The present invention relates, in part, to the discovery that parenterally administered extended release formulations possess an unexpected advantage in treating patients possessing active CYP 3A4. This advantage is particularly beneficial where the individual is concomitantly administering a CYP 3A4 inhibitor or is in risk of doing so. Thus, the invention relates to a method for treating individuals possessing a functional CYP3A4 gene with an active agent metabolized by CYP3A4 comprising parenterally administering the active agent in a first extended release formulation in a first administration and the formulations for use in such methods. The invention further includes a method for preventing adverse drug reactions in individuals possessing a functional CYP3A4 gene with an active agent metabolized by CYP3A4 comprising parenterally administering the active agent in a first extended release formulation in a first administration.