The Arizona Republic has raised an interesting question. Is it possible to use tapeworms as a diet aid? Won't the tape worm just sit in your stomach and consume all the extra food you eat, and after a while you can pull out the thing? The basic answer is no. This would not be a good idea. They go into some more details:

While a tapeworm might take in some of the food you do, it would at the same time be taking in a lot of vitamins and other nutrients you need to stay healthy. Do you know what ascites are? A big pool of fluid in your tummy caused by an immune response to something in your guts. Something like a tapeworm. It gives you a big potbelly, which runs kind of counter to the look you might be wishing for. And a tapeworm might not necessarily just set up camp in your innards. It can also cause cysts in your muscles, liver and eyes. Your eyes! So don't you think it might be easier instead to just eat a bit less and exercise a bit more?