The American Dietetic Association has challenged three current internet rumors concerning Aspartame. According to it's Evidence Analysis Library, the sweetener does not cause "rebound hunger." Much evidence shows that it has no effect on appetite or food intake. Therefore, it would not lead to weight gain. When used in the reduced calorie diet, it increases weight loss, due to it's lack of calories. There have been no ill effects on health overall as a sweetener, as various myths have reported. Some claims that explain the chemistry behind the catabolic phases of Aspartame are incorrect. The ADA has concluded that in the 40 plus years the product has been on the market, "Aspartame consumption is not associated with adverse effects in the general population."
Aspartame is one of the most studied of all food additives. It is a simple compound made of components common to many foods: the amino acids phenylalanine and aspartic acid, with a methyl group. The combination creates a product with 200 times the sweetness of table sugar (sucrose & fructose). When digested, enzymes break apart the 3 components. The amino acids are used as proteins, just as any other food proteins. The methyl group converts to methanol, and oxidizes into carbon dioxide. Simple tomato juice yeilds 6 times the amount of methanol as a diet soda.
People with the inherited disease phenylketonuria, or PKU, are unable to metabolize and dispose of phenylalanine. The accumulation of phenylalanine and it's by-products is toxic, therefore a specialized low protein diet is the treatment, and aspartame must be avoided. For this reason, all newborns in the U.S. are screened for the disease.
Aside from the special case of PKU, aspartame is safe.
~American Dietetic Association