Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2019 Publisher: Pfizer Limited, Ramsgate Road, Sandwich, Kent CT13 9NJ, UK
Pharmacotherapeutic group: sex hormones and modulators of the genital system, progestogens and oestrogens, fixed combination
ATC code: G03AA05
The mode of action of Brevinor is similar to that of other progestogen/oestrogen oral contraceptives. COC suppress gonadotropins in a manner that inhibits ovulation, which leads to contraception.
Such activity is exerted through a combined effect on one or more of the following: hypothalamus, anterior pituitary, ovary, endometrium and cervical mucus.
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.
Ethinylestradiol is rapidly and well absorbed from the gastro-intestinal tract but is subject to some first-pass metabolism in the gut-wall. Compared to many other oestrogens it is only slowly metabolised in the liver. Excretion is via the kidneys with some appearing also in the 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.
Long-term continuous administration of oestrogens in some animals increases the frequency of carcinoma of the breast, cervix, vagina and liver.
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