Kicking the caffeine habit abruptly can bring on headaches, tremors, lethargy and a case of the blahs bad enough to look like a possible depression, new studies confirm.

Up to 90 percent of adults in North America consume caffeine regularly, partly because they can feel so lousy when they stop, said researcher Suzette Evans of the National Institute on Drug Abuse's Addiction Research Center in Baltimore.Even people who just don't feel like themselves before they've had their first cup of coffee may be feeling early symptoms of caffeine withdrawal, she said.

Caffeine withdrawal is "a real phenomenon," she said. "That may be one thing that maintains caffeine intake, avoiding the withdrawal."

The research suggests that a person who wants to eliminate caffeine should do it gradually off over a few days, Evans said. Even one cup of coffee has enough caffeine to run the risk of withdrawal if it's suddenly eliminated, she said.

Evans spoke in a recent interview and during a presentation of research results last week at the annual meeting of the American Psychological Association.

As many as 90 percent of adults in North America regularly consume foods containing caffeine, with most caffeine coming from coffee, researchers said at the meeting.

He said scientists don't know what percentage of the population is vulnerable to caffeine withdrawal. But the new work shows doctors should consider it when patients complain of headaches or fatigue, Hughes said.

Caffeine withdrawal is no fun, as Evans' study of 62 people illustrated. Study participants averaged about 235 milligrams a day of caffeine before the study, a fairly typical amount.

A second study of 17 people found that when they abruptly stopped taking 100 milligrams of caffeine a day, they had less severe headaches than when they stopped taking 300 milligrams or 600 milligrams a day. But their fatigue and sluggishness were just as bad.