<p>There are strong indications that estrogens, estriol included, can increase the pharmacologic effects of certain corticosteroids. If necessary, the dosage of the corticosteroid should be reduced. There are also some indications, mainly obtained with other estrogens or oral contraceptives, that concurrent use of estriol with activated charcoal, barbiturates, hydantoins and rifampicin may possibly decrease the effectiveness of estriol.</p>