People who down several diet sodas per day are hardly rare. Government surveys have found that people who drink diet beverages average more than 26 ounces per day (some drink far more) and that 3% of diet-soda drinkers have at least four daily. Are these diet-soda fiends true addicts? And if so, what are they addicted to? Research suggests that the artificial sweeteners in diet soda (such as aspartame) may prompt people to keep refilling their glass because these fake sugars don't satisfy like the real thing. Your senses tell you there's something sweet that you're tasting, but your brain tells you, 'Actually, it's not as much of a reward as I expected,'" says Martin P. Paulus, MD, a professor of psychiatry at the University of California San Diego, and one of the authors of the study. "The consequence might be that the brain says, 'Well, I'll have more of this.'" In other words, artificial sweeteners may spur drinkers -- or their brains -- to keep chasing a "high" that diet soda keeps forever just out of reach. It's not clear that this teasing effect can lead to dependence, but it's a possibility, Dr. Paulus says. "Artificial sweeteners have positive reinforcing effects -- meaning humans will work for it, like for other foods, alcohol, and even drugs of abuse," he says. "Whenever you have that, there is a potential that a subgroup of people ... will have a chance of getting addicted."