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The most-widely-used drug in the world (over 125,000 tonnes is consumed globally each year), and one that quickly produces physical dependence. Heavy coffee and cola drinkers will vouch for that, developing severe headaches and other symptoms when the drug is withdrawn; and having anxiety, gastritis, palpitations and insomnia is too much is consumed.
American's over fourteen, are estimated to consume caffeine at an average of 500mg per person per day, in tea, cocoa, cola and especially coffee; in Britain the level is nearer 225mg. Heavy coffee drinkers can reach levels of 7,500mg a day, and this can cause 'caffeinism' symptoms of which include anxiety, constipation, depression, duodenal ulcers, frequent urination, heart disease, high cholesterol, hypertension, insomnia and nervousness. While drinking during pregnancy, is thought to increase the risk of miscarriage.
People who are sensitive to caffeinism can exhibit some of these symptoms from a few as two cups of coffee a day.
Caffeine, like theobromine and theophylline, is a xanthine derivative. All stimulate the cortex (outer grey matter of the brain), especially to the psychic and sensory functions, allaying drowsiness and fatigue; it also has diuretic effects, causing diarrhoea in some people. It keeps some people awake at night, but has little effect on others.
It occurs naturally in coffee beans, cocoa beans (cacao), tea leaves and cola nuts. Quantities vary with types - for example, Brazilian coffee has much more caffeine than Colombian or Central American; while that from the Philippines has the most of all.
A South American drink, guarama, made from the seeds of a large woody climbing plat native to the Amazon valley, has three times as much caffeine, as most coffee.
Tea leaves, in their natural state, have a higher caffeine content than coffee beans, but as a prepared beverage tea has generally only half the caffeine content - allowing some people to think they can sleep after tea, but not coffee.
Even experts can't agree on precise caffeine quantities in a cup of either drink, as so much depends on its strength and size. For example - a 6 fl oz cup of coffee can vary from 72-150mg of caffeine; and the same-sized cup of tea from 36-48. Cocoa may vary between 10 for a "drinking chocolate" variety to as much as 50mg for strong cup of the real thing.
Coca-Cola has about 4mg per fl oz, giving the same-sized measure 24mg. See "Caffeine; Medicinally"