Are you talking about taking HCG injections and then going on a 500-calorie a day restrictive diet?
I don't want to wee on your parade, but that screams dangerous fad diet and weight loss that will not last.
HCG itself is not approved by the FDA for weight loss.
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“HCG is a hormone extracted from urine of pregnant women. It is approved by FDA for treatment of certain problems of the male reproductive system and in stimulating ovulation in women who have had difficulty becoming pregnant. No evidence has been presented, however, to substantiate claims for HCG as a weight-loss aid.”
Aside from all of the potential side effects of the injections themselves the 500-calorie a day diet is not sustainable; your body will most likely regain all that weight and more when you resume a normal daily routine (a normal HEALTHY daily routine).
I'd encourage you to read what the Mayo Clinic says about it:
http://www.mayoclinic.com/health/hcg-diet/AN02091
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"Although researchers have studied the HCG diet for years, no high-quality studies have shown that the hormone itself helps weight loss. Following any very low calorie diet is likely to result in weight loss, regardless of taking HCG. And the HCG diet can have drawbacks. Severe calorie restriction can make it hard to meet all of your nutritional needs. And rapid weight loss can lead to gallstones. Also, HCG can cause side effects, including headache, fatigue, irritability and male breast enlargement. And be careful about buying HCG products on the Internet — they might not be what they say they are.
Once you stop the HCG diet, you're likely to regain any weight you lost. The key to permanent weight loss is developing healthy eating and exercise habits — not following fad diets."