The following is excerpted from an article which can be read in its entirety here:
http://articles.mercola.com/sites/articles/archive/2009/07/14/Study-Links-Aspartame-To-Leukemia-and-Lymphoma.aspx

Aspartame is the technical name for the brand names NutraSweet, Equal, Spoonful, and Equal-Measure. It is one of the first generations of artificial sweeteners and is 180 times sweeter than sugar. It is also one of the most dangerous chemicals that should never have been allowed in food. At the end of 2008, aspartame was found in over 6,000 products; everything from soft drinks to frozen desserts, yogurt and chewable vitamins. Products containing aspartame are sold in over 100 countries and are consumed by over 250 million people worldwide.

Initially, the FDA actually denied the approval of aspartame due to:
• Flawed data
• Brain tumor findings in animal studies
• Lack of studies on humans to determine longer-term effects
Unfortunately, despite these valid concerns, including evidence of its neurotoxicity, aspartame was successfully pushed through to the market by heavy financial investment and political clout.

A study, performed in 2007, which showed that aspartame-exposed rats developed so much formaldehyde in their bodies that their skin turned yellow. It is this formation of formaldehyde that is the link between aspartame and lymphoma and leukemia. It is well known aspartame turns into wood alcohol when it is consumed, however, few people realize this wood alcohol morphs into formaldehyde – the same stuff used for embalming – in the cells of your body. Formaldehyde is a Class 1 cancer-causing agent, responsible for everything from sick house syndrome to birth defects, and research shows it causes lymphoma and leukemia in both lab rats and humans.

Aspartame intake has also been shown to increase your risk of breast or prostate cancer. Incidents of both types of cancer have been on the rise at a pace closely associated with the expanding use of aspartame throughout the world. There are over 900 published studies on the health hazards of aspartame. (You can find a list in the National Library Medicine Index.) So to still insist that aspartame is safe today is a staggering example of coming to the wrong conclusion in the face of a mountain of evidence.

Aspartame has generated unprecedented numbers of FDA Complaints. There have been more reports to the FDA for aspartame reactions than for all other food additives combined. Less than a year after aspartame was granted approval for use in soft drinks in 1983, the FDA had recorded 600 consumer complaints of headaches, dizziness and other health-related reactions from aspartame consumption.

The unprecedented number of complaints caused the FDA to call in the Centers for Disease Control (CDC), who concluded that adverse reactions to aspartame were occurring in “unusually sensitive” individuals, but that there was insufficient evidence to prove existence of wide-spread health problems attributable to its consumption.

Health problems initially reported included:
• Neurological/behavioral symptoms (67 percent), including headaches, dizziness, and mood changes such as depression
• Gastrointestinal symptoms (24 percent)
• Allergic type and/or dermatologic symptoms (15 percent)
• Alterations in menstrual patterns (6 percent)
• Other symptoms of various types (9 percent)
Today, there are some 10,000 documented reports of adverse reactions to aspartame, including death. But since it is estimated only about one percent of people who experience a reaction report it, it is safe to assume at least a million people have had a reaction to this chemical.

Migraines are by far the most frequently reported reaction. Other commonly reported symptoms of an aspartame reaction include:
• change in vision
• convulsions and seizures
• sleep problems/insomnia
• change in heart rate
• hallucination
• abdominal cramps/pain
• memory loss
• rash
• nausea and vomiting
• fatigue and weakness
• dizziness/poor equilibrium
• diarrhea
• hives
• joint pain