Excess drinking of coffee and tea may lead to addiction to caffeine as a stimulant, resulting in tension, headache, insomnia, racing heart and in severe cases, malnutrition through loss of appetite. Caffeine is used medically in tablet form as a stimulant, against headache and in certain cases for the treatment of asthma, but its abuse induces tolerance and mental dependence. Caffeine abuse, which is often coupled with nicotine abuse, may go unrecognized because coffee drinking is commonplace and socially acceptable; it is fully recognized as a drug problem.
Caffeine belongs to a group of drugs called methyxanthines which is found in over 63 plant species worldwide. It is found in coffee, tea, chocolate products, soft drinks, cough and cold remedies, pain relievers, diuretics and weight-control products. The caffeine content of these products varies. A can of cola, for instance, has 35 to 55 milligrams (mg) of caffeine - much less than a cup of drip coffee which has 110 - 150 mg or a cup or glass of tea which has have 45 - 75 mg. An cup of cocoa may contain up to 56 mg of caffeine; a chocolate bar has about four mg.
The caffeine content of coffee depends on preparation: 23 grams (8 ounces) provides up to 280 milligrams of caffeine; drip coffee contains between 88 and 280 mg; percolated contains between 27 and 64 mg.
As a drug, caffeine is not addicting but mildly habit-forming owing to its stimulating effect. It stimulates the heart and brain, causing improved performance, decreased fatigue, and increased alertness. Caffeine's stimulative effect on the central nervous system, the 'lift' one experiences from a caffeine-containing beverage, is a primary reason for its consumer popularity. Of course, not all people react positively to caffeine. In some, the drug causes irritability, nervousness, tremors, restlessness, headaches, insomnia, heartburn, diarrhea, and heart palpitations. These are the symptoms of caffeinism or coffee nerves and they usually occur with the consumption of 600 mg of caffeine.
Caffeine blocks the brain's receptors for adenosine, a molecular messenger that damps nerve cell activity. Hence the stimulant effect. Even those who consume only moderate amounts of caffeine may suffer withdrawal symptoms – headaches, fatigue, depression, poor concentration – after 18 to 24 hours of abstinence.
According to a 1991 FDA report, the average American consumes 200 mg of caffeine (2-3 cups of coffee) per day.