Scientific studies suggest you should probably lay off the sugar-packed Big Gulp.
That's why places including New York City and the Bay Area city of Richmond are weighing taxes or other measures to limit the consumption of sugar-sweetened beverages.
But what about their zero-calorie counterparts?
"There is this cultural lore that has people thinking that diet soda is what's really bad for you and that the other stuff (sugar-sweetened soda) isn't as bad," said Harold Goldstein of the Center for Public Health Advocacy in Davis. Worries include that artificially sweetened drinks will cause cancer or diabetes.
Though most science dissuades drinking regular soda, diet soda's comparatively ambiguous research has not completely dispelled the lore.
Here are the answers, some rather nuanced, to some of the common questions about diet soda.
Is diet soda poisonous?
No, it won't poison you.
Most diet sodas are made with artificial sweeteners such as aspartame, which are several hundred times sweeter than real sugar or high-fructose corn syrup. Because not much of these is needed, a diet soda can get away with negligible calories.
Artificial sweeteners "have probably been the most intensively studied food ingredient ever," said Richard Mattes, professor of foods and nutrition at Purdue University. "Every panel has cleared them."
So what's the big deal?
Humans crave sweetness.
"There are some cues in foods that would normally be good predictors that calories are going to show up," said Susan Swithers, professor of psychological sciences at Purdue.
Sweetness is one of them.
But with artificial sweeteners, "all of a sudden, for the first time in our evolutionary history, sweet does not mean calories," said Dana Small, a Yale University neuroscientist.
Does diet soda make me eat more?
Barry Popkin, professor of nutrition at the University of North Carolina at Chapel Hill, said the big question is whether drinking diet beverages "means you'll eat more sweet foods."
There is some evidence that suggests so. Several years ago, Purdue's Swithers gave one group of rats a consistent diet of pudding with sugar, and another group pudding with artificial sweeteners.
She then fed all the rats a real-sugar snack. At the next meal, rats with the diets of artificial sweeteners ate more than those with a sugar diet – not compensating for the extra calories of the snack they'd had.
"They were used to getting sweet-tasting stuff that didn't give them as many calories," Swithers said.
For diet-soda drinkers, said Swithers, "their history with diet soda would say, this piece of cake tastes really sweet, but I have no idea if it has calories or not."
And they might overeat it, even if it has calories.
Perhaps as a result, Swithers also found higher blood glucose, a sign of diabetes, in rats with artificial- sweetener diets.
Animal research like this "helps us to understand what might happen in humans," but not the whole story, said David Allison, nutrition professor at the University of Alabama at Birmingham. Lab conditions may be unrealistic, and there are big size and cognitive differences between rats and people.
"It's cute, but it has no relation to humans," said North Carolina's Popkin of such animal studies. "To date, nothing has shown that a diet beverage consumer consumes more sweet food than a water consumer."
So what about evidence in people, in the brain?
"You cannot fool your brain. It's as simple as that," said Guido Frank, a neuro- scientist at the University of Colorado at Denver. He found that brain images were different after people ate artificial sweeteners than after they ate sugar.
Yale's Small found that after eating real sugar, those who regularly drank artificially sweetened beverages had different brain responses than those who didn't.
"The brain knows it as soon as you eat it. And the brain knows it after you eat it, and as a result of eating it, seems to change your reward system," she said. She called this effect of artificial sweeteners "worrisome."
But not all scientists agree with the implications.
"I'm singularly unimpressed by brain-imaging studies," said Alabama's Allison. "I'm not sure what it tells me other than that the brain is involved in everything we do."
Will diet soda make me gain weight or get type 2 diabetes?
To determine whether these findings have long-term implications, over decades scientists have followed the diets, lifestyles, and health of large groups of people.
In some of these "observational studies," statistics have shown that drinking diet soda might increase risk for weight gain, heart attacks or diabetes.
But others show no effect.
Why doesn't science seem to have the answer?
Allison explained the inconsistent results.
"When you look across studies, what you see is that it's always a different analysis."
Relationships might also be misinterpreted, he said. If a study showed a relationship between headaches and aspirin use, "it would be foolish to conclude that aspirin causes headaches."
Analogously, people who drink diet soda might be heavier, but it may be because they are drinking diet soda as part of an effort to lose weight.
Even with animal experiments, questions remain.
"If it was an effect that was that obvious, we should have figured it out," said Swithers.
It's "very context-specific, (and) the science is completely mixed," said Mattes.
Most agree that controlled experiments in humans, though they haven't been done, would give the most definitive answers.
What does science know?
Not surprisingly, overall diet and health are important.
Lawrence de Koning, nutrition researcher at the Harvard School of Public Health, studied diet soda's relationship with type 2 diabetes.
When he accounted for underlying risk factors, such as a participant having high cholesterol, "the apparent relationship between diet soda and type 2 diabetes disappeared." (It remained for regular soda.)
Popkin, author of another study of diet soda and type 2 diabetes, found that switching to diet soda could have a negligible or slightly positive effect for a healthy person, but is not the solution for an unhealthy one.
"Right off the bat, it depends where you start from," agreed Allison.
Future actions matter.
"To me, a good analogy is what causes traffic accidents: cars or drivers?" said Mattes. "There's nothing inherently wrong with high-intensity sweeteners; it's how people choose to use them."
He said a good example would be if you craved sweets, and then drank diet soda "to suppress the desire to eat further."
But that might not automatically happen. People might "indulge themselves in other calories," said Mattes.
Should I drink diet instead of regular?
"There's more and more rock-solid evidence that sugary drinks are the leading contributor to the obesity epidemic," said Goldstein of the Center for Public Health Advocacy.
The average American drinks about 42 pounds of sugar every year, he said.
"That's how much my 5-year-old weighs."
"In some ways the question of diet soda is a compelling one, but it's also a bit of a distraction," he said.
So, research is ambiguous, but diet soda seems the wiser choice, especially if you monitor overall diet.
While diet soda may not necessarily lead to weight loss, "diet soda may allow some people to enjoy their meals more, their food or their lives more, without having a negative effect on health," said Allison, who acknowledged that he receives funding from beverage companies.
"What's out there (about diet soda) is more of a fear than a reality," said Popkin.
Small, of Yale, was cautious, saying there's "certainly enough evidence in it, at least for me, not to feed my son artificial sweeteners."
This is too complicated. What should I drink instead?
"This sweetness addiction that we've developed is not something that we should cater to," said Richmond City Council member Jeff Ritterman, the main proponent of a beverage tax in his city.
His suggestion: tap water.
"It's an intervention where you can see you'll get thinner, you'll be healthier, you'll save money, you'll help save the planet – and the infrastructure already exists."
DIET BEVERAGES HAVE BEEN EASING INTO OUR ROUTINES SINCE 1952
In 1952, Kirsch was the first to sell a diet drink, a ginger ale called "No-Cal Beverage."
In 1963, America's oldest soft drink company introduced Dietetic Dr Pepper. Three years later, its name was changed to Diet Dr Pepper. It was reformulated in 1991.
TaB, Coca-Cola's first sugar-free beverage, was introduced in 1963.
Diet Pepsi was formulated in 1964 and reformulated with NutraSweet in 1984. Pepsi Light, with its lemon flavor, was added to the company's products in 1975. The caffeine-free varieties were introduced in 1982.
The Seven-Up Co. took its sugar-free 7 UP to market in 1970 and renamed it Diet 7 UP in 1979.
Coca-Cola introduced Diet Coke in 1982. A year later, the company rolled out the caffeine-free version of the drink. In 2005, Coca-Cola Zero was introduced.
Sources: Coca-Cola, Pepsi, Seven-Up and Dr Pepper company histories