Pharmacokinetics of norethindrone acetate in women.

The plasma half-life, MCR and plasma metabolite levels at various time intervals have been studied in six women after an intravenous injection of 3H-norethindrone acetate. The disappearance curve due to norethindrone acetate showed an initial rapid disappearance of 3H with an average half-life of 7.5 minutes and a subsequent slow disapperance with a half-life of 51.5 hours. Norethindrone acetate was cleared from the plasma with an average MCR of 495 L/day. Norethindrone acetate is rapidly metabolised after an intravenous injection. Norethindrone, the main metabolite, disappears from the plasma with an average half-life of 34.8 hours. Norethindrone maintains a high level compared with norethindrone acetate at all time intervals up to 24 hours and an equilibrium is reached between the two at 24 to 48 hours.