Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2019 Publisher: Pfizer Limited, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
Pharmotherapeutic group (ATC code) L02AB.
Norethisterone given at intermediate doses (5-10mg) suppresses ovulation via its effect on the pituitary. The endogenous production of oestrogens and progesterones are also suppressed, and the ectopic endometrium is converted to a decidua resembling that of pregnancy. In carcinoma norethisterone may act by pituitary inhibition or by direct action on tumour deposits.
Norethisterone is rapidly and completely absorbed after oral administration, peak plasma concentration 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.
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