Gastrointestinal
Orlistat
A weight-loss drug — Blocks fat absorption to support weight loss alongside diet and activity.
Education and reference only. This is a plain-language class overview — it deliberately contains no doses. Always check the current Summary of Product Characteristics (SmPC), the BNF and your local formulary before prescribing or administering any medicine.
What it is
Orlistat supports weight loss in people who are overweight or obese, used together with a reduced-calorie diet and more physical activity rather than on its own.
How it works
It blocks the gut enzymes (lipases) that break down dietary fat, so about a third of the fat eaten passes through undigested and unabsorbed rather than being stored. The unabsorbed fat is what produces both the weight benefit and the characteristic bowel side-effects.
In practice
In practice orlistat is an adjunct to a reduced-calorie, lower-fat diet and increased activity, not a stand-alone fix, and it is used within eligibility criteria based on BMI. Its mechanism makes the counselling self-enforcing: by blocking the absorption of some dietary fat, it causes oily stools, urgency and flatulence that are markedly worse after fatty meals — so the side-effects themselves reinforce a low-fat diet, and they ease as eating habits improve. The practical safety points are that it reduces the absorption of fat-soluble vitamins (a multivitamin taken at bedtime is advised) and of some important drugs — it can reduce the effectiveness of levothyroxine, ciclosporin, some antiepileptics and the combined contraceptive pill (especially if the diarrhoea is severe), and it can enhance the effect of warfarin — so timing and monitoring matter. Treatment is reviewed for benefit and continued only if meaningful weight loss is achieved.
Examples
Practical use
How to take it & use it well
- Orlistat is a capsule taken with meals to help weight loss by reducing the amount of fat absorbed from food.
- Take it just before, during or shortly after each main meal; if you miss a meal or it contains no fat, skip that dose.
- Follow a reduced-calorie, lower-fat diet, as eating fatty meals makes the oily side effects much worse.
- Spread your fat intake evenly across the day rather than having one very fatty meal.
- Take a multivitamin containing fat-soluble vitamins at bedtime, separated from orlistat, as it can reduce their absorption.
- Use it alongside diet and exercise as part of an overall weight-management plan, not on its own.
Common uses
- Weight management in overweight/obesity (with diet and activity)
- Adjunct within BMI-based eligibility criteria
Monitoring
- Weight loss and continuation only if meaningful benefit
- Interacting medicines (e.g. INR with warfarin, thyroid levels)
- Diet quality and tolerability
Weighing it up
Advantages & disadvantages
Advantages
- Can help with weight loss when combined with a reduced-calorie, lower-fat diet.
- Works in the gut and is barely absorbed into the body.
- May improve blood pressure, cholesterol and blood sugar through weight loss.
- Available as part of a structured weight-management plan.
Disadvantages
- Causes oily stools, urgency, wind with discharge and oily spotting, especially with fatty meals.
- Can reduce absorption of fat-soluble vitamins.
- Only works well if you stick to a lower-fat diet.
- Can interact with several important medicines.
- Stomach effects can be socially inconvenient.
Key safety principles
What to watch for
- Oily stools, urgency and flatulence — worse with fatty meals; a low-fat diet reduces them.
- Reduces absorption of fat-soluble vitamins (take a multivitamin at bedtime) and of some drugs (e.g. levothyroxine, ciclosporin, some antiepileptics, the combined pill); can enhance warfarin.
- An adjunct to diet and activity, reviewed for benefit — not a stand-alone treatment.
Key interactions
What to avoid or check alongside
- Can reduce absorption of levothyroxine, so thyroid treatment may need separating in time and monitoring.
- May reduce absorption of some contraceptive hormones, particularly if severe diarrhoea occurs, so extra precautions may be needed.
- Can alter the effect of warfarin by reducing vitamin K absorption, so anticoagulation may need closer monitoring.
- May affect ciclosporin and some epilepsy and HIV medicines, so doses are separated and levels watched.
Patient & carer advice
- It works with a low-fat diet — fatty meals cause oily, urgent stools
- Take a multivitamin at bedtime, and use extra contraceptive precautions if you get severe diarrhoea
- Tell us your other medicines, as it can affect some of them
Use with
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Answers
Orlistat: frequently asked questions
Why does orlistat cause oily stools?
Orlistat blocks the absorption of some dietary fat, which then passes out in the stool. This causes oily, loose motions, urgency and wind, and the effects are worse the more fat you eat.
How can I reduce the side effects?
Sticking to a reduced-calorie, lower-fat diet and spreading fat evenly across meals greatly reduces the oily bowel effects. Avoiding very fatty meals is the most effective step.
Do I need to take vitamins with orlistat?
Orlistat can reduce absorption of fat-soluble vitamins A, D, E and K. Taking a multivitamin at bedtime, separated from your orlistat doses, helps maintain your levels.
Can orlistat affect my contraceptive pill?
If orlistat causes severe diarrhoea, it may reduce absorption of the contraceptive pill and make it less reliable. Use additional contraception during such episodes and ask your pharmacist for advice.
Will orlistat work without dieting?
Orlistat is designed to be used alongside a reduced-calorie, lower-fat diet and more activity. Without these changes it is far less effective and causes more unpleasant side effects.
Authoritative sources
Always verify against the source
This overview is for orientation. For doses, interactions, contra-indications and the full monograph, use:
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