Source: Web Search Revision Year: 2010 Publisher: Pfizer Healthcare Ireland, 9 Riverwalk, National Digital Park, Citywest Business Campus, Dublin 24, Ireland
Pharmacotherapeutic Group: Hormonal Contraceptives for Systemic Use; Progestogens
ATC code: G03AC03
Levonorgestrel appears to exert its contraceptive effects by various methods. These include:
Levonorgestrel is rapidly and completely absorbed from the gastrointestinal tract, with an absolute bioavailability of approximately 100%. Peak levonorgestrel serum concentration is reached in about 1-2 hours.
Levonorgestrel is extensively bound to SHBG and albumin and only 1-2% of the total serum drug concentration is present as free steroid.
The most important metabolic pathway occurs in the reduction of the 4-3-oxo group and hydroxylation at position 2, 1 and 16, followed by conjugation. Most of the metabolites that circulate in the blood are sulphates of 3, 5-tetrahydro-levonorgestrel, while excretion occurs predominantly in the form of glucuronides. Some of the parent levonorgestrel also circulates as 17 -sulphate. Metabolic clearance rates may differ among individuals by several-fold and this may account in part for the wide variation observed in levonorgestrel concentrations among users.
The elimination half-life for levonorgestrel is approximately 36±13 hours at steady state. Levonorgestrel and its metabolites are primarily excreted in the urine (40% to 68%) and about 16% to 48% are excreted in faeces.
No preclinical safety data other than those described elsewhere in this document are considered relevant to the prescriber.
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