If you're like 85 percent of the U.S. population, you drink caffeine. It's in your coffee. Your tea. Your soft drinks. Your energy drinks. It's so pervasive in our diets that we hardly think of caffeine as what it really is: a mood-altering drug.

Just like coffee shops in your neighborhood, psychologists' research in the area is proliferating. Researchers are exploring how it affects us and the way we work--and they're finding that caffeine walks a fine line between benefit and bane. Here's a look at a few recent studies.

All in our head

Remember those 1990s commercials in which a wife would secretly switch her husband's regular coffee with decaf and watch as he never noticed the difference? The ads may not have been far off, according to caffeine researcher Laura Juliano, PhD. She found that expecting to receive caffeine mitigated some of the effects of caffeine withdrawal, at least in the short term.

In the experiment, Juliano, a psychologist at American University in Washington, D.C., working with graduate student Peter Kardel, recruited about 90 people to come in to her lab one afternoon in early 2007 and drink a cup of coffee. She then asked them to abstain from eating or drinking all further items containing caffeine until the end of the experiment unless she gave it to them.

Participants returned the next morning and were assigned to one of four groups. One group was told they would receive a cup of caffeinated coffee, and did, in fact, receive regular ol' joe. A second group was also told they would receive regular coffee but instead were given decaf. A third group was told they'd get decaf and were given regular, and the final group was told they'd receive decaf and actually got it.

After 45 minutes, participants completed a questionnaire asking them to rate their caffeine withdrawal symptoms, including headache, fatigue, decreased alertness, drowsiness, irritability and difficulty concentrating, on a scale from 1 to 5. Finally, they returned to the lab a third time that evening for a final assessment.

When Juliano analyzed the data, she found that the groups told caffeine/given caffeine and told decaf/given decaf had the expected withdrawal effects. Namely, the former felt few caffeine withdrawal effects while the latter expressed many withdrawal symptoms. But the group given caffeine that they thought was decaf actually had higher initial withdrawal scores than those who were given decaf but told they got caffeine. Just believing they were being denied caffeine made people feel like they were in withdrawal, even though pharmacologically, they weren't.

By the final evening visit, caffeine showed its true colors. Those who were given caffeine but thought they received decaf reported lower levels of withdrawal symptoms--levels nearly equal to those experienced by the group that was promised and received caffeine. But the symptoms were much more intense for those who drank decaf that they thought was caffeine. Clearly, expectancy had an early effect on withdrawal, but it was only temporary relief.

"In the long run, pharmacology is what matters," says Juliano, who presented the research at the 41st Annual Convention for the Association for Behavioral and Cognitive Therapies in November. "In the short term, expectancy plays a little bit of a role, but your body eventually catches up with your mind. If you haven't had caffeine for 36 hours and you have a headache, you have a headache."

Performance issues

Of course, in addition to staving off withdrawal, there's another major reason people drink caffeine: for the perk. Whether it's tea, soda, coffee or energy drinks, people turn to caffeine to get them through an exhausting day. But can any of caffeine's energetic reputation be credited to an expectancy effect, too?

That's what Sonia Sharma, PhD, wanted to know when she ran her experiment as a graduate student in 2005 at Syracuse University. Using a design setup similar to Juliano's, Sharma tested whether anticipating caffeine could provide the same cognitive boost as actually having it.

Sharma separated people into the same categories as in Juliano's experiment and asked them to participate in a sustained attention task. After receiving either the caffeinated drink or a placebo, they focused on a computer screen and watched single-digit numbers flash by. Any time three even or three odd numbers flashed consecutively, participants pressed a button.

Sharma found no expectancy effect at all. The caffeinated participants performed much better at the task than the non-caffeinated, regardless of whether they believed they received caffeine. But interestingly, the group that was told they received caffeine but instead was given a placebo reported that they could feel the effects of caffeine. They also claimed they were more attentive throughout the task, even though the results didn't bear that out.

"In a way, these results are quite consistent with alcohol research," Sharma says.

In similar experiments involving varying degrees of alcohol consumption, she reports, people who are told they've received a large amount of alcohol but instead are given very little report feeling very drunk, but there's minimal effect on their cognitive skills such as information processing. "You see the behavior and social effects, but not the cognitive ones," she says.

Maximizing attention

Though expectancy alone cannot provide caffeine's attentional spark, it might help sustain it over the long term. A team of psychologists working with the Fourth Military Medical University in Xi'an, China, found that in sleep-deprivation experiments, an initial dose of caffeine followed up by placebos helped sustain cognitive improvement over the course of 28 hours without sleep.

In the experiment, reported in the 2007 Journal of Psychophysiology (Vol. 21, No. 2), researchers found that the placebo effect extended the cognitive boost without incurring the rise in blood pressure and heart rate that are usually associated with caffeine consumption. The psychologists suggest that this caffeine-plus-placebo regimen could be used when work schedules demand long bouts of alertness without sleep to maximize attention but minimize negative health effects. While none of caffeine's effects are life-threatening, they're not entirely benign, either. Too much caffeine can exacerbate existing cardiac problems and anxiety disorders and lead to insomnia, stomach and digestion problems, as well as the bothersome effects of the withdrawal symptoms discussed earlier.

Everything in moderation

So what's the final verdict on caffeine--boon or bane? Roland Griffiths, PhD, a neuroscientist at the Johns Hopkins School of Medicine in Baltimore, has been studying caffeine for over two decades and says there's no easy answer to that question. Humans have been consuming caffeine in one way or another for more than 10,000 years, so it's difficult even to imagine our society without it. That makes it a particularly interesting drug to study, he says.

Discerning its effects on society as a whole is an especially intractable problem because it's difficult to even find someone who's never been exposed to it.

"Caffeine is so well-integrated that some people don't even think of it as a drug," Griffiths says. "But when people say, 'I like the taste,' they're misattributing that sensation because it's really the control of caffeine addiction. No one drank coffee for the first time and said, 'Oh, now this is what I've been missing.'"

Given that caffeine is ubiquitous and has some manner of control over most of us, it's still unclear whether there is a net benefit, Griffiths says. But he points out there are times when caffeine is a handy drug to have around. If you need a quick pick-me-up to stay focused on the job, it's a benefit. When you get addicted and can't function without it, well, it's crossed the fine line and become more a nuisance than a benefit, he says. Ultimately, Griffiths thinks that people need to adjust the way they think about caffeine so they at least make informed decisions when they consume it.

"When you get down to it, caffeine is a drug and it needs to be accorded the respect as a drug," he says. Consuming it is "ultimately an adult decision."