Orlistat or Mounjaro: in XENDOS, the longest orlistat trial, Xenical produced 5.8 kg of weight loss over four years against 3.0 kg on placebo, while Mounjaro (tirzepatide) produced 20.9% weight loss in 72 weeks at the top dose in SURMOUNT-1, against 3.1% on placebo (Diabetes Care 2004; NEJM 2022). The two drugs were never compared head to head: orlistat is a modest capsule with no injection, Mounjaro a different league of treatment with a different route to access.
Orlistat vs Mounjaro: what do the clinical trials show?
Orlistat and Mounjaro were tested in separate trials, and the gap in results is wide: a few kilos over four years for orlistat, up to a fifth of body weight over 72 weeks for Mounjaro. Our reference article on Mounjaro weight loss results in clinical studies breaks the figures down dose by dose; here is what matters for the comparison with orlistat.
For orlistat, the benchmark is XENDOS. It followed 3,305 adults with obesity for four years, randomised to orlistat 120 mg or placebo, all with lifestyle support. Weight loss reached 5.8 kg on orlistat against 3.0 kg on placebo, so the drug’s own contribution was roughly 2.8 kg over lifestyle support alone. Type 2 diabetes appeared in 6.2% of the orlistat group against 9.0% on placebo, mostly in people who already had impaired glucose tolerance.
For Mounjaro, SURMOUNT-1 enrolled 2,539 adults with obesity for 72 weeks: weight fell by 15.0% at 5 mg, 19.5% at 10 mg and 20.9% at 15 mg, against 3.1% on placebo. On a starting weight of 100 kg, 20.9% is roughly 21 kg, though individual results vary widely.
Two caveats. XENDOS aimed at preventing diabetes in people with obesity, while SURMOUNT-1 aimed at weight loss, so populations, durations and goals differ. And XENDOS lost many participants along the way: 52% of people on orlistat completed the study against 34% on placebo, a sign of how hard it is to stay on a treatment with digestive side effects for years.
How does orlistat (Xenical) work on dietary fat?
Orlistat works in the gut rather than the brain: it blocks gastrointestinal lipases, the enzymes that digest dietary fat, so that around 30% of the fat you eat is not digested and leaves in the stool, according to the European product summary for Xenical. It is not an appetite drug; its action is on the fat in your meals.
That is a difference of kind with Mounjaro, not just of degree. Tirzepatide mimics gut hormones (GLP-1 and GIP) that act on fullness and stomach emptying, with one weekly injection. Orlistat is taken as a capsule with a meal, up to one hour after it, and its action depends on what that meal contains, which makes it demanding to live with.
European authorities define the indication as obesity (BMI of 30 or more), or overweight (BMI of 28 or more) with associated risk factors, alongside a moderately reduced-calorie diet. You can see where you stand with the BMI calculator and weight-loss projection, a sensible first step before talking to a clinician.
What digestive side effects should you expect from orlistat and from Mounjaro?
Orlistat and Mounjaro both cause digestive symptoms, for opposite reasons. With orlistat they stem from undigested fat: oily spotting, fatty or oily stools, urgent bowel movements and flatulence with discharge, mostly early in treatment. With Mounjaro they are mainly nausea, diarrhoea or constipation around dose increases.
The NHS page on orlistat lists fatty or oily stools, urgent or more frequent bowel movements and wind with oily spotting among the common effects, and the European summary notes that they appear early and ease with time. For tirzepatide, SURMOUNT-1 reports that the most common adverse events were gastrointestinal, mostly mild to moderate, occurring mainly during dose escalation. We publish no percentages, because we did not verify comparable rates for both drugs.
Two safety points are worth knowing about orlistat. The Spanish product information for Xenical says treatment may alter absorption of the fat-soluble vitamins A, D, E and K, hence the advice to space any supplement at least two hours from the dose. And in 2012 the European Medicines Agency reviewed liver safety at France’s request and concluded that the benefits continued to outweigh the risks, while harmonising leaflets to list hepatitis as a possible effect. Yellow skin, dark urine or abdominal pain during treatment call for a prompt medical opinion.
Is orlistat available without a prescription, and who can get it on prescription?
Orlistat comes in two strengths with two legal statuses: Xenical 120 mg is prescription-only, while Alli 60 mg is the lower-dose version that can be bought in pharmacies without a prescription by adults with a BMI of 28 or more. Rules vary by country, so confirm with a pharmacist what applies where you live.
The European summary for Xenical lists it as prescription-only, and the one for Alli describes it as available without a prescription, for overweight adults with a BMI of 28 or more, for no more than six months. In the UK, the NHS says orlistat can be prescribed as Xenical when BMI is 28 or more with a weight-related condition, or 30 or more, while the lower-dose Alli is bought from pharmacies. It also advises that taking orlistat beyond three months is not recommended if you have not lost 5% of your body weight.
For Mounjaro, access, prescribing conditions and price depend heavily on your country and situation. To see what it could cost you, compare Mounjaro prices and coverage. Whichever you consider, avoid buying orlistat outside the legitimate pharmacy supply chain.
Orlistat or Mounjaro: how do you choose for your situation?
Between orlistat and Mounjaro, the right choice depends on your BMI, associated conditions, digestive tolerance and budget, and it is made with a clinician. Orlistat is less powerful but needs no injection; Mounjaro delivers better results in trials, with more tightly controlled access.
- You want to avoid injections: orlistat is a capsule and meets that criterion, at the cost of a more modest effect and real dietary constraints.
- You want the largest weight loss: trials place tirzepatide far above orlistat, even though the two were not compared in one protocol.
- Cost matters: the price of each depends on where you live and how access is organised; check it locally before deciding.
- Your digestion is fragile: both drugs affect the digestive system in different ways, so raise it openly at your appointment.
- You are already on a treatment: change nothing on your own. Stopping or swapping a prescribed drug is decided with your doctor or pharmacist, never from an article.
If you wonder where orlistat comes from and why it was long the only option, our piece on anti-obesity treatments before GLP-1s tells that story. The angle here is different: choosing today between two existing options.
Can you take orlistat and Mounjaro together?
We found no data in the official sources we consulted establishing the safety or benefit of combining orlistat and Mounjaro, so we neither recommend nor advise against it. It is a question for your doctor or pharmacist, who know your other medicines and your history.
One thing is clear: a combination decided without medical advice would add the digestive effects of two drugs with no demonstrated benefit. If you are on one and considering the other, note how you feel before discussing it.
How can you track your results, whichever treatment you use?
Whichever treatment you and your doctor choose, your weigh-ins and any injections are the basis of honest follow-up: without measurements you cannot tell whether the treatment is doing what you hoped. The NHS rule for orlistat, not continuing beyond three months without 5% weight lost, rests on exactly that kind of tracking.
You can log your weigh-ins and injections in the MounjaGO app and export a PDF report to bring to your review appointment. We do not describe any further feature here: tracking weight and injections is enough to document what you want to discuss.
What are the limits of these data?
The XENDOS and SURMOUNT-1 figures come from separate trials with different populations, durations and goals: in the sources we consulted, no trial compares orlistat directly with tirzepatide. Several limits therefore frame how you should read the figures, the comparisons and the access information on this page.
- Trial averages do not predict your individual result.
- XENDOS dates from 2004: it describes orlistat as studied then, over four years, with a high dropout rate.
- We publish no adverse-event rates, because we did not verify comparable figures for both drugs.
- Legal status, availability and cost differ between countries and change over time; we verified only the European product summaries and the NHS.
- Combining the two treatments is not documented in our sources.
Frequently asked questions
Is orlistat as effective as Mounjaro?
No, according to the trials: 5.8 kg lost over four years on orlistat in XENDOS (against 3.0 kg on placebo), against 20.9% of body weight in 72 weeks on Mounjaro 15 mg in SURMOUNT-1. The trials are separate, so a strict numerical comparison is not possible, but the difference in scale is clear.
Can you buy orlistat without a prescription?
The 60 mg version, Alli, is authorised in Europe without a prescription for adults with a BMI of 28 or more, for up to six months. The 120 mg version, Xenical, needs a prescription. Rules differ by country, so check with a pharmacist.
Does orlistat work without dieting?
No. The European indication pairs orlistat with a moderately reduced-calorie diet, and XENDOS tested it with lifestyle measures in both groups. Its action is on the fat in your meals, not on appetite.
Is orlistat a cheaper alternative to Mounjaro?
It depends on your country and how each is supplied or covered, so no single answer fits. Check prices with the price comparison tool, then discuss it with your doctor.
Should you stop Mounjaro to switch to orlistat?
Not on your own. A change of treatment is assessed with the clinician who follows you, based on your results, tolerance and goals. Never stop a prescribed treatment without medical advice.
Sources
- Torgerson JS, Hauptman J, Boldrin MN, Sjöström L. “XENDOS study”. Diabetes Care, 2004. PMID 14693982, accessed .
- Jastreboff AM, Aronne LJ et al. “Tirzepatide Once Weekly for the Treatment of Obesity” (SURMOUNT-1). New England Journal of Medicine, 2022. PMID 35658024, accessed .
- European Medicines Agency, Xenical: EPAR. ema.europa.eu, accessed .
- European Medicines Agency, Alli: EPAR. ema.europa.eu, accessed .
- European Medicines Agency, Orlistat: referral. ema.europa.eu, accessed .
- NHS, Orlistat. nhs.uk, accessed .
- AEMPS, Ficha técnica Xenical. cima.aemps.es, accessed .
Medical disclaimer. This article is for information only and does not replace medical advice. The figures quoted are clinical-trial averages and do not predict your individual result. Nothing here is a treatment, dose or switching recommendation: any decision to start, change or stop a treatment should be made with a doctor or pharmacist. Information current as of 5 October 2026.