Caffeine is the stimulant found in tea and coffee, soft drinks ( particularly cola drinks ), and as an additive in many over-the-counter chemist preparations. A cup of coffee contains on average 115 mg caffeine ( filter method ), 80 mg ( percolated ) and 65 mg ( instant ). Tea contains approximately 60 mg ( tea bag method ) and up to 100 mg per cup ( loose tea ). Soft drinks vary from brand to brand, usually containing the caffeine equivalent of 2 - 3 cups of filter coffee (200 - 300 mg). Because of their low body weight, a can of soft drink given to a child can increase the concentration of caffeine to a level equivalent to 6 cups of coffee for an adult. Over-the-counter stimulant preparations ( Pro-plus, Reactivan etc) also vary from brand to brand, but as they contain caffeine in its pure form, they generally contain around 200 mg per tablet. Caffeine is also found in chocolate, with a typical bar containing approximately 25 mg.
It has been estimated that including all available sources, the average daily caffeine consumption in Britain is approximately 400 mg per person.
SHORT TERM EFFECTS
In small doses ( up to 250 mg ) caffeine gives the user a lift by helping to relieve fatigue and boredom with no interference with manual or intellectual performance. In larger doses ( 300 mg+) the heart and breathing rate are increased , blood pressure is raised and blood vessels in the brain are constricted. The user is likely to feel alert and possibly anxious or irritable.
The effect of caffeine is evident within 10 - 20 minutes following use and can last several hours, after which time, if the dose is not repeated, fatigue can set in.
LONG TERM EFFECTS
When a heavy user has built up to 500 - 600 mg+ of caffeine a day they may experience anxiety and restlessness with prolonged use. On 15 cups of coffee / tea a day ( approximately 1 gm caffeine ) sensory disturbances have been documented ( ringing in the ears, flashing lights etc ), insomnia, palpitations, nausea, vomiting, diarrhoea, and feelings of restlessness and agitation. Overdose is possible on 100+ cups a day, but is highly unlikely and very rare.
Prolonged use of around 7 cups a day can produce insomnia and headaches, anxiety and irritability, and can involve a withdrawal syndrome if use is ceased abruptly. However, all these symptoms eventually disappear when caffeine intake is reduced or stopped altogether. Tolerance does develop to caffeine, although there seems to be a limit past which tolerance is not affected, or many more people would be drinking 15+ cups of tea / coffee a day, which remains fairly unusual.
Withdrawal tends to follow use of 350 mg+ per day, when the user may experience headaches,. irritability, nervousness and fatigue. Dependence is likely to be psychological as well as physical, with users needing to punctuate the day with regular coffee / tea breaks.
It is unlikely that any permanent damage is done by prolonged use of caffeine, but people who suffer with ulcers, high blood pressure, palpitations or anxiety would be best advised to avoid its use.