Q. I like caffeine, but is it bad for me? What are its effects?

A. Caffeine is an addictive substance and, according to National Geographic Magazine, the world's most popular psychoactive drug.

It is a stimulant that raises your heart rate, elevates blood pressure, increases stomach acids, suppresses appetite and increases dopamine — which can activate your brain's pleasure center.

In certain cases, caffeine may prevent a migraine from becoming full-blown by constricting blood vessels in the brain. However, large amounts of caffeine can cause anxiety, nervousness, heart palpitations, stomach pain and make heartburn worse.

By boosting adrenaline and blocking the effects of the sleep-inducing chemical adenosine, small amounts of caffeine can keep you feeling alert and help you stay awake if you need to. Although caffeine may give you the sensation of having extra energy, if consumed on a daily basis it can add to your body's overall stress load and eventually have negative effects on your adrenal glands.

In pregnant women, caffeine can cross the placenta and adversely affect the unborn child. According to the Organization of Teratology Information Services, "children born to mothers who consumed greater than 500 milligrams of caffeine a day were more likely to have faster heart rates, tremors, increased breathing rate and spend more time awake in the days following birth."

It is recommended that pregnant women avoid high intakes of caffeine (greater than 300 milligrams, which is equivalent to three cups of coffee a day).

Nursing mothers should be aware that caffeine enters their breast milk. The American Academy of Pediatrics warns that "caffeine tends to build up in babies' systems because their bodies cannot get rid of it very easily. Too much caffeine can cause problems such as poor sleeping, nervousness, irritability and poor feeding."

It isn't known what happens when young people, whose bodies are still growing, ingest large amounts of caffeine for prolonged periods of time. According to the April 2007 U.S. News and World Report, "Since scientists have never studied how caffeine affects growing bodies and brains, children who go through the day guzzling soda after iced tea after energy drink are serving as tiny guinea pigs."

At present, there is no requirement for labeling the amount of caffeine in any given product, but the American Medical Association and the Center for Science in the Public Interest are lobbying the FDA for the labeling of caffeine content on products.

The current recommended maximum intake of caffeine for adults is 300 milligrams a day.

Laurie Steelsmith is a naturopathic physician and licensed acupuncturist in Honolulu, as well as author of "Natural Choices for Women's Health" (Random House). Read past columns at www.drsteelsmith.com. This column is for information only. Consult your health provider for medical advice.