<p>Interactions between ethinylestradiol and other drugs may lead to decreased or increased ethinylestradiol concentrations, respectively. Decreased ethinylestradiol serum concentrations may cause an increased incidence of breakthrough bleeding and menstrual irregularities and may possibly reduce efficacy of the oral contraceptive. Example of substances that may decrease serum ethinylestradiol concentrations include rifampicin, phenytoin, primidone, rifabutin, dexamethasone, griseofulvin, topiramate, some protease inhibitors, modafinil, ritonavir and barbiturates. Certain antibiotics including ampicillin, other penicillins and tetracyclines may reduce the efficacy of oral contraceptives. During concomitant use of this tablet &amp; other drugs that may lead to decreased ethinylestradiol serum concentrations, it is recommended that a non hormonal back-up method of contraception to be used in addition to the regular intake of this tablet.</p>