It does not matter why you want to lose weight. Whether you want to become healthier or to be able to fit an old pair of jeans, you probably want to lose the weight fast.
Losing weight fast is very possible, but it can vary quite a lot from person to person. What helped me lose 50 pounds may not be effective at all for my neighbor. Current weight, health, diet, metabolic rate and even genetics are all factors that can influence how much weight you can lose greatly.
Our bodies are mostly water. This is why any diet that will make you lose a lot weight fast will also cause you to lose some muscle tissue too. The ratio is about 75% fat and 25% but can vary from person to person. The problem with these crash diets is that you will gain the weight back when you start eating like before, and you will mostly gain fat. With a reduced body mass, your body will find it harder to burn excess fat and you will lose this new weight a lot slower. This may sound like a useless enterprise, but if you really need to, you can lose up to 20 pounds in a week. If you want to lose only fat without compromising your muscle mass, you can lose about 2 pounds every week. But this is pure fat, and you will notice a gradual increase in the weight loss speed, as the fat/muscle ratio will shift and will work with you.
This weight will also not return as it is gradually removed.