Dr Clare Morrison

Article by Dr Clare Morrison

How Does Orlistat Work?

Orlistat is a medicine that helps obese people lose weight. Normally, fat from your diet is broken down by enzymes in your stomach and intestines and absorbed into your body to be stored. However, Orlistat stops these enzymes from working – this means 30 to 40 percent less dietary fat will be taken up by your body. Instead, the unabsorbed fat will pass directly through your digestive system.

The image below also represents how Orlistat works:


Can Orlistat help me lose weight?

Orlistat isn’t effective for everyone and is by no means a miracle weight loss pill. Orlistat is recommended if you are overweight and if previous weight-reducing efforts have not worked. As long as you follow a sensible, balanced diet, as well as exercise regularly, you’ll have a good chance of losing weight.

How effective is Orlistat?

In clinical trial data, the average weight loss was 50 percent extra with Orlistat in comparison  to placebo with diet. This means that if you could lose 6 kg with diet alone, you could lose 9 kg with Orlistat.  Furthermore, it was shown that those taking Orlistat maintained weight loss better than in the placebo group.  For maximum effectiveness of Orlistat, one capsule is taken with each main meal (a maximum of three capsules a day)  before, during, or up to one hour after each meal. If you miss a meal or the meal does not contain any fat, you do not need to take the capsule.

How do I take Orlistat?

Orlistat is available in two types: Xenical and Alli. Xenical is the prescription dose (120mg) of Orlisat, while Alli contains half the amount (60mg) and is available to buy over-the-counter. Both treatments target and remove fat in exactly the same way.

Take one capsule within an hour of eating each of your three main meals. One at breakfast, one at lunch and one at dinner. You should continue taking Orlistat for at least three months in order to see if you’re effective in losing weight. If you’ve failed to lose any weight, most doctors will advise you to stop taking Orlistat. If you’ve been on Orlistat for a year, we advise you to discuss the potential benefits and limitations of continuing to take Orlistat.

How to get the most from your Orlistat treatment

  • Eat well – but try to cut down your dietary calories by about 500 calories per day. This should result in the loss of 1lb per week, a healthy weight loss
  • Eat slowly and enjoy your food – don’t rush (this will keep any hunger at bay)
  • Exercise regularly
  • Avoid temptation – keep the cupboards free from unhealthy foods
  • Know that weight loss takes time. Have realistic goals in mind, monitor your progress and don’t give up!

What are my targets?

Successful weight loss depends on realistic, achievable goals. When you start taking Orlistat, your first weight loss target should be to lose five percent of your weight in the first three months. If you succeed in losing the weight, you’ll be able to continue with the treatment. However, don’t be disheartened if you don’t lose weight with Orlistat. After a break of six months, you can try again.

What are the side effects of Orlistat?

Side effects related to Orlistat are mainly related to the reduced absorption of fat which passes out in the faeces. The symptoms are often mild and not everyone experiences them, but keeping to your low-fat diet will reduce the risk of them occurring. You could experience the following symptoms:

  • Abdominal cramps
  • Flatulence
  • Loose oil stools
  • Oily spotting
  • Urgency to defecate

These symptoms often improve after a few weeks when your body’s adjusted to the new medicine.

How will Orlistat effect other medication?

As Orlistat affects absorption into your digestive system, other medications may be affected, especially if you experience diarrhoea. It’s very important to check with your doctor or pharmacist first before taking Orlistat if you’re taking other medications, such as the oral contraceptive pill.

It’s also a good idea to consult your doctor if you:

  • Are pregnant or breastfeeding
  • Have problems absorbing food
  • Have reduced bile flow
  • Are allergic to any inactive ingredients in Orlistat (including gelatine)

Can I take vitamins?

Yes! You can take vitamins if you’re taking Orlistat as most are fat soluble and not absorbed well when you take Orlistat. These are vitamins A, D, E and K. It’s strongly recommended that you take a multivitamin containing these vitamins at least two hours before or after you’ve taken Orlistat. Most doctors recommend you take your multivitamin at bedtime.

Decided to purchase Orlistat? Next step might be to work out your diet. Try our Orlistat diet plan.


Dr Clare Morrison
Dr Clare Morrison
Experienced General Practitioner in Hampshire since 1995, with particular interest in Nutrition, Obesity and Smoking Cessation.
Originally published September 12 2014, updated July 05 2019