The concern with fast weight loss is that it usually takes extraordinary efforts in diet and exercise — efforts that could be unhealthy and that you probably can't maintain as permanent lifestyle changes.
A weight loss of 1 to 2 pounds a week is the typical recommendation. Although that may seem like a slow pace for weight loss, it's more likely to help you maintain your weight loss for the long term. Remember that 1 pound (0.45 kilogram) of fat contains 3,500 calories. So to lose 1 pound a week, you need to burn 500 more calories than you eat each day (500 calories x 7 days = 3,500 calories).
Also, if you lose a lot of weight very quickly, it may not be fat that you're losing. It might be water weight or even lean tissue, since it's hard to burn that many fat calories in a short period.
In some situations, however, faster weight loss can be safe if it's done the right way. For example, doctors might prescribe very low calorie diets for rapid weight loss if obesity is causing serious health problems. But an extreme diet like this requires medical supervision.
In addition, some diets include an initiation phase to help you jump-start your weight loss. For example, the Mayo Clinic Diet has a quick-start phase in which you might lose 6 to 10 pounds in the first two weeks. You can lose weight quickly with an approach like this because it combines many healthy and safe strategies at once — no gimmicks or extreme dieting. After the initial two-week period, you transition into the recommended weight loss of 1 or 2 pounds a week, which is not only safe but also realistic and sustainable for the long term.