Norethisterone should not be taken during your period to delay or stop it because it will not stop the period once it has started. It is most effective when taken about three days before your expected period begins. Taking norethisterone after your period has started will not stop it, though it may affect hormone levels and could cause a lighter or shorter period. To delay your period, you should start norethisterone at least three days before your period is due, taking typically one 5mg tablet three times a day until you want your period to start, which usually occurs a few days after stopping the tablets.