Just about everyone uses caffeine—humans have been drinking caffeine-containing beverages for millennia. A pair of researchers reviewed the results of studies on caffeine withdrawal.

What the researchers wanted to know: What happens to people who stop taking caffeine?

What they did: The researchers were reviewing past research on caffeine, not doing a new experiment, so they used article databases to search for studies that contained the word "caffeine" and either "withdrawal," "dependence," "deprivation," or "abstinence." They looked through their own file drawers, too, and also followed up on the references in articles they found. Sixty-seven experiments turned up. Most were double-blind experiments, in which people took either caffeine or a placebo, then were checked for withdrawal symptoms. For example, in a 1964 study, regular, moderate, and light coffee drinkers were told to have no caffeine after lunch, then given a cup of decaf at bedtime, with either caffeine or a placebo; the next morning they reported whether they had a headache.

What they found: About 50 percent of people in experiments had headaches after stopping caffeine, and about 25 percent of people in surveys said they got headaches if they went off caffeine. Withdrawal usually starts 12 to 24 hours after stopping caffeine, and usually lasts two to nine days. Withdrawal is worse for people who were on higher doses of caffeine before stopping, and some people have severe enough symptoms that they have to limit their daily activities. The evidence is good that caffeine withdrawal is a physical reaction to the drug being taken away—not a result of people's expectations about what will happen when they stop taking caffeine. The researchers found good evidence for some symptoms of caffeine that you'd expect, such as headache and fatigue, and more limited evidence for some other symptoms, such as nausea and muscle pain.

What the study means to you: The researchers say that caffeine withdrawal may be bad enough for some people to merit including it in the American Psychiatric Association's official list of disorders. More research, they say, should look at why caffeine affects different people differently—for example, whether gender, genetics, and personality play a role.

Caveats: The researchers were reviewing the results of 67 past studies, each of which had its own quirks and flaws.

Find out more: Caffeine is thought to be safe in moderation, says the National Library of Medicine's Medline encyclopedia entry.

Read the article: Juliano, L.M. and R.R. Griffiths. "A Critical Review of Caffeine Withdrawal: Empirical Validation of Symptoms and Signs, Incidence, Severity, and Associated Features." Psychopharmacology. October 2004, Vol. 175, No. 1, pp. 1–29.

Abstract online: www.springerlink.com