The HCG Diet is another controversial diet plan, most notably because medical professionals disagree on whether it's safe. HCG is a pregnancy hormone that's traditionally been used to treat fertility issues, but hasn't been FDA-approved to be used as a treatment for obesity. Since HCG injections are a part of the HCG Diet, this has caused some controversy within the medical sphere. In addition, the diet that accompanies the injections is extremely low-calorie. Our opinion? Skip this one!