Caffeine
The "brown bottle" Buckfast has a caffeine content about equal to brewed or percolated coffee, while the "green bottle" Buckfast has a caffeine content about equal to black tea.
However, according to Alex Riley's Britain's Really Disgusting Drinks, the "green bottle" Buckfast contains the caffeine normally contained within six cups of coffee. The series also mentions that drop for drop, Buckfast has more caffeine than Red Bull.
Read more about this topic: Buckfast Tonic Wine, Ingredients, Effects