If you've been looking into weight loss treatments, you've likely come across both Mounjaro and orlistat. They're both available in the UK and both used to support weight management - but that's roughly where the similarities end. These two medicines work through completely different mechanisms, produce very different results in clinical trials, and suit different patients. This guide explains how Mounjaro and orlistat compare, so you can have a more informed conversation with your prescriber about which option may be right for you.
How Does Mounjaro Work?
Mounjaro contains tirzepatide, a medicine that acts on two hormone receptors in the body: GLP-1 (glucagon-like peptide-1) and GIP (glucose-dependent insulinotropic polypeptide). Both are incretin hormones - naturally occurring gut hormones released after eating that signal to the brain that you're full, slow the rate at which food leaves your stomach, and help regulate blood sugar.
By activating both receptors simultaneously, tirzepatide produces a stronger appetite-reducing effect than medicines targeting GLP-1 alone. Most people find it significantly easier to eat less without the persistent hunger that makes dieting difficult to maintain. Some people also describe a reduction in "food noise" - the constant preoccupation with eating that can undermine even well-intentioned dietary changes.
Mounjaro is administered as a once-weekly subcutaneous (under the skin) injection using a pre-filled pen. Treatment begins at 2.5 mg per week, with the dose increased every four weeks up to a maximum of 15 mg, depending on tolerability and response.
How Does Orlistat Work?
Orlistat works through an entirely different mechanism. Rather than affecting appetite or hormones, it acts locally in the gut to block the enzymes responsible for breaking down dietary fat. These enzymes - gastric and pancreatic lipases - normally hydrolyse the triglycerides in food into absorbable fatty acids. Orlistat binds to these lipases and inhibits them, preventing around one-third of the fat you consume from being absorbed. That undigested fat passes through the gut and is excreted in stools.
Orlistat is taken as a capsule, typically three times daily alongside meals that contain fat. The prescription-strength version, available as Xenical or generic orlistat 120 mg, requires a clinical assessment. A lower-dose version (60 mg, branded as Alli) is available over the counter from pharmacies without a prescription, following a brief pharmacist assessment.
Because orlistat doesn't affect appetite or satiety, the motivation to reduce calorie intake must come from you. Its mechanism is most effective when dietary fat intake is kept moderate - roughly 30% of daily calories from fat. Consuming a high-fat meal while taking orlistat significantly increases the likelihood of gastrointestinal side effects.
How Do Mounjaro and Orlistat Compare on Effectiveness?
Clinical trial data shows a substantial difference in the degree of weight loss achieved with each treatment.
In the SURMOUNT-1 phase 3 trial - the largest clinical study of tirzepatide for weight management in adults without diabetes - participants achieved average weight reductions of 16.0%, 21.4%, and 22.5% over 72 weeks at the 5 mg, 10 mg, and 15 mg doses respectively, compared to 2.4% in the placebo group.1 These are among the largest weight loss results recorded in phase 3 trials of licensed weight management medicines.
Orlistat produces more modest outcomes. A meta-analysis of 22 randomised controlled trials (n = 5,921) found that orlistat produced a mean weight loss of 2.40 kg more than controls - statistically significant, but comparatively small.2 In real-world use, weight loss with orlistat typically falls in the range of 3-5% of starting body weight when used consistently alongside a reduced-calorie, lower-fat diet.
The gap reflects how differently each medicine works. Mounjaro reduces appetite systemically, making it easier to sustain a significant calorie deficit. Orlistat only reduces calorie absorption from dietary fat and has no effect on overall appetite, meaning adherence to a calorie-controlled diet remains entirely dependent on the patient.
Neither treatment is a substitute for dietary and lifestyle changes. Both medicines are licensed for use alongside a reduced-calorie diet and increased physical activity, and both can produce weight regain when stopped without supporting lifestyle changes in place.
Side Effects: What to Expect from Each Treatment
Mounjaro side effects
The most common side effects of Mounjaro are gastrointestinal and are most pronounced in the early weeks of treatment or following a dose increase. According to Mounjaro's official prescribing information, nausea, diarrhoea, vomiting, and decreased appetite are classified as "very common" side effects, meaning they affect more than 1 in 10 people.3 These effects tend to decrease as the body adjusts.
More serious but less common risks include acute pancreatitis, gallbladder problems (including gallstones, which can occur more frequently during rapid weight loss), and acute kidney injury as a secondary consequence of severe dehydration. These require prompt medical assessment if symptoms develop.
Mounjaro slows the rate at which food leaves the stomach, which can reduce the absorption of oral medicines - including the combined oral contraceptive pill. Women taking oral contraception should use an additional barrier method for four weeks after starting tirzepatide and after each dose increase.
A full overview is available in our Mounjaro side effects guide.
Orlistat side effects
Orlistat's side effects are almost entirely gastrointestinal and are directly related to the amount of unabsorbed fat passing through the gut. The most commonly reported effects include oily or fatty stools, oily spotting, increased bowel movements, flatulence (with or without oily discharge), and in some cases faecal urgency.4 These are more likely after meals with a high fat content.
Gastrointestinal effects with orlistat tend to decrease over time, particularly as patients become more consistent about reducing dietary fat. However, for some people they remain bothersome enough to affect quality of life or treatment adherence.
Orlistat also reduces the absorption of fat-soluble vitamins (A, D, E, and K). A daily multivitamin supplement - taken at least two hours before or after an orlistat dose - is recommended during treatment to maintain adequate vitamin levels.4 There is a small risk of interaction with warfarin, ciclosporin, and amiodaron. Patients taking these medicines should inform their prescriber before starting orlistat.
Who Is Eligible for Mounjaro in the UK?
Access to Mounjaro depends on whether you are seeking treatment through the NHS or privately, and the criteria differ significantly between the two routes.
Mounjaro is officially licensed in the UK for weight management in adults with a BMI of 30 or above, or from 27 where at least one weight-related health condition is present - such as high blood pressure, high cholesterol, obstructive sleep apnoea, cardiovascular disease, prediabetes, or type 2 diabetes.3 This is the licensed basis on which private prescribers can offer treatment.
For NHS prescribing, NICE technology appraisal TA1026 (published December 2024) sets a more restrictive threshold: tirzepatide is recommended for adults with a BMI of at least 35 and at least one weight-related health condition.5 A lower BMI threshold (reduced by 2.5) applies to people from South Asian, Chinese, other Asian, Middle Eastern, Black African, or African-Caribbean backgrounds. NHS England's phased rollout has further prioritised the highest-need patients first: initially those with a BMI of 40 or above and at least four qualifying health conditions, with eligibility expected to expand over time.5
Mounjaro is not suitable during pregnancy or breastfeeding, or for people with a personal or family history of a rare type of thyroid cancer called medullary thyroid carcinoma, or a condition called Multiple Endocrine Neoplasia syndrome type 2 (MEN2) - a genetic disorder affecting hormone-producing glands.
Who Is Eligible for Orlistat in the UK?
Orlistat 120 mg (prescription strength) is licensed for adults with a BMI of 30 or above, or from 28 where at least one weight-related risk factor is present, such as type 2 diabetes, high blood pressure, or high cholesterol.4 Treatment is typically initiated alongside dietary advice and continued for up to a year, with ongoing prescribing dependent on demonstrating consistent weight loss.
Alli 60 mg (over the counter) is available without a prescription from pharmacies for adults aged 18 or over with a BMI of 28 or above, following a brief pharmacist assessment.
Because orlistat's eligibility criteria are less restrictive and it is available without a prescription at the lower dose, it is often more accessible to people who do not yet meet the BMI or health condition requirements for Mounjaro.
Orlistat is not recommended during pregnancy or breastfeeding, and should be used with caution in patients taking ciclosporin, warfarin, or amiodarone.
Which Treatment Is Right for You?
There is no single answer, and the decision should always be made with a prescriber based on your individual health profile. That said, some general principles apply.
- Mounjaro - tends to produce significantly greater weight loss and is better suited to people with obesity, particularly those with a higher BMI or weight-related conditions who are likely to benefit from more substantial reduction. The once-weekly injection format suits people who find daily tablet regimens difficult to maintain.
- Orlistat - typically more accessible and lower cost, with no injection required. It may be a practical starting point for people who are overweight rather than obese, who prefer an oral treatment, or who do not yet meet the eligibility criteria for Mounjaro. Its effectiveness depends heavily on consistency with a low-fat diet.
The best outcomes with either treatment come from using medication as a tool within a broader approach to diet, physical activity, and habit change - not as a standalone solution.
When to Seek Further Advice
Contact your GP or prescribing clinician if you:
- Experience severe or persistent nausea, vomiting, abdominal pain, or diarrhoea on either treatment
- Develop symptoms consistent with pancreatitis while taking Mounjaro: severe upper abdominal pain, particularly radiating to the back
- Notice signs of gallbladder problems: pain in the upper right abdomen, especially after fatty meals
- Are taking oral contraceptives and have recently started Mounjaro or increased your dose
- Are taking warfarin or ciclosporin and are prescribed orlistat
- Have lost less than 5% of your starting body weight after six months on either treatment at the maximum tolerated dose - your clinician may need to reassess suitability
Do not stop either treatment without speaking to your prescriber first, as a structured dose review may be appropriate.
Frequently Asked Questions
Is Mounjaro better than orlistat for weight loss?
Clinical trial data shows Mounjaro produces significantly greater weight loss than orlistat. In the SURMOUNT-1 trial, tirzepatide produced average weight reductions of up to 22.5% over 72 weeks, compared to the modest additional weight loss of around 2.4 kg beyond diet and exercise seen with orlistat in clinical studies. However, "better" depends on your individual circumstances. Mounjaro has stricter eligibility criteria, requires a weekly injection, and costs more. Orlistat is more accessible and available without a prescription at the lower dose, making it a practical option for people who are overweight rather than obese, or who prefer an oral treatment over a weekly injection.
Can I take Mounjaro and orlistat together?
No. There is no clinical evidence supporting the combined use of Mounjaro and orlistat, and taking both simultaneously would not be appropriate. If you are already taking orlistat and want to switch to Mounjaro, speak to your prescriber. They will assess whether you meet the eligibility criteria and guide you through the transition.
Can I get Mounjaro or orlistat on the NHS?
Orlistat 120 mg is available on the NHS for adults with a BMI of 30 or above, or from 28 with a weight-related health condition. Mounjaro is available on the NHS following NICE approval (TA1026, December 2024), but access is currently restricted to those with the highest clinical need: initially adults with a BMI of 40 or above and at least four weight-related comorbidities. Both treatments require a clinical assessment. Mounjaro is also available privately with less restrictive criteria, in line with its officially licensed indications.
What happens when you stop taking Mounjaro or orlistat?
Weight regain is common after stopping either treatment if the underlying diet and lifestyle changes are not maintained. With Mounjaro, the appetite-suppressing effects of tirzepatide cease when the injections stop, and hunger typically returns to baseline levels. Clinical studies have shown significant weight regain following discontinuation without continued lifestyle support. Orlistat's effect also stops immediately on discontinuation, as it has no systemic action. In both cases, speaking to your prescriber before stopping is strongly recommended.
Which has fewer side effects - Mounjaro or orlistat?
Both treatments commonly cause gastrointestinal side effects, but the nature of those effects differs. Mounjaro's side effects - nausea, diarrhoea, vomiting - are systemic, driven by its effect on gut hormones and gastric emptying, and tend to improve over time as the body adjusts. Orlistat's side effects - oily stools, flatulence, faecal urgency - are directly caused by unabsorbed fat passing through the gut, and are more controllable through diet: keeping dietary fat intake low significantly reduces their likelihood. Neither is clearly "better" on side effects. It depends on which profile a patient finds more manageable.
References
- Jastreboff AM, et al. Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine. 2022;387(3):205-216. doi:10.1056/NEJMoa2206038
- Bhaskaran K, et al. Efficacy of orlistat in type 2 diabetes: a systematic review and meta-analysis. BJGP Open. 2025. doi:10.3399/BJGPO.2025.0058
- Eli Lilly Nederland B.V. Mounjaro (tirzepatide) Summary of Product Characteristics. Electronic Medicines Compendium (eMC). medicines.org.uk/emc
- Roche Products Ltd. Xenical (orlistat 120 mg) Summary of Product Characteristics. Electronic Medicines Compendium (eMC). medicines.org.uk/emc
- National Institute for Health and Care Excellence. Tirzepatide for managing overweight and obesity (TA1026). December 2024. nice.org.uk/guidance/ta1026
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.