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Alli: How This Weight Loss Aid Works and Tips for Use

Dr Karen Martin
Reviewed by Dr Karen MartinReviewed on 20 Feb 2025 | 3 minutes readNext review due: 20 Feb 2028
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Alli is a branded product that contains the medication orlistat, which is used for weight loss. You can buy it from your pharmacy, although it is only suitable for overweight adults with a body mass index (BMI) of 28 kg/m2 or above who will commit to a reduced-calorie and lower-fat diet.

Exercise and other healthy living choices should also form an essential role in your weight loss plan. The medication can help those who are overweight to reduce their increased risk of developing a range of health problems such as diabetes, high blood pressure, heart disease, and even some cancers.

How do I take Alli?

Alli is available as capsules. You can take one capsule three times a day at breakfast, lunch, and dinner. You should take Alli just before, during, or up to one hour after meals. If you miss a meal or your meal does not contain any fat, you should not take Alli. Make sure you swallow the capsules whole with a glass of water.

The medication orlistat found in Alli can decrease the absorption of fat-soluble vitamins (namely vitamins A, D, E, and K). Therefore, you should take a multivitamin supplement containing these vitamins. It is best to take the multivitamin supplement in the evening before you go to bed so orlistat does not stop the fat-soluble vitamins from being absorbed into your body.

Do not take Alli for longer than six months without consulting your doctor. If you have not lost weight (at least a 5% reduction) after taking Alli for 12 weeks, you should speak with your doctor or pharmacist, as they may not want you to continue treatment.

How does Alli work?

Alli (orlistat) is an over-the-counter weight loss aid designed to help reduce the amount of fat your body absorbs from food. Unlike appetite suppressants or stimulants, Alli works specifically in your digestive system. It targets the enzymes that break down dietary fat, making it easier to manage calorie intake when combined with a low-fat, reduced-calorie diet.

The active ingredient in Alli, orlistat, blocks the function of lipase enzymes in your stomach and small intestine. These enzymes are essential for breaking down fat into smaller molecules that your body can absorb. By inhibiting their activity, Alli prevents around 25% of the fat you consume from being digested. Instead, the unabsorbed fat is passed through your system and excreted. This targeted approach makes Alli an effective tool for weight loss, especially for those who struggle with managing fat intake in their diet.

Who should avoid Alli?

Do not take Alli if you are allergic to orlistat or another ingredient listed in the medicine. Alli is unsuitable for anyone under 18 or with a BMI lower than 28 kg/m2. Do not take Alli if you are pregnant or breastfeeding, have problems absorbing food, have a medical condition called cholestasis, take cyclosporin, or take warfarin or another medicine to thin your blood.

Speak with your doctor before taking Alli if you have kidney disease or take amiodarone, levothyroxine, or other medications for diabetes, epilepsy, high blood pressure, high cholesterol, or HIV. If you would usually check with your pharmacist or doctor before taking new medications, it may be best to speak to them first to ensure Alli is a safe option for you.

Are there any side effects?

As with all medications, some people may experience side effects. Some common ones include flatulence, fatty or oily stools, oily spotting on your underwear, an urgency to have a bowel movement, and abdominal pain. Eating lower-fat meals can help to reduce side effects. You should speak with your doctor or pharmacist if you are concerned about any side effects.

Remember that Alli isn’t a ‘magic bullet’ that will instantly get you to your ideal weight. It’s only part of a general weight loss program that includes a healthy calorie-controlled diet and regular exercise.

The AI in healthwords.ai refers to conversational search, while the only AI tools used for articles are grammar and language improvements -  medical content remains rigorously authored by healthcare professionals.

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This article has been written by UK-based doctors and pharmacists, so some advice may not apply to US users and some suggested treatments may not be available. For more information, please see our T&Cs.
Dr Karen Martin
Reviewed by Dr Karen Martin
Reviewed on 20 Feb 2025
Next review due: 20 Feb 2028
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