No more than 2 pounds per week should be lost if you want to avoid a rebound effect or malnutrition. There are many negative effects on the body by losing weight too quickly, including but not limited to: Metabolism, Diabetes, Hair Loss, Loose Skin...etc.
If you lose weight too quickly, your body may perceive that it is being starved, and slow the speed of your metabolism. This in turn may make it more difficult for you to lose weight in the long run. Other negative side effects of a state of starvation include feeling tired, irritable and low on energy. Dizziness and fainting are relatively more likely if you are starving and losing weight too fast.
The Medline Plus medical encyclopedia indicates that crash dieting, starvation or sudden weight loss may cause hair loss. ...and losing more than 2lbs per week will do just that; make your body believe it is being starved.
Losing weight too quickly can lead to diabetes mellitus and the development of diabetes in people who have participated in studies was believed to be an increase in stress hormones, as a result of crash dieting.
Losing more than 1-2 pounds of actual body fat per week is not only unhealthy, but just plain dangerous. Great question that many should know the answer to!