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I like my coffee like I like my women, cold & bitter.

And for the impatient ones amongst us, caffeine is added to hundreds of over-the-counter drugs. We're not just talking No-Doz or CoffeeGo, but a vast array of cold, headache and diet concoctions.

Must of us don't "do" caffeine to stimulate our gastric juices, though it does that. Nor are we particularly interested in its proven capacity as a diuretic. So potent an enemy of muscle fatigue and enhancer of strength is caffeine that the Olympic Committee regularly checks athletes for excessive levels, and the American Institute of Medicine recommended that the pentagon add it to troops' rations.

But most of the time, that's not why we take it either. We consume caffeine for the brain. "The most widely consumed psychoactive drug in the world," is how the experts describe it. Brain medicine, pure and simple. It takes about 30 minutes for the caffeine from a decent cup of coffee to arrive full force in the seat of reason. Once there it does its work mainly by stealth.

According to accepted scientific wisdom, caffeine doesn't directly stimulate your cortex so much as prevent it from receiving a chemical signal called adenosine, a naturally occurring depressant.

The brain has special receptors that, when plugged with passing adenosine, slows the body down. But caffeine is so similar in molecular shape to adenosine that it slips into the receptor instead, blocking the adenosine. Caffeine is like oil between your tires and the pavement; the light is red but you're not stopping at the intersection.

Of course, for those of us with one eye always cocked on the coffee maker in the corner, this suppression of adenosine certainly feels like stimulation, (and pharmacologists do classify caffeine as a stimulant! ) The problem is, the adenosine is still there in the nervous system, floating around, building up, waiting. Yup, crash.

The half-life of caffeine-the time it takes your body to eliminate 50% of the amount ingested-is about five hours. But long before that you probably drank another pot of coffee.

There's a theory about caffeine tolerance-the verifiable increased desire for caffeine amongst us ten-plus cups a day crowd-that's gaining approval among neuroscientists. This postulates that in order to make up for the bush-wacked adenosine, the brain grows more receptors in a process called upregulation.

If this extra-receptors theory proves correct, researchers believe it will help account for some of the very real caffeine withdrawal symptoms. The common headache is a case in point. Caffeine contracts the brain's blood vessels while adenosine expands them. When the caffeine supply is cuts off and adenosine hits the extra supply of receptors, your brain veins swell up to quite a painful degree.

The solution is an aspirin and a strong cup of caffeine!

The brain's role in other documented symptoms of severe caffeine withdrawal-muscle aches, runny noses, nausea, fatigue, vomiting-is less clear.

"Just as with withdrawal from more dangerous drugs like cocaine, we're not sure of the exact location of the effect, whether it's actually generated in the brain or at a specific site in the body," offers J. Hopkins Medical University professor Roland Griffiths.

"But we know it's very real. Performance is impaired in withdrawal. Caffeine withdrawal is not in any way just a 'psychological phenomenon. '"

And the good news is... no study has yet found a convincing connection between caffeine and a host of serious diseases. So far, caffeine has beaten the rap on charges of causing heart disease, birth defects, various cancers, even PMS.

The evidence on caffeine and miscarriage is inconclusive, but it only makes sense that women shouldn't drink so much jolt during pregnancy, and women with a family history of osteoporosis should be aware of caffeine's role in reducing body calcium. But even though caffeine ranks as one of the world's most researched drugs, that's about all you have to worry about.

Best of all, those odd folks which study the sex lives of senior citizens discovered that elderly folk who consume coffee often are much more likely to be getting it on in the bedroom more regularly than their peers that don't drink it.

So, cut back a couple cups a day, maybe, but quit? Not on your life, grandpa.