My sister had a 2-4 liter a day coke habit and her pancreas blew up.
She ended up in ICU for a week, with the highest recorded blood sugar the hospital had ever had on record. She's now on insulin for life, even though she's down to 120 lbs (around the BMI/weightish range that many diabetics can get off insulin and control it solely with diet/exercise.)
I still drink about 12 oz at a time, every other day or so, maximum. And it has to be real Coca Cola. Pepsi tastes like dirt to me. (The flavor from the coca leaves is also said to still containe trace amounts of cocaine, and I can tell the difference.)