500 calories is starvation and your body will eventually slow metabolism and hang on to fat for self preservation.
Since this hcg stuff isn't coming from an official lab, what are the guarantees that the drops actually contain hcg?
Just another fad diet, when you go back to your regular diet you metabolism is now slowed down and will pack weight back on in a hurry to protect you from future starvation.
Start eating on a small plate, dessert sized. Read labels, eat six times a day, 3 meals, 3 snacks, restrict refined white carbs, eat more unprocessed foods, an orange instead of orange juice. The fiber will help fill you up. Eat lean protein, that will also fill you up. Don't skip meals, eliminate all soda and alcohol along with the sweets.
There is no magic bullet. Lifestyle changes must happen, and for some of us, weight loss surgery along with those lifestyle changes are needed. Surgery is NOT the easy way out, it requires a lot of work and change for it to succeed, but along with diet changes, exercise, it is a valid and good tool for those of us who are too far down the obesity road.