Anything this good must be bad. That's the prevailing attitude when it comes to caffeine, isn't it? You crave it. You guzzle it. It makes you feel good - better able to handle an overbearing boss or an unruly pack of toddlers. But then... you feel guilty about it, suspecting that sooner or later, it's going to do you in.

In reality, it's not the guilty pleasure everyone makes it out to be - in fact, a little caffeine can do you good. So feel free to grab a tall breakfast blend while we separate fact from fiction.

Long-term caffeine consumption is just plain bad for you

FALSE. This myth exists in part because smokers, who metabolise caffeine twice as fast as non-smokers, tend to drink more coffee. And smokers have more health problems. But nicotine, not caffeine, is the culprit. As long as you don't have high blood pressure, heart arrhythmia or anxiety, drinking caffeinated beverages for years is fine, says Dr Harris Lieberman, a research psychologist for the US Army Research Institute of Environmental Medicine.

Caffeine alters your body chemistry

TRUE. Caffeine is a natural stimulant, and 250mg of it - about one strong cup of filtered coffee - will triple the amount of adrenaline in your bloodstream, upping your respiratory rate. In the brain, caffeine intercepts adenosine, a chemical that slows down your nerves and signals the need to sleep. It also increases dopamine levels, stimulating pleasure centres. So caffeine has the ability to make you feel both alert and relaxed, says Dr Ernesto Illy, biochemist and founder of coffee producer Illy Caffe.

It makes you smarter

TRUE. A cup of coffee before you write your performance review isn't a bad idea. "It improves cognitive functions by blocking that brain-slowing adenosine," says food chemist Dr Joe Vinson. When given caffeine equal to two cups of coffee, severely sleep-deprived US Navy Seal trainees improved their alertness, vigilance, learning and memory by as much as 60 per cent, says Dr Lieberman, who administered the test.

It improves your game

TRUE. In terms of athletic performance, caffeine "might be the difference between first place and last," Dr Vinson says. Without caffeine, not only are you a little slower mentally, you're lagging physically. "Caffeine stimulates you to exercise 10 to 15 per cent longer" because it keeps you from getting as tired, explains Dr Terry Graham, a nutritionist who specialises in caffeine at Canada's University of Guelph. It's also a mild analgesic, so you can work out longer before you start to feel sore. However, a study by the University Hospital Zurich, Switzerland, showed that 200mg of caffeine may decrease blood flow to the heart during exercise. More research is needed, but study author Dr Philipp Kaufmann says healthy people shouldn't be at risk. The main lesson? People with heart disease or those at risk should avoid caffeine before a run or at least check with their doc first.

It causes high blood pressure

FALSE. The Journal of the American Medical Association reported that caffeine doesn't put you at risk of hypertension, although something else (as yet unknown to scientists) in sugared and diet colas does. In people with normal blood pressure, any change in heart rate is "too small to measure," says Dr Graham, and disappears within minutes of your first cup.

It leads to bone loss

FALSE. There's evidence that calcium is lost through urine, and by increasing the amount of urine you produce by drinking coffee, you decrease your body's calcium stores. But the effect is negligible. "A coffee drinker may lose a few milligrams of calcium, but one drop of milk makes up for the loss," says food toxicologist Dr Jim Coughlin. But cola could have an effect. "Research suggests the phosphoric acid in soft drinks could counteract the effects of calcium," says WH nutrition expert Sharon Natoli.

You can overdose on it

TRUE. But most of the documented cases are of people who have intentionally overdosed on caffeine pills (the lethal dose is about 50). To OD on caffeinated beverages, you would have to down dozens of cups of coffee, Coke and tea at the same time. The excess water would kill you before the caffeine, Dr Illy says.

You can drink all you want, with no consequences

FALSE. A Greek study suggests moderate to heavy coffee intake is associated with a higher level of inflammation. An immune system response, inflammation can be harmful because too much of it can produce chemicals in the body that have been linked to heart disease and diabetes. Another study, from the US's Harvard School of Public Health and Canada's University of Toronto, found that certain people, referred to as "slow metabolisers", are genetically programmed to break down caffeine at a slower rate, and caffeine placed these people at increased risk for heart attacks, while "rapid metabolisers" were protected. Scientists believe excess caffeine may lead to heart disease but that other benefits from coffee, such as antioxidants, outweigh the risk, as long as the caffeine doesn't linger in your system. Researchers don't discourage caffeine consumption, but warn against it in excess. A safe limit, says Natoli, is two regular cups of espresso or five to six cups of instant a day.

It's addictive

TRUE. You develop a tolerance to caffeine over time, and when you no longer take any in, your body, expecting that daily dose, reacts with headaches and irritability. The more caffeine you consume, the more severe the withdrawal symptoms.

If you have tea after dinner, you'll never fall asleep

FALSE. Tea has so little caffeine (25mg per cup of weak brew) that for most, it soothes rather than agitates. But beware of highly caffeinated drinks, especially if you're pregnant or on the pill: it takes four to six hours for an average adult to metabolise caffeine, and almost twice that long for a woman taking oral contraceptives. (For a pregnant woman, this increases to 18 hours.) If you have trouble sleeping, stop drinking caffeinated beverages at least six hours before bedtime.