If you're taking orlistat to support your weight loss, it helps to know which side effects to expect and what to do about them. Most are digestive, and they come from how the medicine works: it stops some of the fat you eat from being absorbed, so that fat passes out in your stools.1
The good news is that you have a lot of control over orlistat side effects. How much fat you eat makes the biggest difference.2 Here's what to expect, how to manage each effect and which rarer problems need medical advice.
Most orlistat side effects are manageable, and what's on your plate is the biggest lever you have.
What are the most common orlistat side effects?
Orlistat blocks some of the fat in your food from being absorbed. The NHS says around a third passes out in your poo.1 That's why the side effects are mostly digestive, and why they're stronger when you eat more fat.2
The UK product information lists these as very common, meaning they affect more than 1 in 10 people:2,3
- Oily or fatty stools
- Oily spotting or discharge from your back passage
- Wind with discharge
- An urgent need to open your bowels, or needing to go more often
- Liquid stools
- Tummy pain or discomfort
- Headache
People with type 2 diabetes may also get low blood sugar levels.2,3 Your diabetes treatment may need closer monitoring while you take orlistat, so tell your prescriber or diabetes team.2
Other effects are common (1 in 100 to 1 in 10 people). They include soft stools, leaking stools, rectal pain, tooth or gum problems, tiredness and irregular periods.3 The leaflet in your box has the full list.
These digestive effects are usually worst at the start of treatment and after meals high in fat. They tend to settle if you keep to your recommended diet.3
Oily spotting and urgency are expected rather than alarming. It's sensible to plan for them in the first weeks, with a change of underwear and a toilet nearby.
Do side effects differ between orlistat, Xenical and Orlos?
The side effects in this guide come from the UK product information for Xenical 120 mg capsules.2 Xenical contains 120 mg, and prescription orlistat is usually 120 mg too.
Orlos contains 60 mg and is a pharmacy medicine, which means it's sold in pharmacies rather than prescribed.4 It's licensed for adults with a BMI above 28, and its product information says treatment should not last longer than 6 months.5 If you haven't lost weight after 12 weeks, it says to speak to a doctor or pharmacist.5
The Orlos product information lists the same kinds of effects as Xenical, including oily spotting, wind with discharge, urgent bowel movements, oily stools and tummy pain. It also lists hepatitis, kidney problems and allergic reactions, with the frequency not known.5
Follow the instructions for the exact strength you've been supplied, and check the leaflet in your box for its full list of side effects.
Why does diet make such a big difference?
Fat is what sets orlistat side effects off, so the amount on your plate matters most. The product information says digestive effects may increase on a high-fat diet, and more so after a meal very high in fat.2
It recommends a mildly reduced-calorie diet with about 30% of calories from fat, spread across three main meals.2 For scale, on a 2,000 calorie day, more than 30% from fat means over 67g of fat.2 Saving most of your fat for one big meal is the easiest way to trigger side effects.
The NHS says to avoid high-fat foods such as fried food, biscuits and cakes. It also says you still need to eat some fat.1 On food labels, 3g of fat or less per 100g counts as low fat, and more than 17.5g per 100g counts as high.6
A few swaps help quickly:
- Grill, bake, poach or steam instead of frying
- Choose leaner cuts of meat and trim the visible fat
- Swap full-fat dairy for lower-fat versions
Our guide to 5 healthy meal swaps for weight loss has plenty more ideas.
Spreading your fat across three meals beats saving it all for one big one.
How can you manage specific orlistat side effects?
Most side effects respond to the same fix: less fat at each meal. Here's what else helps with each one.
“Most orlistat side effects are digestive and relate to unabsorbed fat passing in your stools. They often ease over time, and avoiding high-fat meals can help. Follow a balanced, lower-fat diet and skip the dose if you miss a meal or it contains no fat. Get urgent medical help for severe tummy pain or yellowing of your skin or the whites of your eyes.
Ana Carolina GoncalvesSuperintendent Pharmacist
Oily stools mean some of the fat you've eaten is passing through, which is what orlistat is designed to do.1 They're expected, but they don't tell you how much weight you're losing. Cutting the fat in each meal is the most reliable fix, and effects like these usually settle as your eating habits adjust.3
Wind and urgency
Wind and urgency can feel unpredictable early on. Lighter meals and fewer high-fat snacks help. If you miss a meal, or a meal contains no fat, leave that dose out.2 The NHS adds that you should never take two doses together to make up for a missed one.1
Tummy discomfort
Mild cramping is very common and usually settles as your gut adjusts.3 Eating more slowly and keeping each meal lower in fat can help. Severe tummy or back pain that does not go away needs urgent advice, so call NHS 111.1
Headache
Headache is very common with orlistat.2 If headaches are frequent or severe, speak to your pharmacist or GP.
Do not take orlistat with a meal that has no fat, or when you have skipped a meal. Leave that dose out.
Are there long-term side effects of orlistat?
In the longest trial, which ran for four years, the pattern of side effects stayed similar. Digestive effects became less common each year.2
Orlistat is barely absorbed into your body, which is why its main effects stay in your gut.2 Treatment should be stopped after 12 weeks if you haven't lost at least 5% of your starting weight (more on that below).2
Digestive effects tend to fade with time rather than build up, so a bumpy start isn't a verdict on the whole course.
Will orlistat affect my vitamin absorption?
It can. Orlistat may reduce how well your body absorbs the fat-soluble vitamins A, D, E and K.2 In clinical studies lasting up to four years, the vast majority of people stayed within normal ranges for these vitamins.2
The product information says to eat a diet rich in fruit and vegetables, and that a multivitamin could be considered.2 If you take one, take it at least two hours after your orlistat dose, or at bedtime.2 A multivitamin such as OneVit Complete is one option to discuss with your pharmacist.
Does orlistat interact with other medicines?
Orlistat can change how some medicines work. These are worth knowing about:2
- Blood thinners such as warfarin: your INR (a blood clotting test) should be monitored.
- Epilepsy medicines such as valproate or lamotrigine: convulsions have been reported, so your prescriber will want to monitor for changes in seizures.
- Levothyroxine: rare cases of poorer thyroid control have been reported.
- Ciclosporin: the combination is not recommended, because orlistat can lower ciclosporin levels.
- HIV medicines: orlistat may reduce how well they're absorbed and could make them work less well.
- Diabetes medicines and acarbose: your diabetes treatment may need closer monitoring, and acarbose should be avoided.
- Some antidepressants, antipsychotics (including lithium) and benzodiazepines: there are case reports of reduced effect when orlistat was started.
The combined pill needs a mention too. Severe diarrhoea could stop it working properly, so use extra contraception such as condoms while you have it.2,1
On warfarin, epilepsy medicine, levothyroxine or ciclosporin? Tell your prescriber before you start orlistat.
Is orlistat dangerous? What are the serious side effects?
Most orlistat side effects are digestive and mild.3 But some serious reactions have been reported. Their frequency cannot be estimated from spontaneous reports.2 Seek prompt medical advice for rectal bleeding, and urgent help for severe, persistent tummy pain or yellowing of your skin or eyes.
- Liver problems (hepatitis): some cases have been serious. Signs include yellow skin or eyes, itching, dark urine and tummy pain. Stop taking orlistat and tell your doctor if you notice them.3
- Kidney problems (oxalate nephropathy): this can lead to kidney failure. The risk is higher if you have chronic kidney disease or you're dehydrated.2
- Rectal bleeding: get medical advice promptly if you notice bleeding from your bottom.1
- Pancreatitis: severe tummy or back pain that does not go away. Call NHS 111.1
- Gallstones and diverticulitis: both have been reported.2
- Allergic reactions: signs include a swollen throat or tongue, a raised itchy rash, difficulty breathing or skin blistering. Get urgent help.1,3
Do not wait it out if you notice yellowing of your skin or eyes, or severe tummy pain that will not go away.
When should you stop orlistat or get advice?
Contact your GP or prescriber if:
- You notice yellowing of your skin or eyes, dark urine or severe tummy pain
- You see rectal bleeding or blood in your stools
- Side effects are severe, persistent or affecting your daily life
- You get severe diarrhoea while taking the combined pill
- You take blood thinners, epilepsy medicine, levothyroxine or ciclosporin and have concerns about how orlistat may affect them
- You haven't lost at least 5% of your starting weight after 12 weeks of treatment^2^
If you use Pharmica's orlistat service, its assessment requires you to stop when your BMI falls below 28. Keep following the separate review rule: stop after 12 weeks if you have not lost at least 5% of your starting weight, and talk to your prescriber about what to do next.
The full list of known side effects and interactions is in the leaflet supplied with your medicine. Comparing orlistat with an injection? Our Mounjaro vs orlistat guide covers how they differ.
Frequently asked questions
Do orlistat side effects go away?
For most people, yes. Digestive effects are usually worst at the start of treatment and after fatty meals, and they tend to settle as you keep to your recommended diet. In the longest trial, they became less common each year. If they're severe or not easing, speak to your pharmacist or GP.
Does oily stool mean orlistat is working?
Oily stools mean some of the fat you've eaten is passing through, which is what orlistat is designed to do. They're expected, but they don't measure how much weight you're losing. If you haven't lost at least 5% of your starting weight after 12 weeks, speak to your prescriber, as treatment will need to be reviewed.
Is Orlos a different medication to orlistat?
Orlos is a brand of orlistat, so the active ingredient is the same. But Orlos contains orlistat 60 mg, while prescription orlistat and Xenical usually contain 120 mg. Follow the instructions for the exact strength you've been supplied, and check the leaflet in your box for its full list of side effects.
Should I stop taking orlistat if I get side effects?
Not necessarily. Mild digestive effects are expected and usually manageable by cutting back on fat. If symptoms are severe or do not settle, speak to your pharmacist or GP before stopping. Stop taking orlistat and seek medical advice straight away if you notice yellowing of your skin or eyes, dark urine or severe tummy pain.
Can I drink alcohol on orlistat?
The UK product information found no interaction between alcohol and orlistat. That said, drinks often come with fatty food, and high-fat meals make digestive side effects more likely. If you do drink, keep an eye on what you eat alongside it, and speak to a pharmacist if you're unsure.
What happens if I eat a high-fat meal on orlistat?
You're likely to notice stronger side effects, such as oily stools, urgency and wind. This can be uncomfortable. Go back to your lower-fat diet at your next meal. If your symptoms are severe or do not settle, speak to a pharmacist or GP.
References
- NHS. Orlistat: a medicine to treat obesity. NHS website; 2026. nhs.uk
- Neon Healthcare Ltd. Xenical 120 mg hard capsules: Summary of Product Characteristics. Electronic Medicines Compendium; 2023. medicines.org.uk/emc
- Neon Healthcare Ltd. Xenical 120 mg hard capsules: Package leaflet. Electronic Medicines Compendium; 2023. medicines.org.uk/emc
- Medicines and Healthcare products Regulatory Agency. Promotion of medicinal products for weight loss by Menwell Ltd t/a Voy: July 2026. GOV.UK; 2026. gov.uk
- Crescent Pharma Limited. Orlos 60 mg hard capsules: Summary of Product Characteristics. Patient Info; 2026. patient-info.co.uk
- NHS. Facts about fat. NHS website; 2023. nhs.uk
This article is intended for informational purposes only and does not replace professional medical advice. Always read the patient information leaflet supplied with your medication and speak to a healthcare professional if you have specific concerns.