It is like a dug because there is a change in the brain cells that grow more adenosine receptors, which is the brain’s attempt to maintain equilibrium in the face of a constant onslaught of caffeine, with its adenosine receptors so regularly plugged. This explains why regular coffee drinkers build up a tolerance over time, it takes more caffeine to block a significant proportion of them and achieve the desired effect which is why it is so addicting. It is not as bad as a worse drug because the effects of caffeine are relatively short-term. Caffeine is a stimulant that affects the nervous system in various ways.