Abstract Norethisterone (Chemical Abstracts Service Registry Number 68-22-4) is a synthetic progestin used as a contraceptive or in the treatment for amenorrhea and endometriosis. It has moderate potential for bioconcentration. The main route of exposure is ingestion of the drug whose metabolites are mostly excreted in the urine. After binding to its intracellular receptor, norethisterone stimulates transcription of related proteins. Norethisterone does not have any serious acute side effects; induction of thromboembolic states is the most deleterious aspect of its chronic toxicity. Despite having low potential for genotoxicity, it is considered to have carcinogenic and teratogenic effects. Norethisterone can also be secreted in the milk.