Mounjaro vs Wegovy vs Orlistat: Comparing UK Weight-Loss Medicines

Prescription medicines for obesity are no longer confined to a single medication. New incretin‑based injections have joined older gastrointestinal drugs to offer people with obesity more options. Tirzepatide (Mounjaro) and semaglutide (Wegovy) are injectable hormones that suppress appetite, whereas orlistat is a capsule that blocks fat absorption. Understanding how these medicines differ in mechanism, dosing, eligibility and side‑effect profiles is essential for patients and clinicians deciding which treatment might be appropriate. This comparison draws on official UK leaflets, NHS guidance and regulatory sources to present an objective overview.

Summary table

How the medicines work

Mounjaro: dual incretin action

Tirzepatide acts on two incretin receptors: glucagon‑like peptide‑1 (GLP‑1) and glucose‑dependent insulinotropic polypeptide (GIP). These gut hormones increase insulin secretion after eating and signal satiety in the brain. The European Medicines Agency notes that tirzepatide binds to GLP‑1 and GIP receptors, increasing insulin release and reducing appetite[1]. By activating both receptors, Mounjaro appears to deliver greater weight loss than single‑hormone GLP‑1 drugs. Because it slows gastric emptying and reduces hunger, users must eat smaller meals and adjust their diets accordingly.

Wegovy: GLP‑1‑only appetite suppression

Semaglutide is a synthetic version of GLP‑1, a hormone released from the gut after eating. The package leaflet explains that Wegovy acts on receptors in the brain that control appetite, causing you to feel fuller, less hungry and less likely to crave food[17]. Like tirzepatide, it slows stomach emptying and should be used with a reduced‑calorie diet and increased physical activity[18].

Orlistat: blocking fat absorption

Unlike the two injections, orlistat targets the digestive system rather than appetite. NHS guidance states that orlistat prevents around a third of the fat from the food you eat being absorbed, so the undigested fat passes out with your stool[11]. Because it does not suppress hunger, you must learn to adopt a low‑fat diet to avoid gastrointestinal side‑effects.

Administration and dosing schedules

Mounjaro dosingMounjaro safe use, dosing, injection tips

Mounjaro comes in single‑use injection pens. Adults typically start on 2.5 mg once weekly for four weeks, then increase to 5 mg once weekly. If more weight loss is needed and tolerated, the dose may be increased by 2.5 mg every four weeks up to 15 mg once weekly[2]. The injection should be given under the skin of the abdomen, thigh or upper arm on the same day each week[19]. Missed doses should be taken as soon as possible within four days; otherwise skip and resume the regular schedule[20].

Wegovy dosing

Wegovy pens deliver semaglutide once per week. The dose is escalated gradually over 16 weeks: 0.25 mg weekly for weeks 1–4, 0.5 mg for weeks 5–8, 1 mg for weeks 9–12, 1.7 mg for weeks 13–16 and 2.4 mg from week 17 onward[7]. Users should inject into the upper arm, stomach or upper thigh, preferably on the same day each week, and the day can be changed provided at least three days have passed since the last injection[21]. If you miss a dose by 5 days or less, take it as soon as you remember; if more than 5 days have passed, skip the missed dose and continue on the next scheduled day[22].

Orlistat dosing

Orlistat is a 120 mg capsule taken three times per day. The NHS Lanarkshire leaflet advises taking one tablet with each main meal, immediately before, during or up to one hour after eating[12]. If you skip a meal or eat a meal with no fat, you should skip the dose. The medication begins blocking fat absorption within one to two days[23]. To continue taking orlistat beyond three months, patients must lose at least 5 % of their body weight[14].

Eligibility criteria and access Mounjaro NHS and private eligibility criteria

Mounjaro

NICE has recommended tirzepatide for adults with obesity (BMI ≥30 kg/m²) or a BMI between 27 and 30 kg/m² with weight‑related health problems. In England the medicine is being rolled out through specialist weight‑management services and is not widely available from GPs. Before prescribing Mounjaro, clinicians will discuss benefits and limitations and emphasise that you must eat a balanced, reduced‑calorie diet and take part in physical activity while using the medicine[3].

Wegovy

According to NHS guidance, semaglutide (Wegovy) can only be prescribed by a specialist weight‑management service. It is generally offered to adults with BMI ≥35 kg/m², or a BMI ≥32.5 kg/m² in people of Asian, Black African or Middle‑Eastern heritage, or with a BMI ≥30 kg/m² if they have significant weight‑related health issues[8]. Treatment is reviewed after six months and continued only if at least 5 % of body weight has been lost[24]. Semaglutide is not recommended for pregnant or breastfeeding women[25].

Orlistat

Orlistat is usually prescribed after lifestyle measures have been tried. NHS guidance says it is only considered if you have BMI ≥30 kg/m², or BMI ≥28 kg/m² with weight‑related conditions such as hypertension or type 2 diabetes[13]. The medicine is not typically recommended for pregnant or breastfeeding women[26]. Treatment should continue only if you lose at least 5 % of your body weight within three months[14]. Lower‑dose versions (Alli®, Orlos®) can be purchased from pharmacies under pharmacist supervision[27].

Side‑effect profiles

Mounjaro

The patient information leaflet lists nausea, diarrhoea, vomiting, abdominal pain and constipation as very common side‑effects[4]. Injection‑site reactions and hair loss are common[28]. More serious, but rarer, risks include pancreatitis, severe allergic reactions, gallstones and an increase in heart rate[29][30]. Low blood sugar can occur if tirzepatide is used with insulin or sulphonylureas[5].

Wegovy

Like tirzepatide, semaglutide commonly causes nausea, vomiting, diarrhoea, constipation and stomach pain[9]. The leaflet warns of serious side‑effects such as complications of diabetic eye disease, pancreatitis, kidney or bladder stones and severe allergic reactions[10]. Patients should seek medical attention if they experience severe stomach pain, vomiting that will not stop or signs of gallstones.

Orlistat

Because orlistat blocks fat absorption, gastrointestinal side‑effects are the most common. The NHS lists fatty or oily stools, urgent bowel movements, more frequent stooling and oily discharge[15]. Patients who maintain a low‑fat diet are much less likely to experience these problems[16]. Women taking the oral contraceptive pill should use an additional method of contraception if they experience severe diarrhoea while on orlistat because diarrhoea may prevent absorption of the pill[31].

Pros and cons

Mounjaro

Advantages: Dual action on GLP‑1 and GIP receptors may result in greater weight loss than GLP‑1‑only agents, and it is taken only once weekly, which many patients find convenient. It also improves blood‑sugar control for people with diabetes. Disadvantages: It is a relatively new medicine with limited long‑term data, injection‑related training is required, and there is a risk of gastrointestinal side‑effects and pancreatitis. Access is currently restricted to specialist services such as medcare healthclinic and Phamracy UK and Preston, and there are strict eligibility criteria.

Wegovy

Advantages: Evidence from clinical trials shows substantial weight loss when used with diet and exercise. Once‑weekly injections may be easier than daily medications. Wegovy has been on the market longer than Mounjaro and may become more widely available. Disadvantages: The dose‑escalation period of 16 weeks means it takes time to reach the effective 2.4 mg dose. Side‑effects similar to Mounjaro are common, and the drug may aggravate or unmask diabetic retinopathy. Access through the NHS is limited to specialist services and requires meeting strict BMI thresholds[8].

Orlistat

Advantages: Oral capsules may suit people who prefer not to inject themselves. Orlistat is widely available and can sometimes be purchased over the counter at a lower dose. Disadvantages: Gastrointestinal side‑effects are unpleasant and require strict adherence to a low‑fat diet. The medication must be taken three times a day with meals, which can reduce adherence. It tends to produce more modest weight loss than incretin‑based injections and should be stopped if weight loss is insufficient[14].

Which medicine might be right for you? Mounjaro weight-loss guidance

Choosing between Mounjaro, Wegovy and orlistat depends on your health status, weight‑loss goals, lifestyle and access to specialist services. Individuals with type 2 diabetes may benefit from tirzepatide’s dual action, while those intolerant of the new drug may respond well to semaglutide. People who cannot inject themselves or prefer an oral option may choose orlistat, though they must accept the need for a low‑fat diet. Ultimately, all three medicines require commitment to dietary changes and physical activity; the NHS emphasises that patients taking injections must eat a balanced, reduced‑calorie diet and exercise regularly[3], and the same applies to orlistat[32]. Work with your GP or weight‑management specialist to assess your eligibility, review potential side‑effects and discuss which option aligns with your needs and preferences.

Internal links and further reading

·         Mounjaro: mechanism and benefits -learn how tirzepatide works and the evidence behind its approval.

·         Who’s eligible for Mounjaro? -check our guide to NHS and private prescription criteria for tirzepatide.

·         How to use Mounjaro safely -read detailed instructions for injection technique, dosing and lifestyle tips.

·         Managing side‑effects and maintaining weight loss -our next article explains how to recognise and manage adverse effects and sustain your results.

·         Weight‑management landing page -explore our services and other medicines such as Wegovy, Saxenda and orlistat.

Conclusion

Mounjaro, Wegovy and orlistat are powerful tools to support weight loss when combined with lifestyle changes, but they are not interchangeable. Understanding their mechanisms, dosing schedules, eligibility criteria and side‑effect profiles helps you have informed conversations with your healthcare provider. Injectables like tirzepatide and semaglutide work on gut hormones to reduce appetite and slow digestion, while orlistat blocks fat absorption. Each medicine has advantages and drawbacks: some offer greater weight loss but require injections and may have more side‑effects, while others are easier to take but yield modest results. Always seek professional medical advice before starting any weight‑loss medication and remember that healthy eating, physical activity and behavioural support are the cornerstones of long‑term success.

References

 [1] Mounjaro | European Medicines Agency (EMA)

[2] Mounjaro dosage: Form, strengths, how to inject, and more

[3] NHS England » Weight management injections

[4] [5] [28] [29] [30] pil.15481.pdf

[6] [7] [9] [10] [17] [18] [21] [22] [25] Hqrdtemplatecleanen v10.1

[8] [11] [13] [14] [15] [16] [23] [24] [26] [27] [31] [32]  Obesity – Treatment – NHS

[12] Dietary advice whilst taking Orlistat | NHS Lanarkshire

[19] [20] Tirzepatide (subcutaneous route) – Side effects & dosage – Mayo Clinic