Caffeine is a drug in the sense that it has pharmacological effects on the body and it has properties similar to other substances that are generally referred to as drugs, things like nicotine, alcohol and even hard drugs. Tea and chocolate contain a significant amount, as do cola drinks, which can pack a big caffeine punch. However, the amount of caffeine in these foods varies considerably, and how they're prepared alters the level again. For example, a cup of brewed coffee comes in at about 85 mg, while instant coffee contains about 65mg. As drugs go, caffeine is remarkably potent stuff.
Coffee is absorbed by the body very quickly, reaching peak levels within an hour.
It makes everything speed up. It increases urine production, dilates blood vessels, opens airways to the lungs, strengthens muscles and stimulates the cortex of the brain. It does this by competing with a substance called adenosine. Normally, adenosine acts like an amber light, and stops impulses from speeding along the nerves. But if caffeine is around, it hijacks the system and lets the impulses zip through.