified prescribing pharmacists; we do not use automatic approvals. This helps ensure treatment is safe, appropriate, and clinically suitable before prescribing.
How tirzepatide differs from other GLP-1 medications?
Most weight loss injections currently available in the UK, such as semaglutide (Wegovy), work by targeting a single hormone receptor called GLP-1 (glucagon-like peptide-1). Tirzepatide goes a step further by also activating the GIP (glucose-dependent insulinotropic polypeptide) receptor, making it a dual agonist.
This distinction matters clinically. This dual action is important because the two hormones work through complementary pathways to help regulate appetite, blood glucose, and energy balance.
Why the dual mechanism matters
Most GLP-1 medications, such as semaglutide (Wegovy), act on a single hormone receptor. Tirzepatide (Mounjaro) activates both GLP-1 and GIP receptors. By activating both pathways, research suggests tirzepatide (Mounjaro) appears to produce greater weight loss than GLP-1-only medications.
This is supported by head-to-head trial data: in the SURPASS-2 trial, tirzepatide at 15mg consistently outperformed semaglutide 1mg on both weight loss and blood glucose control in adults.
What do the clinical trials show?
The evidence behind tirzepatide is robust. The MHRA’s approval was based on two large-scale international trials, SURMOUNT-1 and SURMOUNT-2, which together involved over 3,400 participants.
SURMOUNT-1 followed 2,539 overweight or obese adults without diabetes over 72 weeks.
SURMOUNT-2 looked at 938 adults with type 2 diabetes. Even in this group, where weight loss is typically harder to achieve, patients on the 15mg dose lost an average of 15.7% of their body weight, compared to 3.3% on placebo.
These aren’t modest improvements. At the highest dose, nearly all participants lost at least 5% of their body weight, and the majority lost significantly more. The full MHRA approval summary is publicly available for anyone who wants to read the detail.
Who is tirzepatide suitable for?
Tirzepatide is licensed for adults aged 18 and over who meet specific BMI criteria. According to MHRA guidance, it’s approved for:
- Adults with a BMI of 30 kg/m² or above (classified as obese)
- Adults with a BMI of 27–29.9 kg/m² (overweight) who also have at least one weight-related health condition, such as prediabetes, high blood pressure, high cholesterol, or heart problems
It should be used alongside a reduced-calorie diet and increased physical activity, not as a standalone solution.
Who should not take tirzepatide?
Tirzepatide isn’t suitable for everyone. It’s not recommended if you:
- Are pregnant or breastfeeding
- Have a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2
- Have a history of pancreatitis
- Are taking certain other medications that may interact with it
One of our prescribing pharmacists will assess your full medical history before issuing a prescription, which is why the clinical review step matters. At Phlo Clinic, every order is individually reviewed by a qualified independent prescribing pharmacist to confirm suitability before anything is dispensed.
What are the side effects?
Like all medicines, tirzepatide can cause side effects. The most common ones are gastrointestinal and tend to be most noticeable in the early weeks of treatment as your body adjusts.
Common side effects include:
These typically improve over time and are often managed by starting on a lower dose and gradually increasing. The dose titration schedule (starting at 2.5mg and increasing every four weeks if tolerated) is specifically designed to minimise these effects.
Less common but important to be aware of:
- Low blood sugar (hypoglycaemia), particularly in people with type 2 diabetes
- Injection site reactions
If you experience any side effect that concerns you, you can report it directly to the MHRA Yellow Card scheme. Your prescribing pharmacist is also there to support you throughout treatment, not just at the point of prescription.
How is tirzepatide taken?
Tirzepatide is injected once a week, on the same day each week, using a pre-filled pen. You inject it under the skin of your stomach, thigh, or upper arm. The injection itself is straightforward and most people manage it comfortably at home after an initial introduction.
The standard dose schedule is:
- 2.5mg for the first four weeks (starter dose)
- 5mg from week five onwards
- Doses can then be increased in 2.5mg increments every four weeks, up to a maximum of 15mg, based on how well you’re tolerating the treatment and your progress
You don’t need to reach the highest dose for tirzepatide to work. Many people see meaningful results at lower maintenance doses. The goal is to find the dose that works for you, not to push to the maximum.
Many patients begin seeing meaningful progress before reaching the highest dose. You can read more about real patient experiences and results in our Mounjaro reviews and weight loss success stories.
One practical note: if you’re taking the oral contraceptive pill, you should use an additional barrier method of contraception for four weeks after starting tirzepatide and for four weeks after each dose increase, as tirzepatide may affect how well the pill is absorbed.
Ready to find out if tirzepatide is right for you?
Tirzepatide is one of the most clinically effective weight loss treatments available in the UK today. It’s MHRA-approved, backed by large-scale trial data, and designed to work with your body’s own hunger signals rather than against them.
If you meet the eligibility criteria and you’re ready to take the next step, Phlo Clinic makes it straightforward. Complete a short medical questionnaire, have your order reviewed by one of our qualified prescribing pharmacists, and if approved, your treatment is delivered directly to your door with free 24-hour tracked delivery.
No waiting lists. No GP appointments. Just clinically reviewed, MHRA-approved treatment, when you need it.
References
- MHRA. Mounjaro (tirzepatide) authorised for weight management and weight loss. GOV.UK. Published 8 November 2023.
- MHRA. Four-dose Mounjaro KwikPen approved by MHRA for diabetes and weight management. GOV.UK. Published 25 January 2024.
- Jastreboff AM, et al. Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1). New England Journal of Medicine. 2022. Available via ACC Clinical Trials.
- Garvey WT, et al. Tirzepatide once weekly for the treatment of obesity in people with type 2 diabetes (SURMOUNT-2). The Lancet. 2023;402(10402):613-626. doi: 10.1016/S0140-6736(23)01200-X.
- MHRA Yellow Card scheme. Report a side effect. Medicines and Healthcare products Regulatory Agency.
- NHS England. Tirzepatide (Mounjaro) in primary care for weight management. Published January 2026.
