Alli is a diet drug that is available today over the counter. Alli is a reduced strength version of a prescription only pill called Orlistat. Orlistat works by preventing the body from absorbing all of the fat that a person has consumed (an estimated 50%).
That sounds good. Right? If the body doesn't absorb the fat, then it doesn't absorb the calories that you've consumed through fat. Fat is higher in calories than protein or carbohydrates. Each gram of protein or carbohydrates, holds 4 calories. Each gram of fat holds more than twice that at 9 calories. So then by taking alli, consuming fat isn't as calorically expensive. If I eat the same number of calories and take alli, I'll reduce the number of calories that my body absorbs, thus giving me a calorie deficit; which is what is needed in order to lose weight.
Well, thinking about it, where does that fat go then? It gets excreted with the rest of the food that your body can't absorb – through your bowels. Excerpt from wikipedia:
Because orlistat's main effect is to prevent dietary fat from being
absorbed, the fat is excreted unchanged in the *** and so the stool
may become oily or loose (steatorrhea). Increased flatulence
is also common. Bowel movements may become frequent or urgent, and rare
occurrences of fecal incontinence have been seen in clinical trials.
So, I'm not going to make any judgements on whether this is a good thing or not. As far as I see it, the cons are the above stated side effect (in addition to the fact that it can also hinder the absorption of some vitamins & doesn't discriminate between Omega-3 fatty acids and Saturated fat) and that if a person doesn't change their eating habits, once they stop taking the drug, they will regain the weight that they had lost while taking it.
However, if by taking the drug, a person observes their body's reaction to eating high fat meals, and learns to eat properly from those cues, then I don't think it's that bad. In a way, it's keeping someone honest in their diet. S/he knows that if they consume X, then s/he will suffer the consequences; which is more real and immediate than gaining fat or increased risk of disease.
Here are two other viewpoints on the topic: