Alli binds with the gut enzymes that break down dietary fats so that they can't be digested, absorbed and utilized by the body as calories. A portion of the fat "passes through" instead. It's not a stimulant or appetite depressant, and its effects are limited to the gut. It renders a portion of of the fat in your meal to be "null and void", as it were. The drug was previously sold by prescription under the name Orlistat. The over the counter version is half the dose of the original prescription version.

Here's my take:
Alli isn't a magic bullet. If you consume too much fat, you are likely to experience the oft mentioned unpleasant side effects. If you normally eat a low-fat diet, there's nothing for the drug to do. The drug does nothing about carb calories. Alli can interfere with the absorption of fat-soluble vitamins. And it's expensive. IMO, If you follow their diet guidelines, Alli really isn't doing all that much for you.

Let's do the math. The Alli web site recommends no more than 15 fat grams per meal. At 9 calories per fat gram, that's a maximum recommended 135 calories from fat. If Alli blocks 25% of fat calories as promised, then you are blocking a whopping 35 calories per meal, or 101 calories per day. Theoretically, then, Alli will help you lose one additional pound in 35 days (3500 calories). That's a pretty high cost per pound ratio if you ask me. I can more than pay for a gym membership with that and burn 100 calories in 15 minutes a day!!

Sure you could eat a pepperoni pizza and block 25% of mega fat grams, but you'd still be absorbing 75% of those mega fat grams and will likely lose the rest in a rather unpleasant manner.

I researched Alli, even tried it for a while. But it really didn't do anything for me that a balanced, calorie-controlled diet and exercise wasn't already doing.

How long does it take to the side effect start?
If I eat pizza for lunch, when am I start going to the bathroom?

Hmmm...

I eat pretty low fat anyhow...raw food/vegan....in fact for me my fat intake, for good fat, is too low. *thinks about your comments*

EXCELLENT QUESTION Ringmaster! And the answer is YES, it blocks a portion of ALL fat. Good and bad. It is a terrible drug to take if you have any sort of autoimmune dysfunction. People with MS and RA have a particularly hard time.