Human Chorionic Gonadotropin (HCG) is a hormone that is produced by the placenta when a woman becomes pregnant. It is the hormone that shows up on a pregnancy test when the test is positive. This hormone increases each trimester a woman is pregnant.
To everyone out there who thinks the HCG hormone aids in weight loss, it's most likely the 500 calorie diet you are on that is making you lose weight, not HCG. If you normally eat approximately 2,000 calories a day and then all of a sudden start eating only 500 calories a day your body will start quickly shedding the pounds off. It takes eating 500 calories less per day to lose 1 pound per week, 1,000 calories less per day to lose 2 pounds per week, and 1,500 calories less per day to lose 3 pounds per week. So 1,500 calories less per week for 6 weeks will give you a 18 pound weight loss over the course of 6 weeks. Don't inject something you don't need to inject!
If you want to lose weight the best thing to do is eat a healthy, well balanced diet, exercise for at least 20-30 minutes a day for 5-6 days a week (incorporating both cardio and weight training), and eat less than you normally do (500-800 calories less). Taking the weight off slowly will help you better maintain your weight loss in the future.
A great article posted on the ADA's website:
http://www.everydayhealth.com/diet-nutrition/0310/hcg-diet-controversy-strikes-again.aspx