Estriol phosphate (E3P), or estriol 17ò-phosphate, also known as estra-1,3,5(10)-triene-3,16ñ,17ò-triol 17ò-(dihydrogen phosphate), is an estrogen which was never marketed. It is an estrogen ester, specifically an ester of estriol with phosphoric acid, and acts as a prodrug of estriol by cleavage via phosphatase enzymes in the body. Estriol phosphate is contained within the chemical structure of polyestriol phosphate (a polymer of estriol phosphate), and this medication has been marketed for medical use (brand names Gynäsan, Klimadurin, Triodurin).