Sure they work.. But they aren't sustainable. No one can eat a 'fad diet's' diet for their whole life, which is the point of losing weight (life change). But when people are desperate to lose weight, it doesn't really matter. I've sometimes hoped to get sick eating something just so I could lose weight and somehow relate that food to a bad thing in my head so I wouldn't eat it anymore.. Kind of sick, but I guess desperation does that.
After all, how many people do we all know who get a stomach bug or just the flu and say "I lost 10 pounds while being sick!" because they just weren't eating.
I've heard of the tapeworm thing but I don't think many people do that, if ever, and thought it might be a myth and don't think it would ever be sustainable. Yucky.