What happens if you drink a soda expired over six months?
in diet sodas will naturally break down over time, especially if stored in higher temperatures, and in both diet and regular sodas the carbon dioxide will produce further carbonic acid (incidentally making itself flat), but aside from a change in flavour, the beverage is likely to be completely safe to drink -- and will probably still taste good.
It would take a very, very long time for the flavour to be affected enough to be completely unacceptable -- and even then it would still be safe.
Even if all of the remaining carbon dioxide has broken down into sour acid (which would take years in an unopened can), it's still far more dilute and less acidic than your stomach acid, assuming you can handle the terrible flavour.
Soda is usually consumed well before this becomes a problem, of course.
One caution should be made: any sodas that contain real fruit juice, like orange soda, grape soda, etc., should be viewed as potentially unsafe after their expiration
. They can mould or ferment rapidly once the container is opened, in the case of aluminum cans, and glass or plastic bottles which have less perfect seals should be viewed as unsafe even before opening.
PS: The date on the packaging is the "Optimum Taste Date." This is the date by which companies recommend you to consume the beverage in order to enjoy the optimum taste.