What Are The Side Effects Of Norethisterone?

Hi , I take Zeletta 75mg contraceptive pill, until recently it stopped all periods and i started having, breakthrough bleeding, spotting and some heavy prolonged periods. I have been prescribed Norethisterone as I go on holiday and there is no pattern to the bleeding or knowing when it will start. I was told to take this 3 days before I go away, but the possible side affects concern me, and so far have had no bleeding so am kind of in the mind state of don t fix something that doesn t need fixing I m wondering if when I m away and bleeding starts would it be ok to start taking Norethisterone on that day and would it stop any breakthrough bleeding etc if i take it once it starts?