Caffeine is numero uno, the most widely used psychoactive drug on Earth. In the United States, more than 80 percent of adults consume the substance in tea, coffee or caffeinated sodas. Average intake is 280 milligrams per day, an amount equal to three cups of coffee.

People who have a morning cup of java to get their motor running are mostly just staving off the effects of caffeine withdrawal. In other words, heavy coffee drinkers seem to be no more alert after their first or second cup than people who aren't regular drinkers.

If you suddenly stop taking a steady dose of caffeine, you can experience headaches, fatigue and difficulty concentrating.

For most people it's a pretty benign substance. However, heavy caffeine use can raise blood pressure. For women who overuse, there is some research to suggest that caffeine can make conception more difficult. In pregnant women, there is concern about lower-birth-weight babies and an increased rate of spontaneous abortion, although these effects haven't been definitively proven. Caffeine can also aggravate insomnia, increase anxiety and the frequency of panic attacks, cause frequent urination and make your hands jittery - a distinct disadvantage for all you keyboard jockeys.

If you're not pregnant or experiencing insomnia or excessive anxiety, it's OK to drink a couple of cups of coffee a day. The real problem comes when you suddenly stop consuming caffeine. Caffeine withdrawal can start as soon as 12 hours after your last dose, so if you drink regularly at work, you may find you need a cup or two on Saturday and Sunday to stay awake and keep headaches at bay. If you're going to quit, taper off slowly by mixing your beverage with decaffeinated brews.

It does not cause birth defects, but that doesn't mean caffeine is harmless when it comes to making babies. According to some studies, women who drink lots of coffee may have a harder time getting pregnant, and may be more prone to spontaneous abortions or giving birth to smaller infants. This is why doctors recommend avoiding coffee and tea or cutting back on consumption to avoid complications.

Many parents blame caffeine and sugar, either together or separately, for their kids' rambunctious behavior. Not that we're saying you shouldn't monitor your kids' consumption of caffeine, especially in the evening, when it could cause sleeplessness. But the truth is, caffeine in small to moderate doses does not have any harmful effect on children's behavior.

Ounce for ounce, most teas have less caffeine than coffee, but heavy tea drinkers can develop withdrawal symptoms if they quit.