By Lisa Drayer, CNN

(CNN) -- Are you one of those people who can't get your day started without a cup of coffee?

Perhaps you need an espresso at work to keep you alert in the late afternoon. Maybe you grab energy drinks when cramming for an exam or late-night work project. Or maybe you have a habit of drinking caffeinated soft drinks.

Regardless of the form, an estimated 90% of the US population regularly consume caffeine, a stimulant and ingredient that has been enjoyed for thousands of years. It's not hard to do so, as caffeine is ubiquitous in our food supply, found in beverages, chocolate and pain medications, to name a few.

"Caffeine is the most commonly consumed psychoactive drug," said Mary M. Sweeney, an instructor who researches caffeine's effects on individuals in the Department of Psychiatry and Behavioral Sciences at the Johns Hopkins University School of Medicine. "When we consume caffeine, it has positive effects on mood and alertness, and people like these beneficial effects."

Although we know it can be counted on for a pick-me-up, more and more research is revealing other upsides of caffeine, including improved memory, enhanced athletic performance, beneficial effects on liver health and possible protection against Parkinson's disease. For some, caffeine may have the unintended consequence of staving off hunger, and it may even make foods seem less sweet, though few professionals would endorse it as a diet aid.

But can there be too much of a good thing when it comes to caffeine? And if so, how much is too much? When might it be wise to cut back on caffeine consumption?

What are the recommendations on caffeine?

A recent systematic review involving nearly 400 studies looked at adverse health effects associated with caffeine consumption, including general toxicity, cardiovascular effects, effects on bone and calcium, behavioral effects, and reproductive and developmental effects. The research evaluated caffeine intake from any source and was supported with grants from the American Beverage Association and the National Coffee Association, though neither association participated in any aspect of the review.

The researchers concluded that healthy adults can safely consume up to 400 milligrams of caffeine daily, the amount in about four 8-ounce cups of coffee. (Depending on the source, an 8-ounce cup of brewed coffee can contain 75 to 165 milligrams of caffeine.)

The review also revealed that healthy pregnant women can consume up to 300 milligrams of caffeine daily, an amount that is "generally not associated with adverse reproductive and developmental effects," though the American College of Obstetricians and Gynecologists cautions pregnant women to limit caffeine to 200 milligrams daily.

Researchers also recommend that children and adolescents not exceed an intake of 2.5 milligrams of caffeine per kilogram of body weight per day, though they state that the available literature for these groups was "scant." For a child weighing 55 pounds, this translates to a daily limit of 62.5 milligrams of caffeine.

Caffeine tolerance is individual

Though caffeine recommendations are based on a specific amount of milligrams, the effects can vary from person to person, and different individuals may be able to tolerate different amounts.

"People vary in their tolerance to caffeine," Sweeney said. "Many people consume caffeine without negative consequences, but for some individuals, either regularly consuming too much caffeine or consuming too much at once can cause distress."

An individual's tolerance to caffeine may depend on body size and how they metabolize caffeine, which can be related to genetic differences. "Some break it down quickly and may be less likely to experience negative effects," Sweeney said.

Lifestyle factors also affect caffeine metabolism, including pregnancy and the use of oral contraceptives, both of which slow caffeine metabolism in the body, according to Sweeney.

When your daily fix becomes a 'problem'

Research shows that most Americans consume less than the daily 400-milligram limit, and stronger effects of too much caffeine are typically seen at much higher doses, according to Sweeney. Still, too much is any amount that leads to gastrointestinal problems, trouble sleeping, nervousness, anxiety, irregular heartbeat or excessive urination, she said. It's also a red flag if one's caffeine consumption is causing a meaningful impairment at work or at school.

"Caffeine is so ingrained in our day-to-day habits that we don't of think of it as a source of a potential problem," Sweeney said. "It's not to say everyone will have the same effects, but it's important to be aware that it has psychoactive effects, and it can interfere with things in ways that we don't expect."

As caffeine can aggravate and accelerate one's heart rate, it can be a problem for those with an existing heart condition, according to Dr. Vince Bufalino, a spokesman for the American Heart Association and senior vice president and senior medical director of Cardiology-AMG, Advocate Health Care, in Naperville, Illinois. For example, if you have atrial fibrillation (commonly known as irregular heartbeat) or hypertension, he recommends limiting caffeine intake to one to two cups daily, but if you are sensitive to caffeine, you should cut it out completely.

Other research points to the fact that caffeine may become indirectly harmful if its consumption provokes other unhealthy habits, such as when coffee drinking promotes doughnut eating or cigarette smoking, or when energy drink consumption promotes alcohol intake.

For some, the idea of dependency in and of itself, along with its associated symptoms of withdrawal, can be enough of a reason to cut back.

"Withdrawal symptoms can interfere with day-to-day functions and can include severe headaches, difficulty concentrating, irritability, depressed mood or even flu-like symptoms," Sweeney said. Symptoms of caffeine withdrawal have been observed after discontinuing doses as low as 100 milligrams per day, though in general, the higher your daily dose of caffeine, the greater your severity of withdrawal symptoms, she said.

So whether you think you're a little too dependent on caffeine or you have a medical or other personal reason to cut back (perhaps you're planning on becoming pregnant, and that triple shot venti latte will be on the "do not drink" list), here are some tips to help you get started.

Tips for cutting back on caffeine

1. Keep a caffeine diary. It can inform you of how much caffeine you are consuming, and it may be more or less than what you may think, Sweeney said. This strategy was a helpful intervention for people seeking treatment to reduce their caffeine use in a study conducted by Sweeney and other researchers at Johns Hopkins University. (In the study, which also involved brief counseling, people reduced their caffeine intake from an average of 600 milligrams per day to 50 milligrams during a six-week period.)

2. Know all of the sources of caffeine in your diet. Remember, caffeine is found in coffee, tea, soft drinks, energy drinks and shots, as well as cocoa and chocolate. It's also present in fortified snack foods, some energy bars (like Clif Bar's Cool Mint Chocolate and Peanut Toffee Buzz) and even some pain medications, including some forms of Excedrin and Midol. (For a more extensive list of caffeine content from various sources, check the chart from the Center for Science in the Public Interest.)

3. If you drink coffee, gradually cut back on the number of cups per day. "The key recommendation we have suggested to people looking to cut back is to gradually reduce caffeine consumption over a period of days or weeks," Sweeney said. "If you're drinking four cups of coffee per day, you may reduce it by one cup per week. You might also substitute one cup with decaf, or blend in some decaf with each cup," she said.

4. Try coffee alternatives, such as green or black tea. Tea can still give you a boost but has less caffeine than coffee. An 8-ounce cup of black tea contains about 47 milligrams of caffeine, and green tea has about 25 milligrams per cup, compared with 75 to 165 milligrams in an 8-ounce cup of brewed coffee.

5. Anticipate when caffeine cravings may occur. As part of the counseling component in the Johns Hopkins study, and as part of the unit's ongoing work, individuals identify situations or moods in which they are most likely to crave caffeine. The unit advises avoiding situations that trigger cravings, especially during the first few weeks of modifying caffeine use, and having a plan for when cravings occur, like taking a five-minute relaxation break involving deep breathing exercises.

Remember to always discuss any major lifestyle or dietary changes with your health care provider first, as changes may affect your mood or medical conditions.