Inhibitors of CYP3A4 ostaa estraderm ilman rx such as erythromycin, clarithromycin, ketoconazole, itraconazole, ritonavir and grapefruit juice may increase plasma concentrations of estrogens and may result in side effects. Estrogens are widely distributed in the body and are generally found in higher concentrations in the sex hormone target organs. This medicine should not be used by women who are pregnant or breastfeeding. Estrogens act through binding to nuclear receptors in estrogen-responsive tissues. This information is not intended as individual medical advice and does not substitute for the knowledge and judgment of your health care professional.