There are isolated reports in the medical literature that metronidazole may weaken the effect of contraceptive pills and therefore it is recommended to use additional protection for 7 days after the end of treatment. More information in the attached thread: https://www.gdziepolek.pl/opinie-i-pytania/42495/metronidazol-jak-metronidazol-wplywa-na-skutecznosc-antykoncepcji When using birth control pills, withdrawal bleeding can be of varying severity and it may happen that it does not occur at all or only in the form of spotting. If the tablets were taken correctly, this is not a cause for concern. A pregnancy test taken 2 and 3 weeks after intercourse provides a reliable result, and birth control pills do not falsify the results. I enclose helpful materials: https://www.gdziepolek.pl/artykuly/test-ciazowy-jaki-wybrac-i-kiedy-zrobic https://www.gdziepolek.pl/artykuly/jakie-leki-oslabiaja-skutecznosc-antykoncepcji