Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2019 Publisher: Pfizer Limited, Ramsgate Road, Sandwich, Kent, CT13 9NJ, United Kingdom
Pharmacotherapeutic group: sex hormones and modulators of the genital system, progestogens
ATC code: G03DC02
Norethisterone administration increases the protein and sialic acid content of cervical mucus which prevents penetration of the mucus by spermatozoa. It causes changes in the structure of the endometrium such that implantation of blastocysts is impaired. It also reduces numbers and height of cilia on cells lining the fallopian tube, which could delay tubal transport of ova.
Norethisterone is rapidly and completely absorbed after oral administration, peak plasma concentrations occurring in the majority of subjects between 1 and 3 hours. Due to first-pass metabolism, blood levels after oral administration are 60% of those after i.v. administration. The half life of elimination varies from 5 to 12 hours, with a mean of 7.6 hours. Norethisterone is metabolised mainly in the liver. Approximately 60% of the administered dose is excreted as metabolites in urine and faeces.
The toxicity of norethisterone is very low. Reports of teratogenic effects in animals are uncommon. No carcinogenic effects have been found even in long-term studies. In subacute and chronic studies only minimal differences between treated and control animals are observed.
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