Kidney Problems
Diet soda might be bad for your kidneys. In an 11-year-long Harvard Medical School study of more than 3,000 women, researchers found that diet cola is associated with a two-fold increased risk for kidney decline. Kidney function started declining when women drank more than two sodas a day. Since kidney decline was not associated with sugar-sweetened sodas, researchers suspect that the diet sweeteners are responsible.

Messed-Up Metabolism
Even just one diet soda a day is linked to a 34% higher risk of metabolic syndrome, the group of symptoms including belly fat and high cholesterol that puts you at risk for heart disease. Whether that link is attributed to an ingredient in diet soda or the drinkers' eating habits is unclear. But is that one can really worth it?

Obesity
You read that right: Diet soda doesn't help you lose weight after all. A study found that the more diet sodas a person drank, the greater their risk of becoming overweight. Downing just two or more cans a day increased waistlines by 500%. Why? People who consume diet foods might be more likely to overeat, because your body is being tricked into thinking it's eating sugar, and you crave more.

A Terrible Hangover
Your first bad decision was ordering that whiskey-and-diet-cola -- and you may make the next one sooner than you thought. Cocktails made with diet soda get you drunker, faster. Sugar-free mixers allow liquor to enter your bloodstream much quicker than those with sugar, leaving you with a bigger buzz.

Cell Damage
Diet sodas contain something many regular sodas don't: mold inhibitors. They're in nearly all diet sodas. But many regular sodas, such as Coke and Pepsi, don't contain this preservative. These chemicals have the ability to cause severe damage to DNA.

Rotting Teeth
With a pH of 3.2, diet soda is very acidic. (As a point of reference, the pH of battery acid is 1. Water is 7.) The acid is what readily dissolves enamel, and just because a soda is diet doesn't make it acid-light. Adults who drink three or more sodas a day have worse dental health. Soda drinkers had far greater decay, more missing teeth, and more fillings.

Reproductive Issues
Diet or not, soft drink cans are coated with the endocrine disruptor bisphenol A (BPA), which has been linked to everything from heart disease to obesity to reproductive problems. That's a lot of risktaking for one can of pop.