Your Brain on Frappuccino

The U.S. government allocates many billions of dollars a year to the "War on Drugs," but it spends hardly a penny on the most insidious, most omnipresent psychoactive drug of all. I refer, of course, to caffeine (C8H10N4O2), the little alkaloid that made Starbucks' Howard Schultz a billionaire.

Drug? Indubitably: Even before it became endemic in the human diet through use in candy bars and soft drinks, doctors prescribed caffeine as a decongestant and mild painkiller; users discovered its efficacy as an appetite suppressant on their own. But it would be just another minor entry in the pharmacopeia were it not for another aspect: its powerful impact as a stimulant.

That's where the psychoactive aspect of caffeine kicks in. Unlike most small organic molecules, caffeine slips through cell walls as if they weren't there. An hour after your cuppa, caffeine can be found in every cell in your body, including those of the nervous system; even the famed "blood-brain barrier" is impotent against its stealth attacks.

After more than a century of concentrated study, scientists are still not entirely sure what happens when caffeine hits the brain. The current best guess is that it plugs into receptors in cells that modulate "excitability," the propensity of neurons to fire, sending messages to other nerve circuits in the brain. Caffeine fits these receptors well enough to prevent their proper trigger (adenosine) from plugging in, but not well enough to mimic the downstream calming effect of adenosine. Result: The brain remains in a state of higher excitability, alertness, and clarity, not to mention irritability, than it would maintain without caffeine's intrusion.

So far, so good; everybody recognizes the energizing jolt a good cup of coffee delivers. (The size of the jolt depends on the mode of delivery: An espresso contains about a fifth of a gram of the stuff, drip coffee only half as much.) The trouble is that most of us users don't stop with one cup, and the spread of fancied-up ways of absorbing your jolt – lattes, Frappuccinos, and the like – has made it perilously easy to saturate the system with a drug that, its agreeable stimulation apart, is pretty hard on the nervous system.

Some people can't even handle that one espresso without experiencing feelings of anxiety. When the dosage rises above 600 milligrams (only about three shots' worth), a majority of imbibers experience side effects like nervousness and irritability; many also experience higher blood pressure without realizing it.

Even if your system is highly tolerant to caffeine, a gram a day can cause irregular heartbeat and ringing in the ears, not to mention insomnia, outbursts of temper, and heightened distractibility. Ten grams of it and you're dead. Granted, it's almost impossible to absorb 10 grams of caffeine by the usual methods, but it's still a little worrisome that the difference between a useful dose and a deadly one is a mere matter of 50 to one.

Another worrisome aspect of caffeine is that many of its users develop a tolerance to its effects – in others words, you start with a single short and eventually only a triple grande will do. This happens with most drugs that interfere with normal neurotransmitter pathways, which are linked in intricate loops of potentiation and feedback.

When we block adenosine from its target receptors, the nervous system tries to restore its balance by producing more adenosine to compete with the caffeine that's blocking it, so over time it takes more caffeine to overcome the additional adenosine's calming, soporific effect. No two people exhibit exactly the same pattern of tolerances, so there's no way to establish a "safe" dose except through trial and error, leaving plenty of room for the insomnia, heart flutters, and sour stomach that result from an "overdose."