Orlistat Side Effects: Oily Stools, How Long They Last and Is Orlistat Safe?

Short answer: Orlistat side effects are mostly in the bowel. With orlistat, common side effects include oily or fatty stools, oily spotting, wind with discharge, and needing the toilet urgently or more often. They are more likely after a meal high in fat. They come from the fat orlistat stops your body absorbing, which passes out in your stools, and in the orlistat 60mg trials they were generally mild and short-lived, usually came within the first 3 months, and most people had only one episode (alli product information, section 4.8). Serious side effects such as hepatitis (an inflamed liver), pancreatitis, gallstones, bleeding from the bottom and kidney problems have been reported, and how often they happen is not known (Xenical and alli product information, section 4.8).

This guide sets out what the UK product information for orlistat 120mg (Xenical) and 60mg (alli) and the NHS say, as of October 2026: each side effect at each strength, why they happen, how long they last, how to reduce them and when to get help. A GPhC-registered pharmacist reviews every orlistat consultation before anything is supplied, and orlistat 60mg, a pharmacy medicine, is supplied after a pharmacist consultation.

What are the side effects of orlistat?

The common side effects of orlistat are changes to your stools and bowel habits, and the UK product information for both strengths calls them "largely gastrointestinal in nature" (section 4.8 of each). This is the full list, as of October 2026. Very common means more than 1 in 10 people, common up to 1 in 10, and not known that the frequency cannot be estimated.

The 120mg bands count events in the first year of 1 and 2 year trials that were reported by more than 2% of people on orlistat and at least 1 percentage point more often than on a dummy capsule (placebo); the 60mg bands are the alli product information's (section 4.8 of each). The last row was reported after the medicine was licensed. Stomach pain or discomfort is very common with 120mg, but severe stomach pain is different: Get urgent medical advice if you have severe stomach pain, vomiting that will not stop, or signs of pancreatitis or gallbladder problems.

What does the NHS say about orlistat side effects?

The NHS lists the common side effects of orlistat as "fatty or oily poo", "needing to poo urgently or more frequently", "farting with or without oily spotting", stomach pain and diarrhoea, and says you are less likely to get them if you stick to a balanced diet and avoid meals that are very high in fat. Speak to a pharmacist or doctor about any side effect that bothers you or does not go away (nhs.uk, reviewed 6 May 2026).

Why does orlistat cause oily stools?

Orlistat causes oily stools because it stops part of the fat in your meal from being digested, and fat that is not digested cannot be absorbed, so it passes out in your stools instead. It blocks the lipases, the enzymes in your stomach and gut that break fat down (Xenical and alli product information, section 5.1). At 120mg it blocks about a third of the fat you eat, and at 60mg about a quarter (Xenical and alli leaflets, section 1).

The Xenical leaflet calls these "harmless changes in your bowel habits, such as fatty or oily stools, due to the elimination of undigested fat in your faeces", and says they are more likely if you take it with a diet high in fat (section 2). How the medicine works, step by step, is in our guide to how orlistat works.

What is the orange oil when you take orlistat?

Oil in or after your stools on orlistat is undigested fat from your food: the UK product information calls it oily spotting or an oily discharge, and lists it as very common at both strengths (Xenical and alli product information, section 4.8). The UK leaflets do not describe its colour; what they describe is oil, from the fat orlistat stops you absorbing, and lower-fat meals make it less likely (alli leaflet, section 4).

Oil is not blood. Bleeding from your bottom is a different thing, and it has been reported with orlistat (Xenical and alli product information, section 4.4). Tell us or your GP if you notice bleeding from your bottom, especially if it is heavy or keeps happening. If you are not sure whether what you see is oil or blood, tell us or your GP.

How long after taking orlistat do you poop?

There is no set time: the UK product information does not give one. What it does say is that the fat in your stools starts to rise "as early as 24 to 48 hours after dosing" (Xenical product information, section 4.2; alli, section 5.1), and the fat that is not absorbed passes out in your stools (alli leaflet, section 1). Needing the toilet urgently is very common at both strengths, and needing it more often is very common with 120mg and common with 60mg. Side effects in the gut are more likely after a meal high in fat. So it helps to avoid meals high in fat, as the alli leaflet advises (section 2).

Does orlistat cause diarrhoea?

It can. Liquid stools are listed as very common with orlistat 120mg and common with 60mg (section 4.8 of each), and the NHS lists diarrhoea among orlistat's common side effects. Speak to your pharmacist if it is severe or does not settle. Severe diarrhoea can also make the contraceptive pill less reliable:

How long do orlistat side effects last?

Orlistat's bowel side effects usually come early and ease with time. In trials of orlistat 60mg lasting 18 months to 2 years, they were "generally mild and transient", "generally occurred early in treatment (within 3 months)", and "most patients experienced only one episode" (alli product information, section 4.8). With orlistat 120mg, side effects became less frequent the longer people took it (Xenical product information, section 4.8).

The Xenical leaflet puts it this way: the symptoms "are generally mild, occur at the beginning of treatment and are particularly experienced after meals containing high levels of fat. Normally, these symptoms disappear if you continue treatment and keep to your recommended diet" (section 4). If you stop taking orlistat, the fat in your stools usually returns to its usual level within 48 to 72 hours (Xenical product information, section 4.2).

If a side effect has not settled, or bothers you, message your pharmacist before you stop.

How can you reduce orlistat side effects?

Eat lower-fat meals and spread the day's fat across your three main meals: the bowel side effects are more likely when orlistat is taken with a meal very high in fat (Xenical product information, section 4.4). This is the diet advice we give every orlistat patient:

• The Xenical product information's example of a diet high in fat, which makes bowel side effects more likely: on a 2,000 kcal day, more than 30% of calories from fat, which is more than 67g of fat (section 4.4).
• Avoid fatty food between meals, "such as biscuits, chocolate and savoury snacks", as the Xenical leaflet advises (section 3).
• If you do eat a high-fat meal, do not take more than the recommended dose (alli leaflet, section 2).

Meal ideas and food swaps are in our orlistat diet plan.

Can orlistat cause constipation?

Constipation is not listed as a side effect of orlistat 120mg or 60mg in the UK product information (section 4.8 of each, as of October 2026). The bowel effects it lists are oily, softer, looser or more frequent stools, and the urge to go.

But the alli leaflet names constipation, with pain in the lower stomach, particularly on the left side, and possibly a fever, as possible signs of diverticulitis, a serious inflammation of the large bowel, and says to stop taking the capsules and tell your doctor (section 4). If you are constipated, or your bowel habit changes in a way that worries you, speak to your pharmacist.

Is orlistat safe? The serious and long-term side effects

Most side effects of orlistat are in the gut and mild, but a few serious ones have been reported since it was licensed, and how often they happen is not known (Xenical and alli product information, section 4.8). Both strengths have a UK licence: orlistat 120mg as a prescription medicine and orlistat 60mg (alli) as a pharmacy medicine. The NHS says "Most adults can take orlistat", and that "Orlistat can sometimes cause serious side effects, but most of them are rare" (nhs.uk, reviewed 6 May 2026). These are the ones to know the signs of.

Can orlistat damage your liver?

Hepatitis, an inflammation of the liver, has been reported with orlistat. The product information says it may be serious: some fatal cases, or cases needing a liver transplant, have been reported, and how often it happens is not known (section 4.8). The Xenical leaflet lists its signs as yellowing skin and eyes, itching, dark coloured urine, stomach pain and liver tenderness, felt under the front of the rib cage on your right, sometimes with loss of appetite, and says: "Stop Xenical if such symptoms occur and tell your doctor" (section 4). Raised liver enzymes can also show up in blood tests. Orlistat is not used if you have cholestasis, a liver condition (section 4.3).

Can orlistat cause pancreatitis or gallstones?

Both have been reported, and how often is not known (section 4.8). The alli leaflet describes pancreatitis as "severe abdominal pain sometimes radiating towards the back, possibly with fever, nausea and vomiting", and lists "Severe stomach pain caused by gallstones" among its serious side effects (section 4).

Can orlistat affect your kidneys?

Orlistat may be linked to a build-up of calcium oxalate in the kidneys, called oxalate nephropathy, which may lead to kidney stones (alli leaflet, section 4) and sometimes to kidney failure (Xenical and alli product information, sections 4.4 and 4.8). How often it happens is not known. The risk is higher if you have chronic kidney disease or are low in body fluids (section 4.4), which is why our consultation asks about kidney problems and dehydration before orlistat is supplied.

Can orlistat cause bleeding from the bottom?

Bleeding from the back passage has been reported with orlistat, and how often is not known (section 4.8 of each; the alli product information calls it mild rectal bleeding). The alli product information says to see a doctor if it happens, and the Xenical product information asks for severe or persistent bleeding to be looked into (section 4.4 of each).

What other serious side effects have been reported?

• Diverticulitis, an inflammation of the large bowel: lower stomach pain, particularly on the left side, possibly with fever and constipation (alli leaflet, section 4).
• Severe allergic reactions: severe breathing difficulties, sweating, rash, itching, a swollen face, a rapid heartbeat or collapse. The alli leaflet says: "Stop taking the capsules. Get medical help immediately." (section 4).
• Skin blistering, including blisters that burst (both leaflets, section 4).
• Changes to blood clotting, seen in blood tests (INR), in people taking warfarin or another blood thinner (Xenical product information, sections 4.5 and 4.8). Orlistat 60mg must not be taken with them (alli product information, section 4.3), and we do not supply orlistat alongside warfarin.

What are the long-term side effects of orlistat?

The product information does not give a separate list of long-term side effects. What it does say is that side effects became less frequent the longer people took orlistat 120mg (Xenical product information, section 4.8), that orlistat can reduce how well you absorb some vitamins (below), and that the serious side effects above were reported after licensing, with no frequency known. That is why treatment is reviewed. Orlistat is reviewed at 12 weeks and only continued if you have lost at least 5% of your starting weight. After that, our pharmacist reviews it at every order. Orlistat 60mg should not be taken for more than 6 months.

Does orlistat affect your periods, mood, headaches or blood sugar?

Orlistat 120mg can affect each of these: its UK product information lists headache and, in people with type 2 diabetes, low blood sugar as very common, and irregular periods, anxiety and tiredness as common (Xenical product information, section 4.8). For orlistat 60mg, anxiety is the one listed, as common, and the product information explains that it may come "in anticipation of or secondary to gastrointestinal adverse reactions" (alli product information, section 4.8).

Periods. Speak to your pharmacist or GP if your cycle changes. If you think you might be pregnant, tell your doctor: orlistat is not used during pregnancy (nhs.uk). Severe diarrhoea can make the pill less reliable, as above.

Mood. If you take a medicine for depression, anxiety or another mental health condition, tell us before you start: there are some reports of these medicines working less well once orlistat was started (Xenical and alli product information, section 4.5).

Headaches. Headache is very common with orlistat 120mg and is not listed for 60mg. Tell your pharmacist if headaches do not go away.

Blood sugar. Orlistat can lower your blood sugar. Your diabetes medicines may need adjusting and your blood sugar checking more often, so your GP needs to know before you start.

Does orlistat affect your vitamins?

Orlistat can reduce how well you absorb vitamins A, D, E and K, the fat-soluble vitamins, although in clinical studies most people's levels of these vitamins stayed within the normal range (Xenical and alli product information, section 4.5). This is the advice we give every orlistat patient: Take a daily multivitamin containing vitamin D, at least 2 hours after a dose of orlistat, or at bedtime. Our orlistat diet plan covers the food side.

Who should not take orlistat?

Orlistat is not suitable for everyone, which is why our consultation checks before anything is supplied. Orlistat is not suitable if you are pregnant, planning a pregnancy or breastfeeding, have chronic malabsorption syndrome, cholestasis, or liver or kidney problems, or your weight gain is caused by a hormone condition or a medicine. The licence also rules it out if you are allergic to orlistat or to any of the capsule's other ingredients (Xenical and alli product information, section 4.3).

Some medicines rule orlistat out or need checking first; which ones, and why, is in how to take orlistat. We do not supply orlistat together with a GLP-1 medicine such as Mounjaro or Wegovy, and why is in orlistat vs Mounjaro and Wegovy.

If orlistat is not right for you: Your GP or a local NHS weight management service can help with other options, and our pharmacist is happy to talk them through with you. The weight-loss tablets and capsules we supply are on our weight-loss tablets page, and all our weight-loss treatments on our weight loss page.

How do you report a side effect of orlistat?

Tell your pharmacist or GP about any side effects. You can also report them to the MHRA Yellow Card scheme. You can report on the Yellow Card website, or by searching for MHRA Yellow Card in the Google Play or Apple App Store (Xenical and alli leaflets, section 4). How we check every medicine we supply is on our safety page.