Is losing weight fast bad for you?

Is losing weight fast bad for your health? I am really worrying about this. I think I am losing weight too fast recently, I have lost about 18 lbs the last two week. That’s much more than 2 lbs per week that most people recommend. Is it true that if you lose too much weight quickly, you will end up gaining it back?
P.S.
I used to eat too much everyday and I exercise occasionally. But I’ve exercised a lot and been very careful with my diet during the last few weeks.

Asked under: Diet | March 7, 2010