Nobody can guarantee such things. It's not a healthy diet, just one that starves you for a few days. The foods you're instructed to eat are not ones you'd want to eat if you want to be healthy. It's not a long-term diet. You eat very little for 3 days of unhealthy foods, I guess so you enjoy what little they let you eat, then you go back to your regular diet and gain the weight back again, because if your regular diet was working you wouldn't need to try this. So even if it works, it's a yo-yo diet program, and these have proven to be unhealthy. A better solution is to change the diet you eat every day, increase your exercise, don't live a sedentary lifestyle, and lose weight slowly as your metabolism changes so it will stay off.