Orlistat vs Semaglutide: Two Very Different Approaches to Weight Loss

The biggest misconception: orlistat and semaglutide are not in the same league for weight reduction

The most common misunderstanding people bring to this comparison is the idea that orlistat and semaglutide are rough equivalents, an older and a newer version of the same type of medicine. They are not. Orlistat works entirely in the gut: it inhibits pancreatic lipase, the enzyme that breaks dietary fat into absorbable molecules, so a proportion of fat passes through rather than entering the bloodstream. It has no effect on appetite, hunger hormones or how the brain registers fullness.

Semaglutide targets the GLP-1 receptor, which is active in the brain, the gut and the pancreas. It slows stomach emptying, reduces appetite signalling and changes the reward response to food. The difference in outcomes reflects that mechanistic gap. In the STEP 1 trial (68 weeks, 2.4mg weekly semaglutide versus placebo) participants lost an average of around 15% of body weight, as published in the New England Journal of Medicine. Orlistat trials consistently show average losses in the range of 3–5% beyond placebo at one year. That is a genuine benefit, but it is a different order of magnitude. The NHS medicines page on orlistat describes realistic expectations plainly, and it is worth reading before making any comparison.

Semaglutide is a prescription-only medicine. So is orlistat at 120mg (Xenical); the 60mg capsule (Alli) is available over the counter, though a pharmacist will still ask screening questions. Neither is a decision to make without professional input.

How their eligibility criteria and UK licensing differ

Orlistat 120mg is licensed for adults with a BMI of 28 or above alongside a low-calorie diet, a lower threshold than most injectable treatments. If you want to understand how semaglutide and Wegovy relate to each other as licensed products, that context matters here: Wegovy is licensed for adults with a BMI of 30 or above, or 27 and above where at least one weight-related condition is present, such as hypertension, type 2 diabetes or high cholesterol. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. NICE's appraisal of semaglutide (TA875) goes further: on the NHS, Wegovy is recommended only within specialist weight management services, for a maximum of two years, with multidisciplinary support, conditions that reflect how seriously the clinical community takes the medicine's power and its cost.

Orlistat has been available on NHS prescription for over two decades and does not carry those service restrictions, though GPs will still assess suitability. If you are exploring the treatment options available through a regulated private service, a prescriber will look at both your BMI and your health history before recommending anything. It is worth knowing that semaglutide is sometimes confused with Ozempic, which is also semaglutide but licensed solely for type 2 diabetes. A direct comparison of semaglutide and Ozempic clarifies why that distinction matters.

Side effects: the practical day-to-day difference

Both medicines carry a gastrointestinal side-effect profile, but the pattern differs. Orlistat's effects are almost entirely digestive and diet-dependent: oily stools, urgent bowel movements, wind and, in some cases, faecal leakage, particularly if dietary fat intake remains high. Managing orlistat is partly about managing what you eat alongside it. Many people find the side effects self-limiting once they adjust their diet, but they can be socially disruptive early on.

Semaglutide's side-effect profile is driven by slowed gastric emptying and appetite suppression: nausea, vomiting, constipation, indigestion and fatigue are the most reported. These tend to be most noticeable in the first few weeks after starting or after a dose increase, and they usually settle. The January 2026 MHRA Drug Safety Update is relevant here: it confirmed that acute pancreatitis is a known, infrequent but potentially serious risk with GLP-1 medicines, severe stomach pain radiating to the back should prompt urgent medical attention. You can report suspected side effects for either medicine through the MHRA's Yellow Card scheme.

Neither medicine is recommended during pregnancy, breastfeeding or if you are trying to conceive. Under-18s are not included in the licensed populations for either weight-loss indication. Our FAQs page covers a number of the contraindication questions our prescribers hear most often.

Orlistat vs semaglutide at a glance

Figures cited from STEP 1 (NEJM) and NHS orlistat guidance. Which medicine suits your situation is a clinical decision. You can explore what a private consultation involves and how our clinical team approaches these decisions. If you are also considering how Wegovy compares with other injectable options, our page comparing Wegovy and Trulicity covers that ground directly, and our overview of GLP-1 medicines sets both treatments in broader context.

When Wegovy is prescribed through nume and your order is placed before 12pm, it is dispensed the same day and delivered by DPD the next working day, you will get a tracking link to your phone, and the pen arrives in a plain box that gives nothing away. Which treatment gets you to that step is something our prescribers work through with you, not something we decide before seeing your full picture. If you would like to read more before speaking to anyone, our detailed breakdown of orlistat versus Wegovy walks through the key differences in one place. Check your eligibility and start a free consultation.