This information is for people with serious weight problems. It tells you about orlistat, a medicine used for weight loss. It is based on the best and most up-to-date research.
Does it work?
Yes. If you're very overweight or obese, a drug called orlistat could help you to lose a small amount of weight and keep it off. But orlistat has side effects. And it doesn't work by itself. You also need to change the way you eat and exercise.
What is it?
Orlistat is a drug that can help people who are obese lose weight. It stops your body absorbing some of the fat from the food you eat. About a third of the fat you eat passes straight out of your body in your stools.
You can get orlistat on prescription from your doctor. The brand name is Xenical. You can also buy orlistat from a pharmacist, without a prescription, under the brand name Alli. The pills you can buy yourself contain half the dose of the prescription version.
Orlistat comes as pills that you take at mealtimes, up to three times a day. If you're going to eat a meal that doesn't contain any fat, you should skip the dose of orlistat with that meal.
Your doctor may recommend orlistat if: 
Your weight is a risk to your health. This means that your body mass index (BMI) is 30 or more, or your BMI is 28 to 30 and you have medical problems because of your weight
You have been following a diet and exercise programme for at least one month, and have already lost at least 2.5 kilograms (5.5 pounds)
You are able to get advice and support from a local dietitian, practice nurse or GP.
You can buy orlistat from a pharmacist if:
Usually, people lose weight quite quickly when they take orlistat. But it doesn't work for everyone. Talk to your doctor or pharmacist if you haven't lost weight after 12 weeks. Whatever happens, you shouldn't take the over-the-counter version of orlistat for longer than six months. If you want to take it for longer, talk to your doctor.
How can it help?
Taking orlistat can help some people lose weight. It's difficult to say for sure exactly how much weight you'll lose if you take orlistat. Some people lose more weight than others.  
After taking orlistat for a year, people lost about 8.1 kilograms (about 18 pounds) on average.
That was about 2.8 kilograms (6 pounds) more than people who took a dummy treatment (a placebo).
If you have high cholesterol, orlistat may decrease the level of harmful fats in your blood. Reducing your cholesterol can cut your risk of heart disease or a stroke. To read more, see High cholesterol and obesity.
If you have diabetes, losing weight with orlistat may help you to control your blood sugar levels. You may be able to manage your diabetes with less diabetes medicine, or without any medicine.
One summary of the research looked at results of studies comparing orlistat, sibutramine (no longer available), and rimonabant (no longer available) with a dummy (placebo) treatment. It found that, in studies lasting at least one year, orlistat reduced weight by 2.9 kilograms (about 6 pounds) more than placebo, but sibutramine reduced weight by 4.2 kilograms (about 9 pounds) more than placebo, and rimonabant by 4.7 kilograms (about 10 pounds) more. But the studies didn't compare the drugs directly with each other, so we don't know for sure which works best.