Sulfates are the key circulating conjugates of ethinyl estradiol and glucuronides predominate in urine. The key oxidative metabolite is two-hydroxy ethinyl estradiol, shaped from the CYP3A4 isoform of cytochrome P450. Part of the initial-pass metabolism of ethinyl estradiol is thought to come about in gastrointestinal mucosa. Ethinyl estradiol may