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In page Norethisterone:

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Norethisterone is used in birth control pills, opposed to progesterone itself, because it is not metabolized as rapidly as progesterone when consumed orally. When progesterone is consumed orally it is rapidly metabolized in the gastrointestinal tract and the liver, and broken down into many different metabolites. Whereas, norethisterone is not as rapidly metabolized allowing norethisterone to be present in higher quantities allowing it to more effectively compete for progesterone receptor binding sites. [2]