To a chemist, caffeine is a purine--one of a family of hundreds of organic compounds built around a basic structure of two interconnected "rings" of alternating carbon and nitrogen atoms.
Until very recently, though, the decision about how much coffee to drink was based on common sense and guesswork, not reasoned argument.
Even pharmacologists and biochemists had little to support their suspicions that any stimulant as potent as caffeine was likely to have drastic negative effects.
Beginning around the mid-80s, enough had been learned about coffee's wide-ranging yet subtle impact on the nervous system to make even the most devoted cappuccino drinker think again about this popular and public addiction.
If the scientists are right, that morning (and/or mid-morning, afternoon, after-dinner) cuppa is doing you more harm than good in the very area in which it's supposed to be helping you out.
Sixty years of intermittent worry and warnings about the deleterious effects of coffee have had some effect on consumption.
Over the last 30 years, Americans have drunk less and less, and over the last ten years, decaffeinated coffee has taken an ever greater share of the market.
Doctors treating hypertension routinely recommend that their patients cut down on or cut out coffee.
And faced with complaints of sleeplessness or general irritability, doctors immediately ask what role coffee plays in the patient's life-style.
They seem to correspond to uric acid in animals: metabolic end products that play no further active role in the plant's life cycle.
But many xanthines become chemically active if ingested by an animal.
Some are potent poisons; most are diuretics, encouraging excretion; in higher animals, many also cause temporary relaxation of the circulatory system, lowering blood pressure by enlarging vessel capacity.