Diet Coke is sweetened with aspartame. A standard 12-ounce can contains about 200 milligrams of it, with zero sugar and zero calories. Unlike Coke Zero Sugar, which uses a blend of aspartame and a second sweetener called acesulfame potassium, Diet Coke relies on aspartame alone.
How Aspartame Works as a Sweetener
Aspartame is roughly 200 times sweeter than table sugar, which is why such a small amount can make a full can of soda taste sweet. It’s made from two amino acids (the building blocks of protein) that occur naturally in many foods. Your body breaks it down the same way it processes those amino acids from other sources, which is why it contributes essentially no calories.
One thing aspartame doesn’t do well is hold up to heat. It breaks down at high temperatures, which is why it’s common in cold beverages but rarely used in baked goods. It also gradually loses sweetness over time, which is why Diet Coke cans carry a “best by” date that matters more for taste than safety.
The Phenylalanine Warning on the Can
If you’ve ever looked closely at a Diet Coke can, you’ve seen the line: “Phenylketonurics: Contains Phenylalanine.” This warning exists for people with a rare genetic condition called phenylketonuria (PKU), which affects roughly 1 in 10,000 to 15,000 people. Their bodies can’t properly process phenylalanine, one of the two amino acids that make up aspartame, so it builds up to harmful levels. For everyone else, the amount of phenylalanine in a can of Diet Coke is well within what the body handles routinely from everyday protein-rich foods.
Is Aspartame Safe?
Aspartame has been one of the most studied food additives in history, and it remains approved by regulatory agencies worldwide. In 2023, the WHO’s cancer research arm (IARC) classified aspartame as “possibly carcinogenic,” placing it in Group 2B. That sounds alarming, but Group 2B is reserved for substances with limited and inconclusive evidence. Aloe vera extract and pickled vegetables sit in the same category.
At the same time, a separate WHO committee reviewed the same body of evidence and found no sufficient reason to change the long-standing acceptable daily intake of 40 milligrams per kilogram of body weight. For a person weighing 150 pounds (68 kg), that works out to about 2,720 milligrams per day, or roughly 13 to 14 cans of Diet Coke. The practical takeaway: at normal consumption levels, the evidence does not point to meaningful health risk.
Aspartame also does not raise blood sugar. It passes through the body without triggering an insulin response, which is why diet sodas are generally considered compatible with blood sugar management for people with diabetes.
What About Diet Coke With Splenda?
Coca-Cola introduced a version of Diet Coke sweetened with Splenda (the brand name for sucralose) in 2009. That product was discontinued in 2024, so the only Diet Coke currently on shelves uses aspartame. If you’re specifically looking for a Coca-Cola product that avoids aspartame, Coke Zero Sugar uses a blend of aspartame and acesulfame potassium, but it doesn’t eliminate aspartame entirely.
How Diet Coke Compares to Coke Zero Sugar
The two drinks use different sweetening strategies. Diet Coke uses aspartame as its sole sweetener. Coke Zero Sugar combines aspartame with acesulfame potassium (sometimes listed as Ace-K on labels), which produces a flavor profile closer to regular Coca-Cola. Acesulfame potassium is about 200 times sweeter than sugar, similar to aspartame, but the two have slightly different taste characteristics. Blending them lets Coke Zero Sugar hit a different flavor target than Diet Coke, which has always had its own distinct taste rather than trying to mimic regular Coke.