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Mysimba or Orlistat: Prescription Weight Loss UK

Published on: June 03, 2026

Choosing between Mysimba or Orlistat UK prescription weight loss treatments depends on your medical history, lifestyle, and weight loss goals. Both medications are MHRA-licensed for weight management in the UK, but they work through entirely different mechanisms—Mysimba targets appetite and cravings in the brain, whilst Orlistat blocks fat absorption in the digestive system.

How Mysimba and Orlistat Work Differently

Mysimba contains two active ingredients—naltrexone 8mg and bupropion 90mg—that work synergistically on the central nervous system to reduce appetite and food cravings [1]. The combination targets the hypothalamus and mesolimbic reward system, areas of the brain that regulate hunger signals and the pleasure response to eating. Clinical trials demonstrate that this dual-action approach helps patients feel fuller sooner and reduces the psychological drive to overeat.

Orlistat, by contrast, works entirely in the gastrointestinal tract as a lipase inhibitor [2]. It blocks approximately 30% of dietary fat from being absorbed, allowing undigested fat to pass through the bowel. This mechanism means Orlistat only works when you consume fat—it has no effect on carbohydrates or protein, and it doesn't suppress appetite or alter brain chemistry. The medication is taken with meals containing fat, up to three times daily.

The fundamental difference matters clinically: Mysimba requires a gradual dose escalation over four weeks to minimise side effects and works continuously once established, whilst Orlistat works on a meal-by-meal basis and demands strict adherence to a reduced-fat diet to avoid gastrointestinal side effects [1][2].

Clinical Efficacy: Which Produces Greater Weight Loss?

In the COR-I trial involving 1,742 patients, those taking Mysimba achieved an average weight loss of 6.1% of body weight over 56 weeks, compared to 1.3% in the placebo group [1]. Approximately 50% of patients on Mysimba lost at least 5% of their initial body weight, and 28% achieved losses of 10% or more—thresholds associated with significant metabolic health improvements.

The XENDOS study, one of the largest Orlistat trials with over 3,300 participants, showed an average weight loss of 5.8kg over four years in the Orlistat group versus 3.0kg with placebo and lifestyle changes alone [2]. However, results vary considerably depending on dietary fat intake and adherence to the low-fat eating plan required for optimal outcomes.

Head-to-head comparisons are limited, but both medications demonstrate modest but clinically meaningful weight loss when combined with calorie restriction and increased physical activity. Your UK prescriber will consider factors including your BMI, previous weight loss attempts, cardiovascular risk factors, and ability to adhere to dietary restrictions when recommending between these options.

Realistic Expectations for Weight Loss

Neither Mysimba nor Orlistat produces the dramatic weight reductions seen with newer GLP-1 receptor agonists like tirzepatide or semaglutide, which can achieve average losses exceeding 15-20% of body weight [3]. However, both remain valuable options for patients who cannot tolerate injectable medications, prefer oral tablets, or have contraindications to GLP-1 treatments. Clinical guidelines emphasise that even modest weight loss of 5-10% significantly reduces risks of type 2 diabetes, hypertension, and cardiovascular disease.

Side Effect Profiles and Tolerability

Mysimba's most common side effects are neurological and gastrointestinal: nausea affects approximately 30% of patients, particularly during dose escalation, alongside headache, constipation, dizziness, and insomnia [1]. These effects typically diminish after the first month. More serious but rare risks include increased blood pressure and heart rate, requiring regular monitoring, and a slightly elevated seizure risk in susceptible individuals. Mysimba is contraindicated in patients with uncontrolled hypertension, seizure disorders, eating disorders, or those taking certain antidepressants or opioid medications.

Orlistat's side effects are predominantly gastrointestinal and directly related to its mechanism of blocking fat absorption [2]. Patients commonly experience oily spotting, flatulence with discharge, urgent bowel movements, and fatty or oily stools—effects that worsen significantly if high-fat meals are consumed. These symptoms can be socially disruptive but are not medically dangerous. Orlistat also reduces absorption of fat-soluble vitamins (A, D, E, K), so supplementation taken at least two hours apart from Orlistat doses is often recommended.

Tolerability often determines long-term adherence. Patients who struggle with the dietary discipline required for Orlistat may find Mysimba's approach more manageable, whilst those concerned about neurological side effects or blood pressure changes may prefer Orlistat's gastrointestinal mechanism despite the bowel-related effects.

Managing Side Effects Effectively

For Mysimba users, taking tablets with food, staying well-hydrated, and following the gradual titration schedule significantly reduces nausea and headaches. For Orlistat, adhering strictly to a low-fat diet (no more than 15g fat per meal) virtually eliminates the troublesome gastrointestinal effects whilst still allowing the medication to work on the reduced fat you do consume.

Eligibility Criteria and Prescribing Guidelines

NICE guidelines recommend prescription weight loss medications for adults with a BMI of 30 kg/m² or above, or 27 kg/m² or above with weight-related comorbidities such as type 2 diabetes, hypertension, or dyslipidaemia [4]. Both Mysimba and Orlistat fall within these prescribing criteria, though specific patient factors influence which medication your UK prescriber will recommend.

Mysimba requires more extensive medical screening due to its cardiovascular and neurological effects. Your prescriber will assess your blood pressure, heart rate, mental health history, current medications, and seizure risk before prescribing. Regular blood pressure monitoring is mandatory during treatment. Mysimba is not suitable during pregnancy or breastfeeding, and contraception is advised for women of childbearing potential.

Orlistat has fewer absolute contraindications but requires assessment of your digestive health, particularly any history of malabsorption conditions, chronic diarrhoea, or cholestasis [2]. It's generally considered during pregnancy and breastfeeding, though evidence is limited. Patients taking ciclosporin, warfarin, or anticonvulsants require careful monitoring due to potential drug interactions affecting absorption.

Cost Comparison and Treatment Duration

At Cured Pharmacy, Mysimba tablets start from £135.00 for a one-month supply, whilst Orlistat is available from £135.00 per month, making it one of the more affordable prescription weight loss options in the UK. The significant price difference reflects the medications' different formulations and manufacturing processes, with Mysimba's combination therapy commanding higher costs.

Treatment duration varies by individual response. NICE recommends discontinuing weight loss medication if less than 5% weight loss is achieved within 12 weeks for Orlistat or 16 weeks for Mysimba [4]. Successful responders may continue treatment for extended periods—clinical trials have safely studied Mysimba for up to two years and Orlistat for four years, though many patients transition to weight maintenance strategies once their goal weight is achieved.

When calculating total treatment cost, consider the required dietary modifications and potential additional expenses. Orlistat necessitates a lower-fat diet, which may reduce food costs, whilst Mysimba requires periodic blood pressure monitoring appointments. Both medications are most cost-effective when combined with comprehensive lifestyle changes that promote long-term weight maintenance after medication is discontinued.

Alternative Weight Loss Treatments Available

Beyond Mysimba and Orlistat, Cured Pharmacy offers newer GLP-1 receptor agonist treatments including Mounjaro, Wegovy, and Saxenda, which demonstrate superior weight loss outcomes in clinical trials but at higher price points and via injection rather than oral tablets. Your UK prescriber will discuss the full range of licensed options during your free online consultation to identify the most appropriate treatment for your individual circumstances.

Making Your Decision: Mysimba or Orlistat UK Prescription

The choice between Mysimba or Orlistat UK prescription weight loss treatments ultimately depends on your medical profile, lifestyle preferences, and tolerance for different side effect profiles. Mysimba suits patients who struggle with appetite control and emotional eating, who can tolerate potential neurological side effects, and who have healthy blood pressure. Its once-daily dosing (after titration to the maintenance dose of two tablets twice daily) offers convenience.

Orlistat is preferable for patients who consume moderate to high dietary fat, who want to avoid centrally-acting medications, or who have contraindications to Mysimba such as seizure disorders or uncontrolled hypertension. Its mechanism provides immediate feedback—consuming too much fat results in unpleasant bowel symptoms—which some patients find helpful for reinforcing dietary discipline.

At Cured Pharmacy, your free online consultation with a UK-registered prescriber takes under three minutes and provides personalised recommendations based on your complete medical history, current medications, weight loss goals, and previous treatment experiences. All prescription medications require clinical assessment and approval by our prescribing team before dispensing, ensuring you receive the safest and most effective treatment for your individual needs.

Starting Your Weight Loss Journey

Whichever medication your prescriber recommends, success requires commitment to sustainable lifestyle changes including calorie reduction, increased physical activity, and behavioural modifications around eating habits. Prescription weight loss medications are tools that make these changes more achievable, not replacements for them. Our clinical team provides ongoing support throughout your treatment to optimise your results and manage any side effects that arise.

Scientific References

1. Greenway, F. L., et al. (2010). Effect of naltrexone plus bupropion on weight loss in overweight and obese adults (COR-I): a multicentre, randomised, double-blind, placebo-controlled, phase 3 trial. The Lancet, 376(9741), 595-605. https://doi.org/10.1016/S0140-6736(10)60888-4 [accessed 13 August 2026]
2. Torgerson, J. S., et al. (2004). XENical in the prevention of diabetes in obese subjects (XENDOS) study: a randomized study of orlistat as an adjunct to lifestyle changes for the prevention of type 2 diabetes in obese patients. Diabetes Care, 27(1), 155-161. https://doi.org/10.2337/diacare.27.1.155 [accessed 13 August 2026]
3. Jastreboff, A. M., et al. (2022). Tirzepatide Once Weekly for the Treatment of Obesity. New England Journal of Medicine, 387(3), 205-216. https://doi.org/10.1056/NEJMoa2206038 [accessed 13 August 2026]
4. National Institute for Health and Care Excellence. (2014). Weight management: lifestyle services for overweight or obese adults (PH53). NICE. https://www.nice.org.uk/guidance/ph53 [accessed 13 August 2026]
5. NHS. (2026). Tirzepatide: a medicine to manage type 2 diabetes and treat obesity. https://www.nhs.uk/medicines/tirzepatide/ Accessed 13 August 2026.
6. NHS. (2026). Overweight and obesity in adults. https://www.nhs.uk/conditions/overweight-and-obesity/ Accessed 13 August 2026.
7. NHS England. (2026). Weight management injections. https://www.england.nhs.uk/ourwork/prevention/obesity/medicines-for-obesity/weight-management-injections/ Accessed 13 August 2026.

Information on this page is for educational purposes only and is not medical advice. All prescription treatments require clinical assessment by a UK-registered prescriber. Always consult a qualified healthcare professional before starting any new medication.