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"Caffeine is rapidly absorbed through the stomach and small intestine into the bloodstream, where it takes about 15-45 minutes to reach peak levels. The level of caffeine in the blood reaching the brain determines the potency of its effects on the body. Usually the central nervous system is maximally stimulated within 30-60 minutes. Caffeine is metabolized by the liver and eventually its metabolites are excreted in urine, although caffeine can also be secreted in saliva, semen, and breast milk.
Caffeine will continue to have an effect on the body as long as it remains in the blood. The time required for the body to eliminate one-half of the total amount of caffeine consumed (or caffeine's half-life) varies from several hours to several days, but for the average non-smoking adult it is about 3-4 hours.
Several factors can lengthen caffeine's half-life, such as some medications, liver diseases, pregnancy, and the level of enzymes in the liver needed for caffeine metabolism. The half-life of caffeine in a pregnant women is 18-20 hours; the half-life in women taking oral contraceptives is up to 13 hours. Other factors, such as smoking can shorten caffeine's half-life. Half-life in a newborn baby may be as long as 30 hours."
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