Diets aimed at really fast weight loss are usually not for long term use and are only temporary. You may be getting ready for a wedding, your high school reunion or any other affair, these diets are useful but as with every diet, you should check with your physician to be sure that you are capable of very limited food intake.
There are 3, 5 and 10 day diets all claiming to make you lose a maximum amount of weight at the shortest possible time. All of these diets are not recommended for long term use. They can cause other health problems if maintained for a long time.
All of these diets WILL make you lose weight for as long as you don’t stray from them. If you follow them strictly, there is no reason to fail. It is also good to know that these diets will make you lose mostly water retained in your body, hence the quick loss but will hardly make you lose real fat.
Understand that losing body fat entails adapting a lifestyle change and not only a diet change. It is an undertaking that will take all of your willpower and asking the people around you to support you in your endeavor.