Because tetracyclines have been shown to depress plasma prothrombin activity, patients who are on anticoagulant therapy may require downward adjustment of their anticoagulant dosage. Since bacteriostatic drugs, such as the tetracycline class of antibiotics, may interfere with the bactericidal action of penicillins, it is not advisable to administer these drugs concomitantly. Concurrent use of tetracylines with oral contraceptives may render oral contraceptives less effective. Breakthrough bleeding has been reported.