How much weight did people lose on orlistat in trials?

In a one-year randomised trial of 743 adults, participants taking orlistat lost an average of 8.5% of their starting body weight, compared with 5.4% on placebo. Both groups were also following a reduced-calorie diet. That is roughly 17 lbs at a 200 lb starting weight and 21 lbs at 250 lbs.

What does the four-year XENDOS trial show?

XENDOS followed 3,305 adults for four years — the longest trial of any weight loss drug. Participants taking orlistat lost about 2.8 kg (6 lbs) more than those on placebo plus the same lifestyle programme, and were 37% less likely to progress to type 2 diabetes.

Is Alli as effective as prescription Xenical?

Alli is the same drug at half the strength: 60 mg over the counter versus 120 mg on prescription. The trial figures on this page come from the prescription-strength studies. The Alli label notes that most weight loss happens in the first six months of use.

How long does orlistat take to work?

Weight effects begin from the first week because the drug blocks fat absorption directly, but significant weight loss typically takes 8–12 weeks. There is no dose escalation period.

What happens if I stop taking orlistat?

Weight is typically regained after stopping. Because the effect is mechanical rather than hormonal, there is no rebound hunger the way there is with GLP-1 drugs — fat from food simply starts being absorbed normally again.

Xenical and Alli: trial results

What the trials measured

Orlistat has the longest trial record of any approved weight loss drug. Two trials sit behind the figures used on this site.

Sjostrom one-year randomised controlled trial

International Journal of Obesity, 2000

Who took part:: 743 adults with obesity
How long:: 1 year (52 weeks)
Compared with:: Placebo
Average weight loss:: 8.5% (placebo 5.4%)
Worth knowing:: This is the most commonly cited one-year efficacy figure for orlistat

XENDOS

Diabetes Care, 2004

Who took part:: 3,305 adults with obesity, including people with prediabetes
How long:: 4 years (208 weeks)
Compared with:: Placebo plus the same lifestyle programme
Weight result:: About 2.8 kg (6 lbs) more weight loss than placebo at four years
Diabetes result:: 37% reduction in progression to type 2 diabetes

What the averages look like in pounds

Averages from trial data. Individual results vary, but these give you a realistic picture of what to expect.

Weight loss milestones by percentage of starting body weight and by pounds lost, for two reference starting weights
Time	% of body weight	200 lb person	250 lb person
1 year (Sjostrom trial)	8.5% average	~17 lbs	~21 lbs
Across trials	5.7%–8.5%	~11–17 lbs	~14–21 lbs

Trial averages converted at starting weights of 200 lb and 250 lb, rounded to the nearest pound. XENDOS reported its four-year result in kilograms relative to placebo rather than as a percentage, so it is not converted here.

The four phases of the year

Knowing where you are in the arc helps you manage expectations — and stops you from panicking when the scale stalls.

Week 1 onward

No dose to build up to

There is no titration — the prescription dose is 120 mg three times a day with meals
The over-the-counter version, Alli, is the same drug at 60 mg
Weight effects begin from the first week because the drug blocks fat absorption directly
The gut side effects also start straight away and are the drug working, not a reaction to it

Weeks 8–12

Significant loss takes longer

Significant weight loss typically takes 8–12 weeks
The effect depends on how much fat is in your diet, since it blocks fat absorption
The Alli label notes that most weight loss happens in the first six months

Months 6–12

The one-year figure

Average weight loss at one year was 8.5%, against 5.4% on placebo
Both groups in that trial were also following a reduced-calorie diet
Roughly 17 lbs at a 200 lb starting weight, or 21 lbs at 250 lbs

Years 1–4

The only four-year dataset

XENDOS followed 3,305 adults for four years — no other weight loss drug has that
Participants lost about 2.8 kg (6 lbs) more than placebo at four years
Progression to type 2 diabetes fell by 37% compared with placebo plus lifestyle change

The first month: managing expectations

The effect starts immediately, the results do not

Orlistat works in the gut rather than the brain: it blocks roughly 30% of the fat in your food from being absorbed. There is no dose to build up to, so the effect — including the gut side effects — starts from the first week.

Significant weight loss still takes 8–12 weeks, and the one-year average in trials was 8.5% against 5.4% for placebo. The Alli label notes that most weight loss happens in the first six months.

What happens if you stop

A mechanical effect, not a hormonal one

Weight is typically regained after stopping. Unlike the GLP-1 drugs, orlistat works mechanically rather than hormonally, so there is no rebound hunger effect when you stop — the fat you eat simply starts being absorbed normally again. Any weight loss you keep depends on the diet changes that came with it.

Common questions

Written and maintained by one person · Built with agentic AI tools · Every number checked against the cited trial · Not medical advice

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Keep reading

Full Xenical and Alli overview →All orlistat clinical evidence →Xenical and Alli side effects →What to expect week by week →Xenical and Alli cost →What happens if you stop →

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Last reviewed: September 2026

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