Acquired tolerance to some behavioral effects of caffeine in humans is widely assumed to occur but is poorly documented and appears, at most, to be of low magnitude. Withdrawal from regular consumption of caffeine has been reported to result in a variety of symptoms, including: irritability, sleepiness, dysphoria, delerium, nausea, vomiting, rhinorrhea, nervousness, restlessness, anxiety, muscle tension, muscle pains and flushed face. Some of these same symptoms have been reported following excess intake of caffeine. The prevalence of symptoms reported on withdrawal in different studies also covers a wide range from 11% or less to 100%. It is suggested that the evidence leads to the conclusion that non pharmacological factors related to knowledge and expectation are the prime determinants of symptoms and their reported prevalence on withdrawal of caffeine after regular consumption.