Common loanword for the stimulant found in coffee, tea, and energy drinks.
from German Kaffein
Coffee contains caffeine.
I try not to have caffeine before bed.
This tea contains a little caffeine.
If you take in caffeine late at night, you won't be able to sleep.
You should cut back on caffeine while pregnant.
In the afternoon, the caffeine wears off and I get sleepy.