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The biological half-life of caffeine – the time required for the body to eliminate one-half of the total amount of caffeine – varies widely among individuals according to such factors as age, liver function, pregnancy, some concurrent medications, and the level of enzymes in the liver needed for caffeine metabolism. It can also be significantly altered by drugs or hormonal states. In healthy adults, caffeine's half-life has been measured with a range of results. Some measures get 4.9 hours,[147] and others are at around 6 hours.[148] Heavy cigarette smokers show a decrease in half-life of 30–50%,[38] oral contraceptives can double it,[38] and pregnancy can raise it even more, to as much as 15 hours during the last trimester.[38] In newborn infants the half-life can be 80 hours or more; however it drops very rapidly with age, possibly to less than the adult value by the age of 6 months.[38] The antidepressant fluvoxamine (Luvox) reduces the clearance of caffeine by more than 90%, and prolongs its elimination half-life more than tenfold; from 4.9 hours to 56 hours.[147]
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