<p>Antacids containing Aluminium, Calcium, Magnesium, Zinc or Iron salts may impair the absorption of Oxytetracycline. In long term therapy, Tetracyclines depress plasma prothrombin activity and reduced doses of concomitant anticoagulants may be necessary. A few cases of pregnancy or breakthrough bleeding have been attributed to the concurrent use of Tetracyclines or Oxytetracyclines with oral contraceptives.</p>