More caffeine than regular Coke
Diet Coke carries more caffeine than the full-sugar original: 46 mg per 12 fl oz can against 34 mg in Coca-Cola, by the Coca-Cola Company’s own figures. That is about a third more per can. It is also more than Diet Pepsi (35 mg), Pepsi (38 mg) and Pepsi Zero Sugar (38 mg), though less than Mountain Dew and Diet Mountain Dew (54 mg each).
USDA’s generic figure for diet cola, 12 mg per 100 g, works out to about 43 mg a can, so Diet Coke sits a little above its category’s average.
When it is several cans a day
If Diet Coke is a several-cans-a-day drink for you, the extra 12 mg per can adds up. Four cans a day come to 184 mg; eight come to 368 mg, close to the 400 mg daily figure the FDA cites for healthy adults. Nine cans, 414 mg, go over it. On a day with a morning cup of drip coffee (95 mg), seven cans are enough to pass the line.
Cutting back from a multi-can habit can bring on withdrawal. The FDA notes that caffeine withdrawal isn’t considered dangerous but can be unpleasant, and that cutting back gradually is the better way to do it. Swapping some cans for Caffeine-Free Diet Coke, which Coca-Cola sells alongside the original, keeps the habit while lowering the dose.
Against coffee
Even with its higher figure, a can of Diet Coke is about half an 8 oz cup of drip coffee at USDA’s reference strength. Coca-Cola’s own FAQ puts a 12 oz cup of coffee at 140 mg or more, about three times a can of Diet Coke. In pregnancy, four cans, 184 mg, fit under the 200 mg daily ceiling ACOG uses; a fifth does not.