Aspartame is a low-kilojoule sweetener composed of two amino-acids, aspartic acid and phenylalanine. Both of these amino acids are found commonly in protein-containing foods, such as eggs, meat, fish, dairy products and nuts.
Because Aspartame is 180-200 times sweeter than table sugar (sucrose), such small amounts are needed to sweeten foods that its energy contribution to the diet is negligible. For example, a mere 190 milligrams of aspartame (about four kilojoules) has the same sweetening power as 40 grams of sugar (680 kilojoules).
Aspartame is available as a tabletop sweetener under the brand names Equal and Nutrasweet. It is also used in low-kilojoule food and beverage products, ranging from soft drinks and chewing gum to gelatins, confectionary, desserts, yoghurts and sugar-free cough drops.
Aspartame contains phenylalanine and should not be consumed by people with a rare genetic disorder called phenylketonuria (PKU). Food and beverage products that contain aspartame carry a statement on the label alerting people with this condition to the presence of phenylalanine. But the fact that a small portion of the population has this rare condition does not mean in any way that aspartame is unsafe for other consumers.
For people without this condition, aspartame is safe. 200+ scientific studies support its safety. You can find more information from Food Standards Australia and New Zealand here.