Doctors are strictly against crash dieting. Any process that defies the natural law cannot be considered as healthy and there is no exception in the case of crash dieting. First of all, your body may have to face a lot of difficulties if it does not get the required calories. People everywhere are trying to avoid calories like an obsession but they do not understand that there is a basic requirement of calories by the human body everyday.
Fatigue will be a very common thing to deal with all the time with crash dieting, your body will not get adequate energy and so it will fail to function in the proper way. Therefore, you will get tired very easily and you may also end up having blackouts. Another problem with crash dieting is that although you will lose weight, you would not have a very good shape and you will end up with loose muscles in your body.