- Routine consumption of more than 2 units of caffeine per day or subjects who experience caffeine withdrawal headaches or have a history of caffeine withdrawal headaches. (One caffeine unit is contained in the following items: one 6oz/180mL cup of coffee, two 12 oz/355 mL cans of cola, one 12 oz/355 mL cup of tea, three 1oz/28g chocolate bars. Decaffeinated coffee, tea, or cola are not considered to contain caffeine).