The effects of caffeine will vary from person to person depending on characteristics of the:
Individual (user) - Mood, physical size, health, gender, previous experience with caffeine, expectations of the drug, personality, whether the person has had food and whether other drugs have been taken.
Drug - The amount used, the strength of the caffeine, what it is mixed with and the way it is taken.
Setting (environment) - the occasion on which the drug is taken (e.g. while studying or using other drugs).
Caffeine begins to reach all tissues in the body within five minutes of being consumed. Peak blood levels are reached within 30 minutes and, on average, half the caffeine in a given dose is metabolised within several hours. Caffeine does not accumulate in the body as almost all is metabolised.