Orlistat vs Mounjaro: Old Pill vs New Injection

If you are comparing orlistat vs Mounjaro, you are really weighing up two very different eras of weight-loss medicine: a decades-old fat-blocking tablet against a modern once-weekly injection. This guide explains how each works, how much weight you can realistically expect to lose, the side effects, who is eligible, and what they cost in the UK — using verified prices last checked on 25 August 2026. It is information only, not medical advice; a prescriber always decides what is suitable for you.

How orlistat and Mounjaro work — two different mechanisms

The core difference in orlistat vs Mounjaro is where they act. Orlistat works in your gut: it blocks the enzyme (lipase) that breaks down dietary fat, so around 30% of the fat you eat passes through undigested rather than being absorbed. It does nothing to your appetite — you simply absorb fewer calories from fatty food.

Mounjaro (tirzepatide) works on your brain and hormones. It mimics two natural gut hormones, GLP-1 and GIP, which slow stomach emptying and reduce appetite. You feel fuller sooner, stay full longer, and generally want to eat less. That appetite effect is why the newer injections produce much larger results than fat-blocking tablets.

How well does each one work?

This is where the two medicines diverge most sharply.

Orlistat is modest. Used alongside a reduced-calorie, lower-fat diet, it typically helps people lose a few percent more body weight than diet alone over a year — useful, but small. It relies heavily on you sticking to a low-fat diet, partly because eating fatty food while taking it triggers unpleasant side effects.

Mounjaro is the most effective weight-loss medicine studied to date. In the SURMOUNT-1 trial, adults with obesity or overweight lost an average of 16.0% of body weight at 5 mg, 21.4% at 10 mg and 22.5% at 15 mg over 72 weeks, compared with just 2.4% on placebo. Around 90% of people on the 15 mg dose lost at least 10% of their body weight.

In short: if losing a meaningful amount of weight is the goal, Mounjaro delivers on a scale orlistat cannot match. For a wider view of how the medicines rank, see our guide to prescription weight-loss drugs in the UK.

Side effects: gut discomfort of two very different kinds

Both medicines have gastrointestinal side effects, but they feel completely different.

Orlistat side effects are driven by unabsorbed fat: oily or fatty stools, urgent bowel movements, wind with discharge, and oily spotting. These get worse the more fat you eat, which is partly the point — it nudges you toward a lower-fat diet. It can also reduce absorption of fat-soluble vitamins, so a supplement is sometimes advised.

Mounjaro side effects are appetite- and digestion-related: nausea, diarrhoea, constipation, vomiting and tiredness are the common ones, usually worst when you first start or increase the dose. Rare but serious risks include pancreatitis, gallstones and dehydration that can strain the kidneys. Anyone using insulin or a sulfonylurea should be aware of low blood sugar.

Whichever you take, report any suspected side effect through the MHRA Yellow Card scheme.

Dosing and format

Orlistat is a capsule taken up to three times a day with meals containing fat — prescription strength is 120 mg, while a lower-dose version (Alli, 60 mg) is available over the counter in pharmacies.

Mounjaro is a once-weekly injection using a pre-filled pen. It is started low to reduce side effects: 2.5 mg weekly for four weeks (a starter dose, not a treatment dose), then 5 mg, rising in 2.5 mg steps every four weeks or more up to a maximum of 15 mg. Your maintenance dose (5, 10 or 15 mg) is tailored to you.

If the choice between a tablet and an injection matters to you, our comparison of weight-loss injections vs tablets weighs up convenience against results.

Who is eligible?

For Mounjaro, private eligibility is usually a BMI of 30 or above, or 27 or above with a weight-related condition such as high blood pressure or type 2 diabetes. It is recommended by NICE (TA1026, December 2024) and is being rolled out on the NHS in phases from 2025, prioritising the highest BMI and most weight-related conditions first, so NHS access varies by area.

Orlistat can be prescribed at similar BMI thresholds and is also available over the counter at lower strength, making it more accessible if cost or injecting is a barrier. A prescriber makes the final call either way. Our weight-loss medication eligibility guide explains the BMI rules in detail.

Orlistat vs Mounjaro: cost in the UK

Orlistat is inexpensive — prescription orlistat costs little more than an NHS prescription charge, and over-the-counter Alli is typically £20–£40 for a pack. Mounjaro is a private medicine bought monthly, and prices vary widely between regulated pharmacies for the identical pen. The table below shows verified Mounjaro starter-dose prices, including our recommended provider, The Weight Clinic.

Option (starter dose)	Cheapest verified	The Weight Clinic	Typical top of range
Mounjaro 2.5 mg	£131 (Next Script)	£160 (first month £125)	£220 (Envigore)
Mounjaro 15 mg	£266 (Next Script / Medicine Marketplace)	£295	£359 (Numan / Voy)
Orlistat (Alli OTC)	~£20–£40 per pack	—	—

For Mounjaro alone the spread is £89 a month for the same pen — £131 at the cheapest to £220 at the priciest starter dose. That is why it pays to compare rather than buy from the first name you recognise. You can see every provider on our live price table, and for a deeper dive read weight-loss medication costs compared.

The Weight Clinic is our recommended provider. It is a GPhC-registered pharmacy offering monthly video reviews with a prescriber, a refund if the prescriber declines your request, and £35 off your first order with code NEWME — taking a first month of Mounjaro to £125. You can check your eligibility with The Weight Clinic here.

Which should you choose?

Orlistat may suit you if you want a low-cost, non-injectable option, are comfortable with a strict low-fat diet, and are happy with modest results. It is a reasonable first step for some people, or an option if GLP-1 medicines are not suitable.

Mounjaro is the stronger choice if you have a significant amount to lose and want the most effective licensed medicine available — accepting the higher monthly cost and weekly injections. Many people who have tried orlistat with limited success move on to a GLP-1 injection. For the full picture across every option, see our complete 2026 guide to weight-loss medications. Never buy either medicine from an unregulated seller or a "compounded" source — legitimate supply always involves a prescriber and a GPhC-registered pharmacy or CQC-regulated clinic.

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