Losing weight too quickly through crash dieting can lead to lose skin developing (the skin does not have the time to adapt to the loss of the fat and will take time to firm up)...
For those who are older, it can also make you more susceptible osteoporosis , a weakening of the bones due to calcium deficiency...
You can also see a lot of muscle tissue loss in addition to the fat loss,
and early aging...as crash dieting speeds up the process of aging. You
may see more fine lines/wrinkles/sagging skin.
You can also see electrolyte imbalance, low blood cell counts, cardiovascular disorders and organ damage...
Not to mention moodyness, mood swings and lethargy.
There is also the possibility of developing an eating disorder due to this such as anorexia or bulimia.
If you're asking such a question in the first place though, it sounds like you're considering doing some kind of drastic diet, which I would advise against as it does not lead to any lasting results.
You can still lose weight fast but in a healthy way, there are many diet systems out there that can yield fast results without leaving you hungry or impacting your health in any negative way.
Losing weight fast is considered too fast and could put you at risk of many health problems, including muscle loss, gallstones, nutritional deficiencies and a sudden drop in metabolism.
Saggy muscles and possible kidney problems.
Well, I have seen a lot of people going on drastic diets and
very intensive workout programs to lose weight in a short amount of time and in
the long run, this was not good for them. It resulted in saggy skin and once they
stopped with the diets, they gained a lot of weight back very quickly.