The main finding
In one sentence: In a one-year trial at five UK centres, adults with obesity who took orlistat (Xenical) with a low-energy diet lost an average of 8.5% of their starting weight, compared with 5.4% for those taking placebo with the same diet.
Group averages, not an individual prediction. A trial result describes what happened to a defined group over a defined time.
On this page
Why this study matters
Orlistat is one of the older medicines used for weight management. It works in the gut by blocking an enzyme that digests fat, so some of the fat in food passes through unabsorbed. This trial, published in 2000, tested it over one year in a UK setting with a structured diet. It is useful for seeing what the medicine added on top of a low-energy diet, and for its detail on side effects and vitamin levels.
Who was in the study
The trial enrolled 228 adults with obesity at five centres in the UK. Their BMI, which is weight in kilograms divided by height in metres squared, was between 30 and 43. Mean body weight was 97 kg, with a range of 74 to 144 kg. The published summary does not report ages or whether any participants had diabetes. The trial ran for 12 months after a four-week run-in period.
What the researchers did
This was a randomized, double-blind, placebo-controlled trial. Randomized means chance decided each person's group. Double-blind means neither participants nor researchers knew who was taking the medicine. A placebo is an inactive capsule that looks like the medicine.
All participants were prescribed a low-energy diet for the four-week run-in and then for 12 months. The diet provided 30% of energy from fat and was designed to create a deficit of about 600 kcal per day, tailored to each person. Participants then took orlistat 120 mg or placebo three times a day with the diet.
The main outcome was change in body weight. The researchers also tracked waist size, side effects, blood fats, insulin, blood sugar, and blood levels of fat-soluble vitamins and beta carotene. The analysis was intention to treat, meaning people were counted in the group they were assigned to.
What they found
After one year, people taking orlistat had lost an average of 8.5% of their starting body weight. People taking placebo had lost 5.4% (R32-C109). The summary does not give a confidence interval for the difference, which would be the range within which the true gap would likely fall.
| Result at one year | Orlistat | Placebo |
|---|---|---|
| Average weight loss | 8.5% | 5.4% |
| Lost at least 5% of body weight | 35% | 21% |
| Lost at least 10% of body weight | 28% | 17% |
Both differences in the share of people reaching 5% and 10% were reported as statistically significant, meaning unlikely to be due to chance.
Total cholesterol, LDL cholesterol (often called "bad" cholesterol) and the LDL-to-HDL ratio fell significantly more with orlistat than with placebo. The summary does not give the numbers.
Side effects and people who stopped
The two groups had similar side-effect profiles except for gastrointestinal events, meaning effects on the gut. These were 26% more frequent in the orlistat group but were mostly mild and passed with time.
Orlistat reduces absorption of fat-soluble vitamins. To keep blood levels normal, vitamin A, D and E supplements were given to 1.8%, 8.0% and 3.6% of orlistat-treated people, respectively, compared with 0.9% of placebo-treated people for each vitamin. After one year, vitamin E and beta carotene had fallen significantly more in the orlistat group. The ratio of vitamin E to total cholesterol did not change significantly in either group.
The summary does not report how many people in each group stopped the medicine or left the trial.
What this study does not tell you
- With 228 people, this was a small trial. Small trials give less precise estimates.
- The summary does not report dropout by group or confidence intervals, so the strength of the result is hard to judge.
- Everyone followed a supervised low-energy diet. Results without that diet may differ.
- The trial is from the late 1990s. Diet support and trial methods have changed.
- The percentages cannot be combined with or compared against other trials, which used different people, diets and time frames.
- The summary we reviewed does not state who funded the trial.
What it means in Canada
This trial shows what orlistat added to a structured diet over one year, along with its main side effects. It reflects a medicine and a supervised diet in a study setting. For information on orlistat and other medicines, see the site's medicines pages, including semaglutide in Canada and tirzepatide in Canada. For coverage, see public drug coverage by province. To find weight-management services, see compare services.
Common questions
How much weight did people lose with orlistat in the UK one-year trial?
After one year, people taking orlistat had lost an average of 8.5% of their starting weight, compared with 5.4% for placebo.
What share of people lost at least 5% of their weight with orlistat?
35% of the orlistat group and 21% of the placebo group lost at least 5% of their body weight. For 10% or more, the figures were 28% and 17%.
Did orlistat affect vitamin levels in this trial?
Yes. Vitamin E and beta carotene fell more with orlistat, and some people were given vitamin A, D or E supplements to keep levels normal.
Related reading
Other studies in this library that bear on the same question.
Orlistat over two years: weight loss and regain in a European trial
COR-BMOD: naltrexone/bupropion with behaviour change
SCALE: liraglutide 3 mg weight management trial
What a large evidence review found about weight-loss programs
Source
Original paper: N Finer, UK multicentre 1-year orlistat, International journal of obesity and related metabolic disorders : journal of the International Association for the Study of Obesity, 2000. PubMed: https://pubmed.ncbi.nlm.nih.gov/10757623/. DOI: https://doi.org/10.1038/sj.ijo.0801128. Funding: The summary we reviewed does not state the funding source. How we summarized it: abstract and extracted data.
This page explains a published study. It is not medical advice and does not describe whether a medicine is right for you. Talk to your doctor, nurse practitioner or pharmacist about your own situation. Reviewed and signed by two pharmacists registered in British Columbia, 2026-09-18.
Evidence records behind this page
Each record is a row in the clinical evidence file, taken from the published paper named above. The caution column is the limit the researcher recorded for that number.
This table scrolls sideways.
| Record | What the paper reports | Type | Population | Caution | Source |
|---|---|---|---|---|---|
| R32-C109 | In a UK trial with a low-energy diet, mean 1-year weight loss was 8.5% with orlistat and 5.4% with placebo. | direct evidence | Adults with obesity, BMI 30 to 43, five UK centres | Small older UK trial; restricted diet support; abstract lacks arm attrition and confidence intervals. Do not combine percentage losses with other trials. | PubMed 10757623 |