One of these days, a mocha frappuccino at Starbucks may be accompanied by a surgeon general’s warning.

Scientists say that as many as 90% of adults in North America consume caffeine regularly, but now millions of American teenagers seem to have switched from viewing coffee as merely the vile, bitter-tasting brown liquid their parents habitually drink in the morning, to adopting the drink as their own. According to NPD Group, a New York-based market research firm, the number of coffee-drinkers age 13 to 17 has increased by 12% in the past year, on top of an increase of 15% the year before.

Nutritionists, meanwhile, are expressing concern that teenagers are unaware of coffee’s high caffeine content and its detrimental effects on still growing bodies.

In a new study released by Johns Hopkins University, apparently as little as one cup of coffee a day can produce caffeine dependence. The researchers identified common withdrawal symptoms as headache, fatigue or drowsiness, irritability, and depression.

“Caffeine is the world’s most commonly used stimulant, and it’s cheap and readily available so people can maintain their use of caffeine quite easily,” said Roland Griffiths, a prominent caffeine researcher. He is a professor of psychiatry and neuroscience at Johns Hopkins and collaborated on the study of caffeine withdrawal with a researcher from American University, Laura Juliano. Their study was published in the October 2004 issue of the journal Psychopharmacology.

Nutritionists agree that caffeine’s impact is dependent on body weight, and therefore the drug packs a more powerful punch for younger, smaller drinkers. Caffeine can increase alertness and improve performance, scientists say, but it can also cause anxiety, nervousness, and sleeplessness.

For adolescents, especially those of African-American descent, medical researchers said that caffeine intake may increase blood pressure and place them at higher risk for hypertension.

In addition, coffee is a diuretic, meaning it naturally causes dehydration. Researchers warn that large amounts of caffeine may cause calcium and potassium loss, leading to sore muscles and longer recovery periods after exercise.

Also, with a nationwide epidemic of child obesity, the caloric intake in popular Starbucks drinks is significant: The 20-ounce frappucino, sweet and cold, has 350 calories, 100 more than a 20-ounce Coke.

In New York City on any given day, many a Starbucks is flooded with chattering teens nursing 20-ounce frappuccinos – which carry about 215 milligrams of caffeine, more than twice the 100-milligram caffeine maximum for children recommended by nutritionists. In comparison, the caffeine content of a 20-ounce Coke is below 60 milligrams.

“All my friends go to Starbucks,” Julia Anrather, 13, of Manhattan said. She said they buy a frappuccino “for the sugar, caffeine, and taste.”

Derek Wikstrom, 17, exhibited the first symptom of caffeine dependency when he said that while he doesn’t feel as though he has a need for coffee, “My head hurts a lot when I don’t have it.”

In a city that offers few hangout spots for kids not old enough to frequent bars or clubs, coffeehouses like Starbucks provide the ultimate social haven.

Besides simply feeling cooler and more grown-up if they are holding a cup of coffee, some adolescents said they think coffee consumption will improve their academic performance. Actually, nutritionists say excessive caffeine can damage their capacity to pay attention.

Karen Slovin, 17, who said she started drinking coffee at age 10, reckoned the drink “helps me start my day properly.”

The teenager, who said she drinks at least three cups of coffee a day, said she was aware of some of the adverse effects of caffeine.

“When I drink a lot of coffee I can feel my blood pumping and my heart rate speeding up,” she said. “I know that’s not how I should feel. So I get sick.”