PIP: A 35 year old woman on hormonal contraception became pregnant during prolonged tetracycline treatment to cure acne. There is only 1 similar case published in the literature. Tetracycline may have been responsible for alterations in the intestinal flora, thus impairing the enterohepatic cycle of mestranol. An enzymatic microsomal induction is also possible, with acceleration of estrogen metabolization at the hepatic level.