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Varied or little effect

Heredity Caffeine metabolism is controlled by many genes and racial differences exist

Gender Exercise and stress have no reliable effect on the absorption or metabolism of caffeine

Pregnancy There are no placental barriers to caffeine so the foetus is continuously exposed

Slowers-down

Oral contraceptives, late pregnancy and liver disease cause caffeine to be eliminated more slowly

It has been thought that grapefruit juice, though not other citrus juices, slows down metabolism. The data for this has now proved controversial

Some drugs like cimetidine, disulfiram, even alcohol, may slow down caffeine metabolism

Speeders-up

Smoking induces liver enzymes which break caffeine down

Rifamprin