P.D. Mangan posted a good article on the risks of rapid weight loss. I agree with him that a max of 1–2 pounds a week is a reasonable weight-loss goal, and that a bit more than that is OK in the first week but highly inadvisable for the long run. Most folks do well if they settle into one pound a week.
A slower metabolism is another side effect of losing weight too quickly; since your body’s metabolism is essentially determining how much fuel you are needing to burn every day based on your activities and food intake, dropping calories out of your diet can considerably lower your metabolism. While this might not seem like a big deal, it is! A lower metabolism essentially causes your body to burn fewer calories each day than it typically would, and some research even shows up to a 23% decrease in calories that are burned each day. This doesn’t always go away when you stop losing weight quickly, either, your metabolism might take a while to get back on track and burning appropriately, which could affect your health and weight for a long time.
Lastly, your body could end up losing muscle as part of your quick weight loss routine. Granted, eliminating calories from your diet will have you lose weight, yes, but aside from a lot of that weight being water, some of it can also be muscle. A recent study done with people on a 500 calorie diet versus a 1250 calorie diet showed significant results; by the end of the trial, the participants on the 500 calorie diet lost six times more muscle than those on the 1250 calorie diet – an astounding loss in muscle in just a five-week span of time.