Alli is used for weight loss in adults aged 18 and over who are overweight, and have a body mass index (BMI) of 28 or above. Alli should be used along with a reduced calorie, lower-fat diet. How alli works The active ingredient (orlistat) in Alli is designed to target fat in your digestive system. It stops about a quarter of the fat in your meals from being absorbed. This fat will pass out of the body in your stools.  It is important that you commit to a lower-fat diet to manage any side effects the treatment may cause. If you do, the action of the capsules will assist your efforts by helping you to lose more weight compared to dieting alone. For every 2 kg (4 lb) you lose from dieting alone, alli can help you lose 1 kg (2 lb) more. 2. What you need to know before you take alli Risk of being overweight Being overweight increases your risk of developing several serious health problems such as diabetes and heart disease. These conditions may not cause you to feel unwell so you should see your doctor for a general health check. Multivitamins and alli − You should take a multivitamin, every day. alli can lower the levels of some vitamins absorbed by your body. The multivitamin should contain vitamins A, D, E and K. You should take the multivitamin at bedtime, when you will not be taking alli, to help ensure that the vitamins are absorbed.