In fact, the Food and Drug Administration has advised consumers to steer clear of over-the-counter weight-loss products that contain HCG. HCG is human chorionic gonadotropin, a hormone produced during pregnancy. Diet recommends severe calorie restriction — typically just 500 to 800 calories a day. People who follow such a very low calorie diet are likely to lose weight, at least in the short term. Diets that so severely limit calories have risks, such as gallstone formation, irregular heartbeat and an imbalance of electrolytes. Side effects have also been reported with the HCG diet and include fatigue, irritability, restlessness, depression, fluid buildup (edema), and swelling of the breasts in boys and men (gynecomastia). Another serious concern is the risk of blood clots forming and blocking blood vessels (thromboembolism).