With the increased use of aspartame as a dietary sweetener, there have also been questions about its metabolic safety. By investigating metabolic pathways and reviewing experimental studies, this paper explores how aspartame is metabolised in healthy individuals and in Phenylketonuria (PKU) patients. Firstly, it was found that aspartame may have a hyperglycaemic effect. Furthermore, excessive intake of aspartame may cause phenylalanine to be transported to the brain. Additionally, the methanol degradation product may predispose to systemic toxicity or metabolic acidosis. In PKU patients, the absence of phenylalanine hydroxylase results in an accumulation of phenylalanine. Despite the paucity of original studies assessing aspartame processing in PKU individuals, aspartame would presumably alter the metabolic rate in the liver. Individuals heterozygous for PKU are thought to suffer no adverse effects of aspartame consumption. Overall, this paper concludes that aspartame has potentially harmful effects and that we should be wary of its widespread consumption.