Orlistat: what it is and how it works
Quick answer
Orlistat is a weight-management tablet that works in the gut to reduce how much fat from food your body absorbs. It is taken alongside a reduced-calorie, lower-fat diet, is usually reviewed regularly by a clinician, and suits some people more than others depending on health history and goals.
What is orlistat?
Orlistat is a weight-management tablet that has been available in the UK for many years, used alongside changes to diet and activity rather than in place of them. Unlike medicines that work on appetite or hunger signals, orlistat acts locally in the gut, where it reduces the amount of fat absorbed from a meal. It does not affect how the body processes carbohydrate or protein, and it has no effect on appetite itself, which is why the diet you eat alongside it matters so much.
It is typically considered as one option within a wider weight-management plan, alongside dietary changes, physical activity and, where appropriate, clinical support. Because it has been used for many years, its effects and side effects are well documented, which is one reason clinicians often discuss it as a starting point when talking through the options available. A clinician can help you understand whether it is a sensible fit for your circumstances, what to expect, and how it compares with the other approaches available to you, including timescales and how progress is usually reviewed.
How orlistat works
Orlistat blocks the action of an enzyme called lipase, which the gut normally uses to break down dietary fat so it can be absorbed. With that enzyme partly inhibited, roughly a third of the fat eaten in a meal passes through the digestive system unabsorbed rather than being taken up by the body. That unabsorbed fat is then passed out in stools, which is also the reason for some of its more well-known side effects, covered further down.
Because the mechanism depends entirely on the fat present in a meal, orlistat only has an effect on meals that actually contain fat. A very low-fat meal or snack means there is little for it to act on, and it does nothing to reduce the calories from carbohydrate or protein, or to blunt appetite. This is why it is prescribed and taken alongside, not instead of, a structured reduced-calorie eating plan; the tablet supports the plan rather than replacing the need for one.
This local, gut-based mechanism is also why orlistat is generally not absorbed into the bloodstream to any meaningful extent, which is a different way of working from medicines that act on the brain or on hormone signalling. For some people, understanding this distinction helps set realistic expectations: the tablet changes how much of the fat you eat is absorbed, but the overall calorie deficit needed for weight change still depends on the food choices and portions behind each meal.
Who orlistat may suit
Orlistat is not automatically suitable for everyone who wants to manage their weight. In UK practice, it is generally considered for adults who meet a combination of criteria, assessed by a clinician on an individual basis, subject to suitability:
- A body mass index (BMI) of 30 or above, or 28 or above alongside another risk factor such as high blood pressure, high cholesterol or type 2 diabetes
- Already making changes to diet and activity, with orlistat considered as an addition rather than a starting point
- Willing to follow a reduced-calorie, lower-fat eating pattern while taking it, since this is what the medicine is designed to work alongside
- Able to attend regular reviews so a clinician can check it is being tolerated well and having the intended effect
These criteria are a starting point for a conversation, not a rigid checklist you need to tick off alone. A clinician will look at your health history, any other medicines you take, and your goals before advising whether orlistat is a reasonable option, and will discuss the realistic pace and pattern of results so expectations are grounded from the outset.
How orlistat is taken
Orlistat is typically taken up to three times a day with meals that contain fat, usually during or up to an hour after eating. If a meal is missed, or contains no fat at all, that dose is generally skipped rather than taken anyway, since there is nothing for it to act on. Your prescriber or the patient information leaflet supplied with your medicine will set out the exact dosing pattern for you, and it is worth reading this carefully before you start.
Sticking to a lower-fat way of eating alongside treatment is not just supportive advice, it also affects your day-to-day experience while taking it. Meals that are noticeably higher in fat are far more likely to bring on the digestive side effects described below, so many people find that adjusting their eating pattern reduces those effects considerably while also supporting the weight-management goals the treatment is meant to help with.
Common side effects
The most frequently reported side effects of orlistat relate directly to how it works: undigested fat passing through the gut. These can include oily or fatty stools, more frequent or looser bowel movements, an urgent need to reach a toilet, oily spotting, and wind with oily discharge. These effects are usually more noticeable in the first few weeks and tend to ease as your diet adjusts to contain less fat, though they can return if a particularly high-fat meal is eaten.
Abdominal discomfort is also common. Most people find these effects manageable and linked fairly predictably to what they have eaten, but they can be enough to put some people off continuing. Serious side effects are rare but can include signs of an allergic reaction, such as swelling of the face, lips or throat, or severe abdominal pain that could suggest inflammation of the pancreas. Seek urgent medical attention if either of these occurs, and always read the patient information leaflet supplied with your medicine in full.
Interactions and who should avoid it
Because orlistat reduces fat absorption, it can also reduce the absorption of fat-soluble vitamins, namely vitamins A, D, E and K. A clinician may advise taking a multivitamin supplement at a different time of day to your orlistat dose, typically at bedtime, to reduce this effect. It can also affect how well certain other medicines are absorbed, including some thyroid medication and ciclosporin, so it is important to disclose everything you currently take when you have your consultation, including over-the-counter products and supplements.
Orlistat is not suitable for everyone. It should generally be avoided by people with chronic malabsorption syndrome, cholestasis, or who are pregnant or breastfeeding, and caution is needed for people with a history of kidney stones or certain digestive conditions. This is exactly the kind of detail a clinical consultation is designed to cover, so your history and current medicines can be checked against the medicine before anything is prescribed, always subject to suitability.
Access and next steps
Orlistat is available in the UK by prescription, and access usually starts with a conversation about your health, your goals and what you have already tried. This lets a clinician confirm whether the criteria are met, check for any interactions with medicines you already take, and agree a sensible plan for monitoring, including how and when you will be reviewed. Bring a note of your current weight, any relevant health conditions, and the medicines and supplements you take, as this helps the consultation move efficiently and means the advice you get is tailored to you rather than generic.
If you are thinking about weight management support and want to understand whether orlistat, or another approach, might be right for you, the My London Pharmacy clinical team can talk this through with you. A consultation is the starting point for a personalised view of your options, subject to suitability, rather than a one-size-fits-all recommendation, and it is also the right place to ask any questions about what day-to-day life on treatment tends to look like.
Frequently asked questions
What does orlistat actually do?
It works in the gut to reduce the amount of fat absorbed from a meal, roughly a third of the fat eaten while taking it. It has no direct effect on appetite and does not act on carbohydrate or protein.
Do I need a prescription for orlistat in the UK?
Yes, at the standard prescribed strength it is a prescription-only medicine, so it requires a clinical assessment and ongoing review before and during use.
What are the most common side effects?
Digestive effects linked to unabsorbed fat, such as oily or fatty stools, urgency, and oily spotting, are the most commonly reported. These tend to ease as diet adjusts to contain less fat.
Can I take orlistat with other medicines?
It can affect the absorption of some other medicines and of fat-soluble vitamins. Always tell your clinician about everything you take so this can be checked before you start.
Do I still need to change my diet if I take orlistat?
Yes. Orlistat is designed to work alongside a reduced-calorie, lower-fat eating pattern, not instead of one, and following that pattern also reduces digestive side effects.
How long is orlistat usually taken for?
This is set individually by your prescriber based on how you respond and tolerate treatment, with regular reviews along the way to check it remains suitable and is having the intended effect.