Looks like caffeine withdrawal is being recognized as a disorder. The study, funded in part by the National Institute on Drug Abuse, was published in September of 2004.
I have experienced firsthand the joys of caffeine withdrawal and the symptoms are most unpleasant. Caffeine is the most widely used drug in the world. In North America, the average intake is about 280 milligrams per day. That is the equivalent of about two cups of coffee or five bottles of caffeinated soft drink.
The report is here. And here is an excerpt from that report:
In experimental studies, 50 percent of people experienced headache and 13 percent had clinically significant distress or functional impairment — for example, severe headache and other symptoms incompatible with working. Typically, onset of symptoms occurred 12 to 24 hours after stopping caffeine, with peak intensity between one and two days, and for a duration of two to nine days.
In general, the incidence or severity of symptoms increased with increases in daily dose, but abstinence from doses as low as 100 milligrams per day, or about one small cup of coffee, also produced symptoms. The research also showed that avoidance of caffeine withdrawal symptoms motivates regular use of caffeine. For example, the satisfying feelings and perceived benefits that many coffee users experience from their morning coffee appear to be a simple reversal of the negative effects of caffeine withdrawal after overnight abstinence.
I was asked some questions about the amount of caffeine in soft drinks. The American Beverage Association has a list of most soft drinks and the amount of caffeine. You can find the list here.