More caffeine than the regular version
USDA’s figure for a generic diet cola with caffeine is 43 mg in a 12 fl oz can, rising to 57 mg in a 16 fl oz can. That is more caffeine than regular Coca-Cola Classic at the same size (34 mg), and close to Diet Coke specifically (46 mg). Manufacturers add caffeine and sugar independently, so cutting sugar for a diet formula doesn’t reduce, and can even come with more, caffeine.
A generic figure, not one brand
This page covers USDA’s broad average across diet colas; specific brands vary; Diet Coke and other named products may differ slightly from this generic figure, so check the can for an exact number when it matters. Against regular cola generically (33 mg in a 12 oz can by USDA’s figure), the diet version again comes out a little ahead.
Why some diet sodas add more caffeine
One reason diet colas sometimes carry more caffeine than the regular version is that caffeine itself has a slightly bitter edge that can help balance the taste of artificial sweeteners, which read differently on the palate than sugar does. That’s a plausible explanation for the pattern, not a rule every diet soda follows, since formulations vary by brand and country.
Where it fits in a day
At 43 mg, a can is far under the 200 mg single dose EFSA assessed as safe for healthy adults. Nine cans, 387 mg, would come close to the 400 mg daily figure the FDA cites. In pregnancy, where ACOG uses a 200 mg daily ceiling, four cans still fit under the day’s allowance.