Skip to content
Compare providers
Information only Current oral options, future GLP-1 tablets and clear provider information kept separate Prescription medicines require clinical assessment

Overview

Why orlistat side effects are almost all digestive

Orlistat works by blocking an enzyme in your gut that breaks down dietary fat. Undigested fat then passes through the bowel and is excreted. Every common side effect of orlistat is a direct result of this mechanism – fat in the stool causes oily, fatty, and sometimes urgent bowel effects. These are not signs that something has gone wrong; they are signs the medication is doing what it is supposed to. The key to managing them is keeping your fat intake consistently low.

Common side effects

What most people experience

Very common (affects more than 1 in 10 people)

Oily or fatty stools – the most characteristic effect; undigested fat in the stool.
Oily spotting on underwear – oily discharge from the rectum, particularly after high-fat meals.
Flatus with discharge – wind accompanied by oily leakage.
Fatty or oily stools with urgency – bowel movements that feel more urgent than usual.
Oily evacuation – oily liquid passed from the rectum.

Common (affects 1 – 10 in 100 people)

Soft or liquid stools – looser than usual consistency.
Increased number of bowel movements – more frequent than your normal pattern.
Abdominal pain or discomfort – stomach cramps, often linked to a high-fat meal.
Rectal pain or discomfort – soreness in the rectal area, particularly with frequent or loose stools.
Headache – less common than gut effects but reported.

How to manage them

Three things that reduce side effects significantly

Keep fat intake low

Aim for no more than about 15 g of fat per meal (roughly 45 g total per day). The more fat you eat, the more passes through undigested, and the worse the effects. If you eat a high-fat meal, expect intensified effects – this is predictable and avoidable.

Spread fat evenly across meals

A single high-fat meal can trigger significant effects even if the rest of the day is low-fat. Keeping fat distributed evenly across meals helps prevent the peak concentrations that cause the most acute symptoms.

Carry a change of clothing early on

Particularly in the first few weeks or when eating out, many people find it practical to carry a spare pair of underwear. Oily spotting can occur with little warning and is not always controllable. This is practical, not alarmist.

Rare but serious

Less common effects to be aware of

Effect Notes
Kidney stones (oxalate) Orlistat can increase oxalate absorption in some people, raising kidney stone risk. Stay well hydrated. People with a history of oxalate kidney stones should discuss this with a clinician before use.
Liver damage (very rare) There have been rare reports of serious hepatic injury associated with orlistat. If you develop jaundice, dark urine, or abdominal pain with nausea, stop the medication and seek medical assessment.
Reduced efficacy of oral contraceptives Severe diarrhoea while taking orlistat may reduce absorption of oral contraceptives. Use additional contraception during and after episodes of severe diarrhoea.
Reduced vitamin absorption Fat-soluble vitamins (A, D, E, K) are absorbed alongside dietary fat. Orlistat reduces their absorption. Taking a daily multivitamin at bedtime – away from orlistat doses – is commonly recommended for anyone using orlistat long-term.
Thyroid medication (levothyroxine) Orlistat can reduce levothyroxine absorption. People with hypothyroidism taking levothyroxine should take it at least four hours apart from orlistat and have their thyroid function monitored.

Who should not take it

Contraindications and cautions

Absolute contraindications

Orlistat must not be used by people with cholestasis (a condition affecting bile flow from the liver), malabsorption syndrome (conditions causing poor nutrient absorption such as coeliac disease or short bowel syndrome), or during pregnancy. It is also not suitable for people under 12 years old.

Cautions and interactions

Use with care if you take ciclosporin (separate doses by at least three hours), anticoagulants such as warfarin (monitor INR), levothyroxine (monitor thyroid function), or antiretrovirals for HIV. People with type 2 diabetes may need blood sugar monitoring adjusted, as weight loss can reduce insulin requirements.

FAQ

Common questions about orlistat side effects

Are the oily side effects dangerous?

No. The digestive effects of orlistat are unpleasant but not medically harmful. They are a direct result of the mechanism – undigested fat in the bowel. They are most intense after high-fat meals and are substantially reduced by keeping fat intake low.

How long do the side effects last?

For most people, the worst effects are in the first few weeks as the body adjusts and as dietary habits change. If fat intake is reduced consistently, effects often diminish significantly over time. They will recur if a high-fat meal is eaten.

Is orlistat safe long-term?

Orlistat has been used for over 25 years and has a well-established safety record for long-term use at the 120 mg dose, based on data from the XENDOS trial (up to four years). The main longer-term consideration is fat-soluble vitamin absorption – taking a multivitamin is recommended. Rare serious effects (liver injury, kidney stones) are documented but uncommon.

Can orlistat affect my other medications?

Yes – several important interactions exist. The most clinically significant are with ciclosporin, levothyroxine, anticoagulants (warfarin), and some antiretrovirals. If you take any of these, discuss with a clinician before starting orlistat.

Related pages

Orlistat UK: full guide

How it works, cost, and where to buy. Read the guide.

Important

Information only

Tablet Compare is information only. If you experience unusual or severe symptoms while taking orlistat, speak to a pharmacist, GP, or clinician. Do not adjust medications without professional advice.

Related orlistat guides