The drug is rapidly and completely absorbed after oral administration, peak plasma concentrations occurring in the majority of subjects between 1 and 3 hr. Owing to a first-pass presystemic metabolism in the intestinal wall and liver, blood levels after oral administration are only 60% of those after intravenous administration. Norethisterone is rapidly and widely distributed throughout the body tissues, with the highest levels accumulating and being metabolized in liver, kidney, intestine and bile. There is no enterohepatic circulation. Less than 5% exists in plasma in the unbound state, with about 60% bound to albumin and 35% to sex hormone binding globulin. Most tissues can metabolize Norethisterone. The half life of elimination varies from 5 to 12 hr with a mean of 7.6 hr. with single daily doses a steady state concentration is reached in 5 to 10 days. Small amounts of norethisterone pass into breast milk, the concentration being 10 to 20% of that plasma.
Norethisterone is metabolized mainly in the liver. A little, probably less than 1%, is excreted unchanged
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