From my discussions with Mormons, Pepsi is fine*. It's another one of those points where a high control religion demonstrates inconsistency. We outsiders see their prohibition of tea and coffee and logically project that it is a prohibition on caffeine. So, back in the day when the Mormon church supposedly owned PepsiCo**, I brought it up and they said "So what?" The prohibition by JS himself is not on Pepsi, but on tea and coffee alone. Apparently the Mormon church is not bound by the "ejusdem generis" principle. Mormons can happily knock back Pepsi by the gallon. We outsiders might wonder if there's any cognitive dissonance going on, but they're right: Pepsi isn't tea or coffee.
* If anyone knows better, let me know
** Probably an urban myth of the time