Aspartame is the second most widely used artificial sweetener in the world. It is found in more than 6,000 products including carbonated and powdered soft drinks, hot chocolate, chewing gum, candy, desserts, yogurt, and tabletop sweeteners, as well as some pharmaceutical products like vitamins and sugar-free cough drops. More than 200 million people worldwide consume it. The sweetener has been used for more than 30 years, having first been approved by the FDA in 1974. Studies of the carcinogenicity of aspartame performed by its producers have been negative.

Researchers administered aspartame to Sprague-Dawley rats by adding it to a standard diet. They began studying the rats at 8 weeks of age and continued until the spontaneous death of each rat. Treatment groups received feed that contained concentrations of aspartame at dosages simulating human daily intakes of 5,000, 2,500, 500, 100, 20, and 4 mg/kg body weight. Groups consisted of 100 males and 100 females at each of the three highest dosages and 150 males and 150 females at all lower dosages and controls.

The experiment ended after the death of the last animal at 159 weeks. At spontaneous death, each animal underwent examination for microscopic changes in all organs and tissues, a process different from the aspartame studies conducted 30 years ago and one that was designed to allow aspartame to fully express any carcinogenic potential.